Welcome to the February 2021 Ask Me Anything episode of Mindscape! These are funded by Patreon supporters (who are also the ones asking the questions). This month is in what has been the conventional format, where I just try my best to answer every question. But it’s growing a bit unwieldy, so going forward I might just try to pick my favorite questions and answer them in greater detail. We shall see.
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Peter Benham
Being able to ask questions is a great bonus of being a patron. I had a feeling it would be the case but Why is it so hard to ask a good question and not sound like an imbecile no matter how much you think you know about something??
Nicholas Walker
Are the answers to dark matter and energy to be found at the quantum level?
John Lounsbery
With our current technology what’s the fastest craft we could build and how long would it be to overtake Voyager I?
Maksym Aleksandrowicz
In “the big picture” you point out a correlation between complexity and entropy. Does it work both ways mathematically? Can entropy be decreased by reducing complexity?
Robert Ruxandrescu
Suppose you’re some distance away from a black hole and you throw an object in it. You’re going to see the object close in on the black hole’s event horizon but never actually “pass into” the black hole. Since from your perspective no object is falling into the black hole, how can we then say that black holes increase their size when objects fall into them?
Doug Orleans
What are your thoughts about Lee Smolin’s book The Trouble With Physics?
Matteo Tanaka
Are we sure that the universal constants like the speed of light have been the same value since the beginning of time?
Deepthi Amarasuriya
What are your ideas on helping undergraduate physics students develop physical intuition, and problem solving skills in tandem?
Jon
When a black hole evaporates is there a point where it no longer has enough mass to be a blackhole and undergoes a phase change into something like a neutron star?
Patrick Hall
As you have mentioned on the podcast, your wife, Jennifer Ouellette, is a science writer and author. Have you and your wife ever collaborated on projects together? If not, is it something you two plan on doing in the future?
Jaime Tan
does physics allow for something to appear out of absolutely nothing, no-time and no-space?? Another way to ask this is, do the laws of physics prohibit such a thing?
Jeff B
How should we conceptualize particle interactions from the many-worlds perspective (especially since wavefunctions are seen as physically real)?
Michael Shillingford
What’s your preferred answer to Van Inwagen’s Special Composition Question?
Jeremy Payne
Could you comment on super-fluid dark matter theories?
https://arxiv.org/abs/1809.00840
Justin Bailey
What was it like going on the Colbert report? Did he prep you for the questions or were you reacting in real time?
MD Murtagh
why is the emergence of space-time specifically *not* like the emergence of a baby bird from its egg?
Joshua Hillerup
Why does black hole evaporation seem to be so much more accepted as true in physics compared to other theories that aren’t confirmed by experiment?
Bill Warner
What is the volume of a black hole with respect to the event horizon? Is a black hole most like a solid, a liquid or a gas?
Scott
In quantum field theory, is it preferable to think of a particle and its anti-particle as arising from the same field that has an extra degree of freedom to allow the conserved numbers like electrical charge to take on opposing values, or do you think of a particle and its anti-particle as arising from different fields?
Patrick Henderson
Have you ever encountered a campaign to systematically misrepresent your views in order to lend credibility to fringe or conspiratorial ideas?
Jacob Arkin
Are you familiar with the recent Watchmen HBO adaptation? If so, what do you think about its portrayal of Eternalism and attempt at a “Laplace’s Demon”-esque character?
Michael Daniels
If the cosmos used to be dense, and closed, and subsequently expanded with decceleration to flat space with an omega of 1, allowing dark energy to take over from gravity and accelerate the expansion toward an eventual omega limit of 0.7, must we not be flying headlong into pringle land?
David H
When the universe splits, à la Everett, is the split instantaneous across the whole pre-existing universe, or does it propagate at the speed of light?
David Maxwell
Could dark matter be the result of the theoretical white holes? That the process involved makes what comes out invisible as normal matter/energy but it still affects gravity?
Paul Hardie
I saw a video discussing the Janus point and that you did work on it. Can you briefly explain what the Janus point is? Thanks
DisconJazz
If you had to redo your entire education over, and for some reason were precluded from studying any discipline of physics proper, what branch of academia would you choose?
Jeremiah M
What are your personal top 10 most interesting unanswered questions in Physics?
Frank Lehman
Do you have a particular stance towards the “unreasonable effectiveness of mathematics” in science, as articulated by Wigner and many philosophers of science and mathematics since? Does it keep you up at night?
Brandon Sherwood
When you were an undergraduate did your first physics classes click with you right away or was it a slow learning process, like it is for myself?
Matjaz Hocevar
In your last solo episode on democracy you mentioned that it is not worth to keep contact with people or even friends that have radically different views and believes of the world/reality/politics and it is best to part ways in those cases. My question is: Did you have such or similar experience and what would have to happen or/and change to resolve this and renew/continue that relationship?
Sam Barta
Why is entropy so important if by yours and others definition it is a measure about how much we don’t know about a system?
Jonny
From your recent conversation with Avi Loeb, did you walk away thinking that looking for alien life is a worthwhile use of time and resources?
Matthew O’Connor
You mentioned on Twitter that you’re leaving CalTech. I’d be interested to know anything you can share on why, what’s next, your thought process, and what this means for the QM textbook.
Hershy Silver
On a scale from 1 to 100, what is the likelihood that humans will find a wormhole (short cut) to travel to distant galaxies within the next 50 years?
Sherman Flipse
How does the “weight” assigned to a given branch of the wave function correspond to the number of microstates that are in superposition in that branch?
Shawn Drew
The visual representation of electron wave functions seems to have some resemblance to the hypothesized dark matter distribution around/in solar systems. Is there anything to this and are there actual smart people thinking about this ?
Charles O’Connor
If the fields of QFT are pervading spacetime in 3 spacial dimensions, how would you accurately describe the geometry of waves in those fields?
Evan West
Can Lorentz contractions occur at the Planck length?
Elias Börjesson
What is true about animals, that if true of humans, would make it ok to breed and kill humans? So for example, if you would answer culture, would it be okay to kill humans if that was part of some culture, etc…
Amanda Lorian
How does the Heisenberg Uncertainty Principle apply to the idea of “fixed” electron orbitals, if at all?
Brian Owenson
do you have a good explanation of why the Lagrangian is T – V and not something else? Leonard Susskind says it’s T – V because that’s what works! Was there no better physical, sort-of intuitive way to get to T – V?
Ondrej
You’ve said that the Big Bang was similar to a white hole, that is, a time-reversed version of a black hole. But doesn’t such a time reversal require gravity to be repulsive rather than attractive?
Richard Moster
Suppose we have three “ideal” clocks (they keep perfect time and never run out of power). (1) The “Earth” clock stays on Earth. (2) “My clock” goes with me on a spaceship. (3) A “smart clock” also goes with me on the same spaceship. The point of the smart clock is that it is fed data regarding spaceship acceleration, location, velocity–whatever it needs to calculate and display the current time of the Earth clock IN THE SENSE that after zipping around the galaxy at near light speeds for awhile I can return back to Earth and the Earth clock and the smart clock will display the same time. Is this possible and, if so, then don’t I have a mapping at all times between my clock and the Earth/smart clocks? Wouldn’t that provide a sense of synchronicity between Earth events and events on the spaceship?
justin wolcott
If I understood correctly, you said in the Big Picture that free will isn’t possible (from the perspective of an omniscient observer) because the current state of the universe equals the prior state of the universe plus the laws of physics.
What about fictional universes where magic and deities exist and don’t obey the laws of physics?
Stefan Bernegger
How does Eric Weinstein’s “Geometric Unity” attempt to unify GR with QFT compare to other attempts like String Theory?
Simon Carter
I recently read “Lost in Math” by Sabine Hossenfelder where she argues that ‘modern physicists’ obsession with beauty has given us wonderful math but bad science’. Stating that There is no known law in nature that says that the universe must comply with fundamental physicists notions of mathematical beauty. Do you have sympathy for her concerns?
Daniele Cortesi
In your podcasts with Chalmers and Goff speaking about philosophical zombies you claim that “since zombies say that they have an experience and are conscious, then none of us know that we’re not zombies because they think they’re conscious like we do”.
So are you saying that consciousness could be an illusion? Or am I misunderstanding something?
Sam
What is one piece of advice you wish someone had given you before you entered grad school?
Chris Fotache
You’ve mentioned many times how we see an object “falling” into a black hole, how it disappears past the event horizon. But can you describe what would happen to someone like me, once I get past it?
Brent Meeker
There have been proposals to use the Sun as a “lens” of a telescope via the Einstein’s ring phenomenon. Of course the Sun is bright and you have to block out it’s visible light to make this work. But the Sun is transparent to gravitational waves AND is has varying gravitational potential, higher at the center. So could you use it as a Luneberg lens of a gravity wave telescope?
Paul Hess
Is empty space scale dependent? Would a cubic parsec of emptiness have 34 times more chance of random quantum stuff happening than a cubic LY would?
Dan O’Neill
On New Years Eve, I found myself trying to explain time dilation in special relativity to my wife, and came up with the following thought experiment. It ignores travel time for light signals to the observers, but is it otherwise valid?
Michael Bukwich
What do you get when you chop an avocado into 6.022 x 10^23 pieces?
Brent Meeker
The diagrams of the solutions describing black holes are nicely symmetric and include inaccessible “other universes”. But this is because they are static, eternal solutions. How does the Penrose diagram for the Kerr-Newman black hole get truncated or changed for one formed by stellar collapse?
Brian Tidmore
Two twins in a spaceship accelerate at 1g from Earth towards Alpha Centauri. One twin resides inside the center hull of the ship. The other twin resides inside a chamber connected to an arm that rotates around the ship so that she experiences 1.5g’s. The twins can see each other through the arm that connects the two locations.
What do the twins see as they travel?
Redbeard
I am curious to know what you think about the category of arguments that includes the simulation and doomsday arguments. I think that they are hard to entertain for two main reasons: the arguments appear to require that we place no evidentiary weight on the input we get from our senses; and I think there is no pathway built in to the arguments that would allow an observer that is either not part of the simulation or not an average observer in the history of our technological civilization to determine that he or she is unusual.
Pavlos Papageorgiou
How does the gravity of a black hole get out of the black hole, if gravitational waves normally move at the speed of light?
Wil Soo
Regarding the way people usually talk about consciousness, could you explain the phrase “what it’s like to be something”?
William E Clark
Are you an advocate of eternalism, i.e., the block universe? If so how do you reconcile this with our experience of the flow of time, the Many Worlds interpretation of Quantum Mechanics, and Special Relativity?
Miron Mizrahi
What is the challenge of figuring out AdS/CFT for a universe with positive cosmological constant?
Wes Clyburn
What is your favorite species to have ever existed in the entire history of the planet, excluding modern human.
Christopher Mathews
As an attorney, I can’t help but wonder about the implications the Many Worlds Interpretation could have on our legal system.
Yavor Trasiev
Your personal opinion of Gödel, Escher, Bach and how it has aged science-wise?
Tim Kennedy
What are your thoughts on the Social Contract as we head into the next 50 years or so, maybe particularly your expectations for both high and low achievers?
Jim Murphy
While reading Feynman’s Lectures, I found it interesting that he essentially posits conservation of energy as an axiom of the universe. After thinking for a while, it occurred to me that the most reasonable explanation for this fact would be based on the anthropological principle. Universes where energy is not conserved would not be able to sustain processes like life in any meaningful way. What are your thoughts on the conservation of energy as evidence for a cosmological multiverse?
Denis Goddard
Suppose for a moment we exclude SUSY. What alternate mechanisms do you think are most likely to explain the low observed value of the cosmological constant?
Kirtland Edward Hobler
What is a good metamathematics or philosophical explanation for why the complex number system is necessary for contemporary science.
P Walder
Within the Popperian worldview when knowledge is ‘created’ (as described by Deutsch) is it being invented or discovered?
Gregory Egan
I can only wrap my head around the “many worlds” interpretation , if I concede to infinity and at least one additional spacial dimension. Do you concur?
Dragan Tubic
Although Wheeler himself denied retrocasuality in the delayed-choice quantum-eraser experiment, I’m still unconvinced. Could you please explain why the casuality is not violated in this particular experiment?
Ken Wolfe
If the expansion of the universe driven by dark energy continues to accelerate and we are headed for the Big Rip, will there be any way to survive that?
Suraj Rajan
In the Time Travel episode you’d mentioned that the “movie versions of” the watch of a time traveling hero running fast is incorrect. And that the watch would actually be showing the passage of time as “personally experienced” by the time traveler. So, what would the time-traveling twin appear to have “aged less”, with respect to? Does this ageing process have something to do with the entropy of the twins’ bodies?
anon
This question is about general advice for science fiction writers. Consider a setting like the one from Star Wars. That society has the technology, budget, and political will to build fleets of space ships and so on. Surely the scientists of Star Wars and similar settings are building equally ambitious, even fantastical physics projects. For example, using an asteroid belt as material to construct a massive circular particle accelerator in space. What kinds of projects would today’s physicists build if a Star Wars technology level and budget suddenly became available, and what are some possible discoveries that could change society?
Aaron McBride
How’s the quantum gravity research going?
Michael Baker
Why is the continuous rotation of the galaxies not considered perpetual motion?
Alan White
What do you think of Stephen Wolfram’s physics project?
Ash Wright
Is the unlikely state of entropy at the big bang the only difference between time and the space dimensions? If one of the space dimensions had a similarly low entropy, would we perceive time-like effects along that dimension?
David Bowland
I watch videos by a gentleman on YouTube named Paulogia who addresses ideas like the Kalam, which I know you know from Bill Craig. Paul’s response is typically that “energy” could have always existed. I’d prefer to phrase it that there could be a first moment of time or there could be a larger system involved. Is either of us in the ballpark of saying something sensible?
Ashley Hayat
How do cosmologists estimate the amount of matter and energy in the universe?
kc
please speculate on whether the Everett branching creates new a Universe quantum field or is there only one?
LINEU D MIZIARA
In Franck Wilczek’s wonderful book “Fundamentals”, he says that the cosmic horizon is expanding. How is this possible, if the expansion of the universe is accelerating?
Enrique Arriola
If black holes are formed in part by the presence of mass and most of the mass is a product of the strong force holding quarks together, what happens to the strong force once the black hole is formed?
(Mike) Briggs
I’ve heard you say that you “believe in” Many Worlds. Wouldn’t it be better to say “convinced by”?
Jason
To what extent is the “spin” of a particle, such as an electron, analogous to the spin of a macroscopic object such as a basketball?
Keith
In EQM, can split wave function branches evolve to an almost identical or identical state, essentially giving that state multiple possible histories within the universal wave function, at least on very small time scales?
Jim Sicilian
Have you considered inviting Colin Woodard (author of “American Nations”) as a guest?
duncan palmer
Do you believe it is possible or even inevitable that computational algorithms will develop some of the next important breakthroughs in theoretical physics and if so does it matter if the human mind understands how the algorithm connects input data to observed output or simply that the algorithms “ work” in fitting the data ?
Elias Grankulla
If you could experience a historical event in person what would it be?
Allen Gebhardt
This relates to the “hard question” of consciousness as presented by Chalmers. Assuming that sensory input increases with the complexity of organisms and that a self-reflective consciousness provides significant survival advantages, how is it a problem to posit that such a deliberative executive functionality is a natural development of Darwinian adaptations across species ?
Jorge
Watching The Biggest ideas in the Universe I understood that, at least in the context of Effective Field Theory, the ‘constants’ like the fine structure constant α, are not constant but functions of the cutoff energy (E). In that case, when you say that, for example, α=1/137, which E is being considered and why?
Krisztian Dobo
It seems to me governments and organizations sometimes choose to distort or simplify facts for the “public’s own good”. When there are not enaough PPEs they might say it’s not effective anyway, when arguing with anti vaxxers they say no vaccine ever causes serious side effects etc. Nothing is ever black or white but what do you think in general of society’s right and capacity to know and handle the truth, where do/would you draw the line?
Tom
what do you think of the name – ‘The second law of thermodynamics’? I studied physics and philosophy at Uni, and we did a short course on entropy, and I never understood it because they kept calling it a law and it seemed obvious it wasn’t a law it was just a likelihood. How can you get anything profound out of a likelihood?
Gary Miller
In your Mindscape with Max Tegmark, you mentioned that the high resolution of our universe may signal that we are in the real thing. However, could there be a selection pressure for high quality simulations, like the creator of the simulation prefers high quality simulations?
–
There are about as many human thought-moments (~.1 seconds) in a human lifespan as there are human lifespans in the age of the universe. Is that some sort of anthropic maximum likelihood thing?
Kristoffer Gustafsson
Is there a globally agreed upon “map-structure” for all the known starts, planets, galaxies, etc. among people working with Astronomy?
Alon G
It seems ‘logical’ that if there is ANY place in the universe where information is not preserved, it is inside a singularity. Why not accept this and face the consequences accordingly?
Samuel Valle
Based on your experience as a student and professor of physics, are there any issues or changes you would make to the post-graduate process of a PhD in physics?
Eric Chen
What’s the argument for why humans should try to reason like Bayesians? Even if we accept something like Solomonoff Induction as the correct account of ideal reasoning, what’s the further justification that boundedly rational agents like us should try to approximate this ideal when reasoning in practice?
Peter A Bamber
Is a wormhole linking space inside the event horizon of a black hole to space outside the event horizon a valid solution to the equations of general relativity? If the answer is yes, does it have any implications for the information loss problem?
Chris Vigorito
How would you describe the difference between an “ether” and a quantum field? On a superficial level they seem to be similar in concept.
Yahel Guberman
Regarding life outside earth, you say that we have a sample size of 1, so life could be ubiquitous or rare. Life appeared on earth fairly early on, and we have no evidence that life evolved independently more than once on earth. Doesn’t that count as evidence against ubiquity?
Damian Aleksiev
What are your thoughts on Hermann Minkowski’s statement that: “space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality”?
David Grimes
It was recently proposed that BHs could have formed orders of magnitude larger than SMBHs – (Stupendously LArge Black holes) are theorized to mass 100 billion solar masses. However, reports also state that it’s been thought BHs could not grow larger than 50 billion solar masses, due to the way accretion works.
What is it about the SMBH accretion process and the special areas of spacetime surrounding such monsters that keep them from growing further?
Allan Rasmussen
Do you see the number of spatial dimensions experienced by humans and used in Newtonian and quantum mechanics (=3) as an emergent or fundemental property?
Anton Hawthorne
I’ve heard talk about a ‘multiverse’ – in the sense of inflation causing the separation of bubble universes – where physical constants can vary between them. FMy question is: what theory predicts these constants to not be constant; why would we expect them to vary over cosmic distances?
Alan Mroczek
Do you think that there is something that we can think of as a “cognitive wall”? By “cognitive wall” I mean a limit to what can be learned, processed, and understood within a human lifespan and/or that certain topics have too many variables to consider.
Alexander Cordova
How is it that some dimensions are smaller than other dimensions (In the context of possible extra dimensions existing)?
Craig Gordon
Are all the force carrier particles virtual? If not, what is the difference between a real photon, gluon, W & Z Boson compared to a virtual one?
John Eastmond
Have you seen the Netflix series Surviving Death episode 6 featuring Jim Tucker’s fascinating research on children with memories of past lives?
Marco Faella
Regardless of current technological limitations, or from the point of view of an extremely advanced civilization, what would be the pros and cons of communicating via gravitational waves?
Jan Smit
Assuming the many-worlds interpretation is a correct interpretation of quantum mechanics, here my question: “ewvif fmarewgknm e bfWF wefwl mytm?”.
I assume in one of these many worlds you will be able to understand this gibberish and anwer my question. Let’s hope it is in this world, I don’t want to waste your time.
anon
Why do you assume that the entire opposition to Democrats is antidemocratic?
Andrei Dinu [edited]
Would you agree to the following description of reality? Ignoring complications related to particle spin, a system of N particles is defined by a complex-valued wavefunction of 3xN variables. If the wavefunction is real then the 3xN dimensional space is real too. And this means that the answer to the question about where Everett’s worlds actually exists, is simple. Everett’s worlds are nothing but peaks of a wavefunction in the 3xN configuration space.
Avi Chein
I’m an undergrad physics major. Last semester I took QM. I’m trying to understand spin of elementary particles. How should I imagine it? Is it similar to a classically spinning object?
Olexiy Zablotsky
I’ve watched that Max Tegmark’s mathematical universe thing, and quickly became convinced.
But there is a problem. There are more complex mathematical structures than simple ones. And our actual laws of physics seems simple.
So Imagine a sequence of random bits with length L. And each subsequence found in it, and then interpreted in some simple formal language – being “breathed fire into equations”.
Jonathan Gordon
Could you do an interview with Judea Pearl on his book about the science of cause and effect called The Book Of Why?
Tim Allman
One of the first things we all learn about QM is that matter and antimatter will annihilate each other in a flash of gamma rays should they contact each other. However, mesons, which consist of a quark and an antiquark exist, even if only for a short time. When they decay they produce particles such as electrons rather than the photons that one might expect.
jason levy
We assume that our senses and our level of intelligence is enough to comprehend any truth about the nature of nature, given time and the proper thinking. Is it possible that that is a misperception? Do you think it is possible that the most fundamental questions of physics are beyond the reach of human understanding?
Anders
If a mouse made it into the house, who would catch it first – Ariel, Caliban, you, or your wife?
Mark Imel
In your recent podcast with Avi Loeb, he mentioned that Oumuamua is more or less at rest in the local galactic frame.
Are there cosmological models that would predict rocks being at rest to local galactic rest frame?
Eric Coker
What inspired the term Mindscape for your podcast? I ask because I just picked up Rudy Rucker’s Infinity and the Mind and the term figures predominantly in the first chapter. Any relationship?
Beth Mowry
Re: the idea of alternate versions of ourselves existing in parallel universes, do these versions “split off” every time a choice is made?
Jack Lyle
Why is the charge on a electron exactly equal to minus the charge on a proton? In 1911 this was a just so story, but is it still one today?
David Whitmarsh
How might someone experimentally test whether the Everett interpretation of QM is correct?
Richard Taylor
Imagine that as an undergraduate you were abruptly transported to the year 2021. What would impress you the most about this world of the future, and what would be your greatest disappointment?
Josh
In Something Deeply Hidden, you mention that the many worlds don’t /really/ exist, and are instead just a useful tool for human thought. Can you elaborate on this point?
Siddhartha
Even if Hilbert space is sufficiently big to contain all branches of the wave function of our universe till now, what would it mean to have run out of room when it eventually happens? Say, our calculations are wrong and the universe runs out of room tomorrow, what would be our subjective experience?
George Sharabidze
How does the Zeno’s dichotomy paradox affect many worlds theory?
Dan Inch
As a non-American, the concept of “legacy admissions” to a university seems very odd, and not in keeping with the goal of excellence in learning or research. I understand that Caltech does not adopt this approach, but wondered what your views are about the practice.
Arthur Adelberg
Does the kind of timepiece being used affect whether it would register time dilation?
Herb Berkowitz
If General Relativity is the physics of the “big world” and quantum mechanics is the physics of the “tiny world” where is the border between the two where one applies on one side and the other applies on the other side?
Robert Murley
I have frequently heard it said that the vacuum energy of the universe is about 120 orders of magnitude lower than it “should” be. But I have never read why physicists think it should be that high. Could you oblige?
robert granese
I’ve often heard you say that the Universe has no center. How is this possible?
Rosemary Fehlner
What do you think are the most interesting frontiers for the intersection between philosophy and science today?
Phil Johnston
I would love to know your thoughts on if you think we are living in a simulation and what it would take to create a universe sized simulation? Is there any clues at the quantum level that would suggest an underlying code structure?
Madox McGrae
If you had to choose, which do you think contributes most to the dark matter puzzle:
WIMPS, axions or other light particles, MACHOS, MOND, or something else?
Jerold Swan
In a recent podcast you expressed some sympathy for Derrida. What are some of his ideas that you think are genuinely original or that actually benefit from his style of writing?
Saad Ahmed
As I have stared using twitter more recently, I have realized that there is no shortage of doctors, scientists, and academics who question the effectiveness of the lockdowns, school closings, and the general narrative around the pandemic. How do you generally approach people or ideas that seem to contradict the mainstream views?
Ferry
In the case of black holes, at what point does our current understanding of physics breaks down?
Ross Hastings
your recent chat with Avi Loeb got me thinking about aliens, and the Fermi paradox. What if Dark Matter was an alien… thing? Could Dark Matter be… aliens?
Bill Seltzer
Might an extension of quantum theory, which enabled the derivation of space and time, also show that the Schrödinger equation to be not fundamental?
Kirk Briggs
I’ve read through your reading list on the PDF download from the Great Courses Plus and all of the books were great except John Bell’s book was over my head. I’d like to learn more about Eternalism and how it’s mapped to Spacetime.
David Lange
I know that the Universe probably isn’t ADS but what do you think are the chances of string theory and a holographic Cosmos leading to a correct formalism?
Ray Waldin
The recently published paper “Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies” (Kyu-Hyun Chae et al 2020 ApJ 904 51) claims to have detected External Field Effect in over 150 galaxies to varying degrees corresponding to the density of neighboring galaxies. If this result can be verified and reproduced independently, will Dark Matter no longer be required to explain the observed rotational curves of spiral galaxies, and is there any other reason to continue looking for it?
Riverside
Let me once again rephrase a question on moral implications of the MWT.
Yoram Ramberg
It is my understanding that the purpose of empirical science is to find out the truth about nature and how it works. This would suggest that progress in science involves the discovery of facts about nature. Yet, when talking about scientific theories you’re always using the language of invention. Why are you using the term ‘invention’ and not ‘discovery’?
douglas albrecht
If I understand, the many worlds hypothesis flows from an ontological starting point that reality is the wave function. And That’s all there is. What’s the evidence for this being a complete description of reality? And Why are you so certain of this assumption?
Jonathon Tucker
Can you invite David Albert back on? Pretty sure I’d be enthralled just listening to you two discussing toasters.
Boris Petrovchich
Regarding “multiverse” and “many worlds” hypotheses which simply can’t be in any way experimentally verified — are they a science or “religions”?
Mark Cark
I’d like to hear musings as if you were talking to a small child. How can the Universe be either finite or infinite? Neither makes sense.
dominique de caluwe
Why does a photon reflect from a mirror under the same angle. If it is re-emitted by an atom, why in this specific direction ?
Steve M.
I seem to be at an impasse. I know my finger will not pass through this table because fermions don’t do well moving other fermions – but wait, those are only waves, perturbations in fields – lots of fields intersecting at my finger tip. Why do I perceive the tables existence when all those fermions are in super positions of being and not being perturbations at this moment?
Allen Proxmire
entanglement is one of a few topics Google and YT haven’t given me the understanding I want. even your greatest ideas leaves me with a lot of questions. would you consider doing another vid on that?
Ramon Durman
If you can imagine for a moment that you are attached to a weather balloon and you’re leaving Earth’s atmosphere, and drift further into space beyond our Solar System and the Milky Way. As you continue to drift in the same direction you pass the galaxies furthest away from earth until you reach the very edge of the universe. What do you see?
Michael Lesniak
Why do the positively charged quarks have double the electric charge as the negative ones? Is this an indication we don’t have the entire picture, and the positive quarks are actually composite particles (composed of two each, perhaps)?[/accordion-item][/accordion]
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0:00:00.9 Sean Carroll: Hello, everyone. Welcome to the February 2021 Ask Me Anything edition of The Mindscape Podcast. I’m your host, Sean Carroll. As probably most of you know, this AMA is something we do almost every month. I don’t do it at the end of December, as I take the holidays off, but every other month, 11 times a year, we do something where patrons, that is to say, supporters of Mindscape on Patreon, can ask me questions. They leave questions in the comments of a post I put up on Patreon, only costs $1 per episode to be a Patreon supporter, and then I try to answer as many of them as I can, and the patrons are the ones who suggested that going forward, we should make the answers, that is to say, the episode you’re listening to right now, public domain. So that is what we’re doing. So this is available a little while after the patrons get it, it gets released into the wild, and anyone can listen.
0:00:54.4 SC: I noticed that over the past couple of months, there have been an increase in the number of patrons, which is a good thing, I’m not gonna argue about that. And also, of course, we skipped last month doing the AMA, so as a result, we got a lot of questions this month. I think ordinarily, the number is around 100, and this month, it’s closer to 175, 180, something like that. And so, it does become difficult for me to answer all the questions in a reasonable period of time, and I really want to do that. So for this one, I did it. I just answered all the questions that I got. I did try to make the answers a little shorter than they usually would be. I reined in my natural inclinations to just go on at great length on somewhat tangential topics. So, going forward, if we continue to grow and there continue to be more questions asked in the AMAs, we can contemplate different strategies for what to do about that. A strategy of just continuing to answer every question is going to be harder.
0:01:53.6 SC: So for the people who support on Patreon out there, if you wanna leave comments about what do you prefer, do you prefer if I just answer some subset of the questions, the first hundred questions, the best hundred questions, or random hundred questions? Should I answer even fewer questions, but at greater length? Should I pick the questions I really think allow me to say interesting things rather than some of the questions I have to say like, “No, [chuckle] I can’t answer that. I don’t know the answer to that.” These are all possible ways going forward. With the constraint that I don’t want the AMAs to go on too much longer than three hours, I’m pretty open to what the strategy should be. Anyway, we did get a lot of questions this month, we got some really good questions, so, let’s go.
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0:02:53.4 SC: Peter Benham asks, “Being able to ask questions is a great bonus of being a patron.” And by the way, I did not ask Peter to say this, this is not a set-up, he just mentions it right off the bat. But then he says, “I had a feeling it would be the case, but why is it so hard to ask a good question and not sound like an imbecile, no matter how much you think you know about something?” So I’m going to deny the premise of this question, Peter. I think that, in fact, if we read over the questions that people ask in the AMAs, they’re generally pretty good. I don’t think any of them are sort of things that I would say, “Oh, you sound like an imbecile.”
0:03:24.5 SC: Some questions are easy to answer, some questions are hard to answer, but the interesting thing is, why would you feel like you sound like an imbecile? And I don’t just mean you; I mean, why do we collectively worry about how we sound when we ask questions? To be fair, I tell this to my graduate students and they don’t always like to hear it, but on the one hand, for graduate students, you wanna tell them, “It’s part of your job to speak up in seminars, to ask questions, to participate in the conversation. You’re a scientist now; you’re not just a student.” On the other hand, they say, “Yes, but I worry that if I ask a dumb question that everyone will be judging me.” And the truth is, the honest response to that is, yes, everyone [chuckle] is judging you. This is part of being a student, part of being a graduate student, especially. People are judging you all the time. For what it’s worth, they don’t stop judging you when you get your PhD. The judging of other scientists is just part of academia as it is right now. Hard to imagine getting away from that.
0:04:21.9 SC: So I get why you would worry that you’re sounding like an imbecile and it’s not something that you can dismiss easily, but ultimately, you’ll be much better off if you just say, “You know what? If I have a question and it’s a good one, I’m gonna ask it. Let the chips fall where they may.”
0:04:39.3 SC: Nicholas Walker says, “Are the answers to dark matter and energy to be found at the quantum level?” The short answer is, I don’t know, since I don’t know what the answers are. The second shortest answer is, I don’t know what it means “to be found at the quantum level.” The quantum level is the whole universe. Everything in the universe is quantum, so in some trivial sense, it is, sure. Probably what you mean is something like, “Are there specifically quantum phenomena that can go into explaining or that are needed to explain dark matter and dark energy?” For dark matter, the answer is probably no. It’s very easy to come up with models of dark matter that fit the data perfectly well. The problem is there are too many of them and we don’t know which one is right. And none of these models involve esoteric quantum phenomena in any central way.
0:05:25.6 SC: For dark energy, it’s harder to say. We have a perfect candidate for dark energy: The cosmological constant that Einstein mentioned a long time ago. You can have a perfectly classical cosmological constant, but there’s a big puzzle about why the value of the cosmological constant is the value that it is. So, it’s possible that the answer to that puzzle will lie in some interesting quantum mechanical phenomena. It’s absolutely possible, and we just don’t know right now, since we don’t know what the answer is.
0:05:53.1 SC: John Lounsbury says, “With our current technology, what’s the fastest craft we could build, and how long would it be to overtake Voyager 1?” I have no idea. You’ve clearly mistaken me for someone who knows something about rockets or engineering, and that is not me.
0:06:06.8 SC: Maxim Alexandrovich says, “In The Big Picture, you point out a correlation between complexity and entropy. Does it work both ways mathematically? Can entropy be decreased by reducing complexity?” Well, I think it’s not quite as simple as a correlation. What I point out in The Big Picture is that complex structures can arise dynamically as entropy increases. If things are very, very, very low entropy, they tend to be simple, there’s not a lot of room to be complex. If they’re very, very, very high entropy, they tend to be simple. So what that means is in between, in the medium entropy regime, that’s where complexity can arise. But it doesn’t necessarily arise, it depends on the dynamical rules that got you from low entropy to high entropy. So there’s no simple straightforward correspondence between an amount of complexity and amount of entropy.
0:07:00.1 SC: Robert Ruckson-Drescue says, “Suppose you’re some distance away from a black hole and you throw an object in it. You’re going to see the object close in on the black hole’s event horizon, but never actually pass into the black hole. Since from your perspective, no object is falling into the black hole, how can we then say that black holes increase their size when objects fall into them?” So it’s actually a bit of a myth that you never see objects fall into a black hole. That would be the case in the idealization where what you’re throwing into the black hole has no gravity itself. It is what we call a test particle when we teach general relativity. But in the real world, everything you would throw in a black hole has some gravity itself. So, when you take that into consideration, what happens is that the event horizon of the black hole grows a little bit to swallow up the object that you’re falling into because the combined gravitational field of the black hole and the object is a little bit more than the black hole itself. So it’s a bit of a myth, a simple… A simplification that works mathematically, but it’s not quite what we see in the real world, to say that you never see things fall into black holes.
0:08:05.6 SC: Doug Orlean says, “What are your thoughts about Lee Smolin’s book, The Trouble With Physics?” I hadn’t thought about that book or read it in quite a while. I had mixed feelings about it. I mean, Lee is a smart guy, and he has interesting things to say about quantum gravity. I was disappointed that so much of the book just went into attacking string theorists personally rather than attacking them for their science. And I don’t mean calling out names and saying they’re bad people, but I mean painting people who do loop quantum gravity as brave truth-seekers trying to understand the world at a deep level, and painting string theorists as just sort of boring people moving incrementally ahead without much imagination. And if you think I’m exaggerating, then I encourage you to check out the book. And there’s a lot in there about group think, and basically, the picture being painted is that string theorists just aren’t nearly as clever and creative and independent as loop quantum gravity people are. I think that is entirely the wrong attitude to take about that particular problem.
0:09:06.2 SC: Mateo Tanaka says, “Are we sure the universal constants like the speed of light have been the same value since the beginning of time?” No, we’re not at all sure. People have been writing plenty of papers suggesting alternatives. So far, all the data are compatible with the idea that the universal constants are the same value, but they might not be. In particular, things like the mass of the electron probably aren’t. In the standard model of particle physics, the electron was massless before the electroweak phase transition and is massive now. But of course, the electroweak phase transition is so deep in the early universe that we’ll never observe it directly.
0:09:41.1 SC: Diptha Amara Soria says, “What are your ideas on helping undergraduate physics students develop physical intuition and problem-solving skills in tandem?” This is a great question, but I don’t have any deep, special ideas about that. I think in practice, it happens by doing it. You don’t develop intuition by reading a book on how to develop physical intuition, it might be a little bit of a help, but you gotta do it yourself. It’s like learning to play the piano by reading a book telling you how to play the piano, but never actually playing it. That’s not gonna work. It’s just never gonna do it. Developing physical intuition, solving problems, is a lot like playing the piano or hitting a baseball or whatever. These are skills that are in some sense, not reducible, at least in our current way of thinking, to an algorithm you can just learn and therefore master. You have to actually physically do it to learn.
0:10:35.4 SC: John says, “When a black hole evaporates, is there a point where it no longer has enough mass to be a black hole and undergoes a phase change into something like a neutron star?” So no, the black hole does not need to be very massive. You can have a black hole as small as the Planck mass. And I know that from particle physics’ perspectives, the Planck mass sounds like a huge number, but when you measure it in grams, it’s something like 1/100,000th of a gram. A Planck mass is not a very big object. So we think that black holes just evaporate away, staying black holes until they go below the Planck mass, and then they just dissolve into kind of a cloud of photons and other elementary particles. There’s no point at which they act like a neutron star.
0:11:19.6 SC: Patrick Hall says, “As you’ve mentioned on the podcast, your wife, Jennifer Ouellette, is a science writer and author. Have you and your wife ever collaborated on projects together? If not, is it something you plan on doing in the future?” We haven’t collaborated on writing projects. We did once give a talk together on black hole firewalls, that was fun. But actually, our interests in writing, even though we both write about physics, we have different interests. Jennifer is much more interested in sort of large-scale condensed matter, fluid mechanical kinds of things, whereas I’m interested in quantum mechanics, cosmology, things like that. So, we have more overlap than the average people do, but not nearly enough yet, anyway, to actually write a project together. But it could be fun, if the right one comes along. You never know.
0:12:02.3 SC: Jamie Tan says, “Does physics allow for something to appear out of absolutely nothing, no time and no space? Another way to ask this is do laws of physics prohibit such a thing?” Well, the short answer, again, is who knows? We don’t know. We don’t know what the ultimate laws of physics are. I would deny that there’s even a sensible way to talk about something appearing out of nothing, no time and no space. ‘Cause you can ask, when would that happen? It doesn’t really make a difference, make sense. What you can do is the other way around: You can approach it from the side of the universe that already exists, or the thing that already exists. Usually this question is asked in the context of the universe itself: Can the universe itself appear out of nothing, no time and no space? So that’s the wrong question to ask, I would say. The right question to ask is, can there be a first moment of time in the history of the universe? Can there be a moment before which there were no other moments? As far as we know, sure. The laws of physics are perfectly compatible with that, but we don’t know for certain, ’cause we dont know what those laws of physics are. I did write a paper that mentions this problem and talks about it more detail, if you’re interested called, Why is There Something Rather Than Nothing? So you can check that out.
0:13:15.2 SC: Jeff B says, “How should we conceptualize particle interactions from the many-worlds perspective, especially since wave functions are seen as physically real?” I’m not sure what you’re asking here there. The many-worlds perspective really doesn’t have anything special to say about particle interactions. It’s just the same kind of particle interactions you would get in any other formulation of quantum mechanics: Copenhagen, or Bohmian, or GRW, or whatever. Many-worlds doesn’t really change that picture very much.
0:13:46.7 SC: Michael Shillingford says, “What’s your preferred answer to van Inwagen’s… ” I think it’s van Inwagen, Peter van Inwagen. It might be Inwagen. I don’t know how the Americanization pronunciation is. “… Van Inwagen’s special composition question?” So as far as I understand it, I haven’t thought about it in a long time, the special composition question is just a question about, at what point, when you combine things together, do you have a new thing? When it’s kind of an inverse mereological question: Instead of saying, you have a big thing, how do you divide it in the small things; you have a small thing, when do they make a big thing? I don’t think there’s any sensible way of thinking there’s a once-and-for-all answer to this question; I think it’s entirely depending on what kind of thing it is that you are making. And also, I don’t think that there’s any metaphysical essence of being a thing that we’re gonna ever put our finger on. It’s just a convenient way of talking.
0:14:41.3 SC: You can always imagine being Laplace’s demon and understanding the world perfectly, and then never talking about composite objects, just talking about the fundamental reality that evolves forward in time or whatever it does. It’s we, human beings, that have access to limited information, limited time, limited powers of observation and description, we find it useful to talk about composite objects. And so, it’s our usefulness that would judge when it is sensible to talk about something being an object or not. And that usefulness is gonna completely depend on the context you’re talking about. Are you talking about galaxies? Are you talking about single-celled organisms? Are you talking about something much more abstract? So, in that sense, I guess I don’t have a preferred answer.
0:15:23.1 SC: Jeremy Paine says, “Could you comment on superfluid dark matter theories?” I don’t know much about them. I did hear one talk by Justin Curry of the University of Pennsylvania, which was very good, actually, about superfluid dark matter. It was very compelling in the sense that if you believe this particular crazy model, you can explain a lot of things. So, superfluid dark matter is a dark matter kind of particle, but it condenses. So it’s not just an individual set of particles, or I should say, it’s not a set of individual particles doing their own thing; the particles interact with each other and form a superfluid. And that actually changes, perhaps, their distribution in galaxies and clusters in interesting ways. So I think it’s a very interesting idea, but as far as I could tell from Justin’s talk, the particle physics models that make it work are not yet very compelling. Maybe we just haven’t thought of the right one yet.
0:16:14.5 SC: Justin Bailey says, “What was it like going on the Colbert Report? Did he prep you for the questions, or were you reacting in real time?” So it was a lot of fun going on Colbert. It’s not gonna happen anymore, because now he’s on a network, he used to be on Comedy Central and the standards are different, they don’t wanna have five minutes of just him talking to scientists anymore. He will talk to scientists, but only if they’re doing sort of crazy visual demonstrations, which is not really my expertise. So, they want you to succeed, actually. They think of scientists and other academics differently than they think about politicians or comedians or whatever, because when it’s a politician on that show, their goal was a little bit to puncture the presumption, the self-regard, that a politician typically has. Whereas with scientists, they actually wanna get the science out. So, what I was told was, “You will be given a chance to say your piece, to actually say the little bit of science that you wanna have, but it’s mostly entertainment, so it’s mostly about telling the jokes.” So, the weird piece of advice that I got was, “Don’t stop talking.” In other words, when you’re on the radio or whatever, when you’re doing a TV documentary, you try to keep things short, sweet, get to the point, do a sound bite.
0:17:36.6 SC: On Colbert, you’re asked to do exactly the opposite. Just keep talking until he interrupts you, ’cause he’s trying to think of jokes. He’s not actually an interviewer trying to get out information from you. But certainly, I’m not given the questions ahead of time. It was absolutely all in real time. Typically, those interviews were six or seven minutes, and they edit it down to about five minutes. So a little bit of the more boring parts were cut out.
0:17:58.8 SC: MD Mertov says, “Why is the emergence of space-time specifically not like the emergence of a baby bird from its egg?” Well, it’s just two different senses of the word “emergence.” The word “emergence” for the baby bird is literally a process happening over time. The bird is in the egg, the egg begins to break, the bird emerges from it. The word “emergence” in philosophy or physics is not a process evolving over time; it’s a relationship between two different theoretical descriptions that can both be simultaneously true. When you say that there’s a theory of the air in the room in terms of atoms and molecules, and there’s a different theory of the air in the room in terms of a fluid, and you say that fluid or gas description is emergent, it’s not like there is some temporal process that takes it from being atoms and molecules to fluid. Both are always true at the same time.
0:18:51.4 SC: Joshua Hillerup says, “Why does black hole evaporation seem to be so much more accepted as true in physics compared to other theories that aren’t confirmed by experiment?” Well, it’s not 100% accepted because it hasn’t been confirmed by experiment, but it is a prediction of theories that are pretty well confirmed by experiment, namely, classical general relativity and quantum field theory. So, generally, it’s not like a completely new theory that we’re starting with when we talk about black hole evaporation. We’re applying ideas from theories that have been confirmed by experiment to new situations that we haven’t specifically observed. And generally, there, I think that if you do the calculations correctly, your attitude should be, “Since the theories seem to be good, I should accept the prediction of those theories until I have some reason not to.” Of course, you don’t accept it 100%, but you put a high credence on it until you have evidence otherwise.
0:19:46.5 SC: Bill Warner says, “What is the volume of a black hole with respect to the event horizon? Is a black hole most like a solid, liquid, or gas?” Well, that’s a very hard question to answer, not because it’s technically difficult, but because what do you mean by volume? You can… We talk about the volume inside, but remember, a black hole is a region of space-time, not just a region of space. And so, because it’s general relativity and space-time is curved, you have to decide how to slice the space-time inside the black hole to calculate the volume. Volume is a three-dimensional concept, not a four-dimensional concept. And the right way to slice the volume inside is not at all obvious, so it’s just a hard question to answer. Having said that, what you would call the density of a black hole depends completely on its size, and the density basically goes down as the black hole goes up. So, if it’s a very small black hole, the density is really, really high; if it’s a very big black hole, the density is relatively low. So it can be whatever you want, depending on the kind of black hole you’re thinking about.
0:20:49.0 SC: Scott says, “In quantum field theory, is it preferable to think of a particle and its anti-particle as arising from the same field that has an extra degree of freedom, or do you think of a particle and anti-particle as arising from different fields?” Well, a little bit, this is up to you, and it depends on what you count as a field. Think about… This is not a directly applicable example, but think about the electromagnetic field. We have the electric field and we have the magnetic field, and each of those have three components, ’cause they are three-dimensional vectors. Or you can say, “Well, they arise from a single four-dimensional field, the vector potential, and the different components of the field are all doing something.” So, in that case, if you have a vector, a four-dimensional vector field, do you say that’s one field with four components, or do you say it’s four different fields that group together to make a four-dimensional field? That’s gonna be important to the answer to your question.
0:21:41.0 SC: So, generally, the particle and anti-particle for something like an electron can be grouped together in one big field. This is a fermionic field. Paul Dirac told us how to talk about these fields in terms of what are known as Dirac spinors, and the Dirac spinor involves both the particles and the anti-particles. But you could also choose, if you wanted to, to divide all those components up into a spin left electron, a spin right electron, and a spin left positron, and a spin right positron, if you wanted to do that.
0:22:13.3 SC: Patrick Henderson says, “Have you ever encountered a campaign to systematically misrepresent your views in order to lend credibility to fringe or conspiratorial ideas?” I think not, exactly. I’ve never been completely hornswoggled like that or misused like that, but close, in some ways. Look, I’ve said enough things online and enough people have read things I’ve said online that all the time, I hear people on Twitter or blogs or whatever, on Reddit, claiming that I believe something that I certainly don’t believe. And there’s no citation, there’s no reference, there’s no quote or anything like that, and it’s impossible to stamp out all of it. All you can say is sort of roll your eyes and go, “I wish people would read what I wrote rather than just making things up.” But that’s not systematic.
0:23:01.5 SC: Only once was I on a TV show where I felt like I was clearly being trapped or lied to. There was a show about… It was supposed to be about extreme crazy phenomena, so, it was basically supposed to be a real-world version of Mulder and Scully in the X-Files investigating interesting ideas. But I wasn’t told that; what I was told was they were doing an episode on quantum teleportation and they wanted me to talk a little bit about quantum teleportation. And I said, “Well, I’m nowhere near the world’s biggest expert on quantum teleportation,” and they said, “That’s okay, you can just talk about quantum mechanics more generally.” And then I get on the show to be interviewed by the two hosts, and it became quickly clear that they had no idea what quantum teleportation is or even heard of it. They had an idea that the US government had teleportation technology and was using it to shuffle astronauts back and forth to Mars, and this was a big cover-up on the part of the government. This was entirely different than what I was told the interview would be about, and so I literally took off my microphone and walked away in the middle of it. I had never done that before, never done that since. These days, I do a lot more homework before I agree to be interviewed by somebody.
0:24:16.5 SC: Jacob Arkin says, “Are you familiar with the recent Watchmen HBO adaptation? If so, what do you think about his portrayal of eternalism and attempt at Laplace’s demon-esque character?” I did see the Watchmen. It was great. I highly recommend it if no one has seen it, if anyone has not seen it. I don’t specifically remember having deep thoughts about Dr. Manhattan or any other questions about eternalism in that particular adaptation. Look, it is often the case, there’s many different stories where there’s a character in sort of a fantasy or a science fiction scenario which is supposed to know everything both about the past, the present, and the future. This is a trope. And it’s never done accurately, because they always change their mind or they decide not to do something that they were gonna do. They’re constantly violating the self-imposed idea that they actually know, with deterministic certainty, what’s going to happen in the past, present, and future. So, I would rather they just not have characters like that. That would be my particular preference.
0:25:24.2 SC: Michael Daniel says, “If the cosmos used to be dense and closed and subsequently expanded with deceleration to flat space with Omega-1, must we not be flying head long into Pringle land?”, by which he means negative curvature. So no, and the answer is because we have not gone from a closed universe to a flat universe. That does not happen, that never happens, in Robertson-Walker cosmology. In Robertson-Walker cosmology, which is sort of just the cosmology of general relativity where space is taken to be similar on all places, so, homogeneous and isotropic, there are three choices for what the geometry of a space can be. It can be positively curved, like a three-dimensional sphere; it can be flat, like three-dimensional flat space; or it can be negatively curved, like a Pringle or a saddle. But is a feature of the equations that you never undergo a transition from one of those to the other.
0:26:19.4 SC: So when people say you have a curved universe, positively curved or negatively curved or whatever, which, because of inflation or because of other dynamics in the early universe, becomes flat, what they mean is it becomes flatter than it was. It never becomes perfectly flat. A closed universe, which is a sphere, has a compact topology, and it’s never going to change into a non-compact topology of a flat universe. So we’re never gonna become negatively curved if we’re not already.
0:26:50.1 SC: David H says, “When the universe splits a la Everett, is the split instantaneous across the whole pre-existing universe, or does it propagate at the speed of light?” So the nice answer is, it’s up to you. And this goes exactly back to what we were talking about, about Laplace’s demon earlier. The branching of the wave function of the universe into separate worlds is not part of the fundamental theory. The fundamental theory is, there’s a wave function and it evolves according to the Schrödinger equation. That’s the entire theory. The splitting into worlds is something that we human beings do for our convenience. So, the right way to ask this question is, is it more convenient to imagine the world splitting all at once across all of space, or propagating at the speed of light?
0:27:31.0 SC: And for that, it’s completely dependent on what your purpose is, right? I actually tend to think of it as simpler just to imagine the universe splitting all at once, pre-existing, simultaneously across the whole pre-existing universe. That bothers some people, because they say, “Well, that’s not compatible with special relativity, which says that signals can’t travel faster than the speed of light.” But there’s no signal traveling faster than the speed of light; it’s just our description is traveling faster than the speed of light, and that’s perfectly okay. A slightly more sophisticated worry is two different observers moving in two different velocities, two different Lorentz frames, would describe the split of the universe differently. And the response to that is just, “Yeah. Sure. They would. Who cares?” They would still end up agreeing on every single prediction for every single observable thing. So that kind of observer-dependence of description doesn’t really bother me.
0:28:28.5 SC: David Maxwell says, “Could dark matter be the result of theoretical white holes, that the process involved makes what comes out invisible as normal matter energy, but it still affects gravity?” So probably not. White holes probably don’t exist. White holes are kind of like black holes but run backward in time, which would violate the Second Law of Thermodynamics, which is generally why we don’t think that white holes exist. And also, if they did exist and something came out of them, there’s zero reason to think it would be invisible or anything like that. It’ll be ordinary photons and particles. So, you’re just making more particles. You just have a very fancy way of filling the universe with particles. So why not just put particles in it and ask about that?
0:29:11.0 SC: Paul Hardy says, “I saw a video discussing the Janus point and that you did work on it. Can you briefly explain what the Janus point is?” Yeah, so, the Janus point is a term coined by Julian Barbour, who is a gravitational physicist. And Julian and his colleagues have this theory of cosmology according to which entropy can grow both toward the future and in the extreme past. So there is some middle point, some Janus point, where entropy was lowest, and it grows in both directions away from that point. And this, of course, is an idea that I had quite a while ago in 2004. Jenny Chen and I published a paper with exactly that kind of idea. The specific example implementation we used of that idea was very different, so, Julian Barbour’s paper is completely separate from ours, but the idea that there is a moment in the history of the universe when entropy is minimum and it increases in both directions around that dates back to our paper in 2004.
0:30:15.3 SC: Discon Jazz says, “If you had to redo your entire education over and for some reason were precluded from studying any discipline of physics proper, what branch of academia would you choose?” That’s a good question. There’s so many good choices. Philosophy would be the obvious one, but I think that what actually happened to me becoming a physicist first and then moving more into philosophy afterward was the better way to go, because there’s more parts of philosophy that I am not that interested in than there are of physics, and I might have gotten very frustrated if I didn’t realize early in my philosophy education that there were the good parts that I really, really like, that are really foundations of physics, really just doing physics, but in a philosophy department. Honestly, that’s the kind of part that I like. But otherwise, I probably could have been very happy studying theoretical computer science or artificial intelligence or complex systems, things that I’m already kind of interested in a little bit now. So now that I’m old, and I’m not doing my education all over, I’m still trying to learn about those different subjects.
0:31:19.8 SC: Jeremiah M says, “What are your personal top 10 most interesting unanswered questions in physics?” So I don’t have a personal top 10, sorry. I don’t have a list of hanging around. This is secretly 10 questions, and the rules of the AMA are you’re only allowed to ask one. So I’ll mention a couple, but they’re obvious. I wanna know, what is the correct formulation of quantum mechanics? I think I know that. I think it’s the Everett interpretation. So maybe that’s not a top 10 one. I would like to know how to quantize gravity, or how to reconcile quantum mechanics with gravity, would be a better way of putting it. Everyone wants to know that, and I think that’s very interesting. I wanna know why the cosmological constant has the size it does. I wanna know if quantum field theory breaks down. I wanna understand non-locality in quantum gravity and things like that. And I also wanna understand a bunch of things about complex systems, how they come to be, how they evolve, how they are related to the evolution of entropy over time. So, that’s nobody else’s list of interesting questions in physics, but those are my personal favorites.
0:32:16.9 SC: Frank Layman says, “Do you have a particular stance toward the unreasonable effectiveness of mathematics and science as articulated by Wigner and many philosophers of science and mathematics since? Does it keep you up at night?” So no, it does not keep me up at night at all. I think I’ve said this before, maybe not in the AMAs, but I don’t think that there is any unreasonable effectiveness of mathematics in physics. I think that no matter what the laws of physics had been, we would end up describing them mathematically. And even if the laws were that things happened completely randomly, we would describe that randomness mathematically. What else could we do? I think that what is meant by the unreasonable effectiveness of mathematics is something like the actual physical laws. It’s not that they’re mathematical, but that the mathematics is particularly simple and elegant. There are many ways the laws of physics could have been messier than they are. They’re a little bit messy, they’re not as elegant as you could imagine them being, as far as we currently know, but there’s many, many ways they could have been much more messy.
0:33:17.7 SC: So, I don’t know about that. It certainly doesn’t keep me up at night. I would like to understand it better, but since I don’t see an immediate path forward on that, it doesn’t keep me up. So the questions that keep me up at night are the ones that are really, really difficult, but yet you can see some kind of progress. You can imagine making progress. So, the ones where I don’t even know how to make progress are not ones that I care about too much.
0:33:41.9 SC: Brandon Sherwood says, “When you were an undergraduate, did your first physics classes click with you right away, or was it a slow learning process like it is for myself?” You know, I think it’s in between. Some things clicked right away, but look, learning physics is always hard. I don’t know anyone who just takes a whole bunch of physics classes and says, “Yeah, this is just easy.” [chuckle] Because it’s new to you, unless you’re cheating, unless you study it ahead of time before you take the class, which, by the way is great, you should cheat. That’s a good thing to do. But I wouldn’t be discouraged by the fact that it’s hard or does not click right away. It’s worthwhile because it’s hard. It takes a lot of effort, and it takes a lot of training, and it takes a lot of patience and the ability to make mistakes and learn from them.
0:34:25.8 SC: Machas Hosavar says… Machas? Sorry. Hosavar says, “In your last solo episode on democracy, you mentioned that it is not worth to keep contact with people or even friends that have radically different views and beliefs about the world/reality/politics, and it’s best to part ways in those cases. My question is, did you have such a similar experience and what would have to happen and/or change… ” A lot of slashes in this question. “What would have to happen and/or change to resolve this and renew/continue that relationship?” So it has happened to me, and I’m not exactly sure what I said. I never can remember what I said in previous episodes. So, I certainly would not wanna go so far as to say, it’s not worth it to keep contact with people who have radically different views than you do. What I wanna say is, it can reach a point with people who have radically different views than you where it is not worth keeping contact with them. In other words, there is no obligation on your part to continue to be friends with someone if your worldviews have diverged so radically that they’re bringing you more stress and sorrow than pleasure and fun. Okay? There’s no obligation of that.
0:35:37.7 SC: But it’s certainly possible to still be friends with people who do have radically different views if you’re still enjoying their company, right? So, what would be required to fix it? You would have to have some change of someone’s point of view, either your point of view or their point of view. This is not… I’m not saying anything very profound here, I think;I’m just saying that it is possible for two people to sort of become friends and then stop being friends because of differences between them. I don’t think this is anything new. This is nothing special about our current moment or hyper-charged or anything like that. When the things that you can’t agree on elevate in their importance to the fact that they color every way in which you relate to that person, it is appropriate to move on. And if you can fix that, if you can stop those things you disagree on from either being that important or being things you disagree on, then you can move forward. That’s all it takes.
0:36:37.1 SC: Sam Barta says, “Why is entropy so important, if by yours and others’ definitions, it is a measure of how much we don’t know about a system?” Well, it’s a very useful thing to keep in mind how much we don’t know about a system because it helps you make predictions for things, like the fact that entropy increases over time is certainly an incredibly useful feature of the universe. We lose information. That’s a real fact about the universe. And you may use to make predictions, to do thermodynamics, to design engines, to think about cosmology, all of these things. So, what we know about the universe is very, very important to what we can do with it and how we think about it.
0:37:22.2 SC: Johnny says, “From your recent conversation with Avi Loeb, did you walk away thinking that looking for alien life is a worthwhile use of time and resources?” I would say it is worthwhile, but I would do it in a minimum level. I don’t think it’s a high priority. I think that I am convinced by the argument that says, “It would have been easy to find ourselves living in a universe where alien life was perfectly obvious and everywhere, and we could find it very simply, and it’s also easy to imagine we live in a world where we’re never gonna find alien life.” So the space available for it to be there and findable but hard to find, I think, is pretty small, but it’s a huge pay-off if you do it. It’s usually hugely important. So I think that like many other high-risk, high-gain things, you should have a little bit of your portfolio devoted to that, but not sell out entirely in that direction.
0:38:12.9 SC: Matthew O’Connor says, “You mentioned on Twitter that you’re leaving Caltech. I’d be interested to know anything you can share on why, what’s next, your thought process, and what this means for the quantum mechanics textbook.” The good news is it means nothing for the quantum mechanics textbook, that had nothing to do with me being at Caltech. My job at Caltech is not that of a tenured professor; I’m a research professor, which is in some ways, an amazingly good job. You have very low responsibilities and huge amounts of freedom. But you do have some responsibilities, [chuckle] and the research professor job is perfect for someone who has a specific kind of research they wanna do, and just does that, just stays focused on that, doesn’t move around intellectually into different areas, stays focused on that sort of one kind of area. So I was hired to be a theoretical particle physicist and cosmologist, and if I still wanted to do nothing but that, Caltech would be a great place for me to do that. But I wanna do other things. And so my job right now is not a good fit for what I want to do. And so I’ll be increasing the time I spend at Santa Fe working on complex things, and I might be doing other things, we don’t know yet, there’s wheels moving in the background, but I will be sure to let people know when I know myself what’s gonna actually happen.
0:39:33.3 SC: Hershey Silver says, “On a scale from one to 100, what is the likelihood that humans will find a wormhole to travel to distant galaxies within the next 50 years?” One, if one means not very likely, then it’s one. [chuckle] If 100 means very likely and one means not very likely, it’s pretty much one. Wormholes are interesting things to think about, but even in the laws of physics, as we currently understand them, you can’t keep them open and travel through them. That’s just science fiction. When you build a wormhole, it collapses right away and you just make a black hole. You don’t make a shortcut to travel to distant galaxies. So probably, wormholes don’t exist macroscopically so that you can find them, and if they did, you couldn’t travel through them anyway. Sorry about that.
0:40:16.3 SC: Sherman Flips says, “How does the weight assigned to a given branch of the wave function correspond to the number of micro-states that are in superposition in that branch?” So, you gotta be a little bit careful. Basically, it is that number, but I wanna be careful here because number of micro-states is a slightly ambiguous concept in quantum mechanics. If what you mean is the number of dimensions of Hilbert space that correspond to that branch, that’s what it means, the number of different directions in Hilbert space that you can add together in some principled way to make that particular vector corresponding to that branch. Whether you wanna call a dimension of Hilbert space a micro-state or not is up to you.
0:41:00.7 SC: There’s another way of thinking about things if you just had like a bunch of spins. So you have a bunch of two-dimensional Hilbert spaces, one for each spin, spin up or spin down, but the dimensionality of the combined Hilbert space is not 2N. If you have N spins, it’s 2 to the N. So you don’t have one dimension of Hilbert space for each dimension of the subspaces; you exponentiate them. That’s why it depends on what you mean by micro-state, but basically, that is what the weight means. You’re on the right track thinking about that.
0:41:32.4 SC: Sean Drew says, “The visual representation of electron wave functions seems to have some resemblance to the hypothesized dark matter distribution in solar systems. Is there anything to this, and are there actual smart people thinking about this?” So, is there anything to this? It depends. There is something to it, but it’s an incredibly trivial thing. It’s just basically that lots of things in nature take big, puffy, spherically symmetric cloud-like shapes. [chuckle] Distributions of dark matter is one, electrons are another. And by the way, for electrons, it’s only the simplest ones that are really spherically symmetric, the higher-level ones, the p, d, orbitals, et cetera, are not spherically symmetric and look nothing like dark matter distribution. So I would not read too much into that similarity.
0:42:20.6 SC: Charles O’Connor says, “If the fields of quantum field theory are pervading space-time in three spatial dimensions, how would you accurately describe the geometry of waves in those fields?” A wave is usually depicted in books as a two-dimensional sine wave, like for electromagnetism. I’ve seen them depicted as what I would call a 3-D bell curves hiding under a blanket. I haven’t heard an explanation of what the wave would actually look like as it moves through its field. So, for questions like this, part of me just wants to say, you reach a point where you can’t visualize things. That’s how science physics works ’cause there’s too many things. So, if space were one-dimensional, then you could easily plot a wave by… Sorry. We live in three-dimensional space. Let me say this exactly right. We live in three-dimensional space, macroscopically. If you imagine a hypothetical one-dimensional world, you can plot a wave in it by plotting that one dimension, but then also plotting the direction of the field orthogonal to it. That’s what you’re ordinarily used to. When you plot a wave moving in one dimension, you actually draw two-dimensional plot, the direction in which it’s moving and the value of the field.
0:43:35.2 SC: So, try thinking about that for a wave in real three-dimensional space. You would need at least four dimensions to plot that, and you can’t visualize four dimensions very easily. So you just have to trust the math or use much simpler examples. That’s why you see one and two-dimensional waves drawn or depicted, and not a real three-dimensional wave. You can do something like it if you really stretch your mind, but then at some point, you’re gonna have some 10-dimensional tensor defined in three-dimensional space, and you’re just not gonna be able to plot that in any interesting way. So, I think that the correct answer is, don’t try. Move beyond that. Think about it in some other way.
0:44:15.6 SC: Evan West says, “Can Lorentz contractions occur at the Planck length?” Short answer is yes, as far as we know. Now, we don’t know what happens at the Planck length. But the Planck length is not some minimum length. That’s just not true. Again, it might be, but we have zero reason to think that the Planck length is a minimum length. It’s the length where quantum gravity becomes important. It’s not like some minimal, un-squeezable amount of distance. In physics, as we currently understand it, Lorentz transformations contracting the length of things happens at all length scales. Otherwise, you just build a large thing out of little Planck-scale things and you wouldn’t Lorentz contract at all. That’s not the way it is.
0:44:54.9 SC: Elias Borjesson says, “What is true about animals that, if true of humans, would make it okay to breed and kill humans? So for example, if you would answer culture, would it be okay to kill humans if that were part of some culture, et cetera?” So I think that what we would need is to remove from humans the ability to conceptualize the future and in hypothetical, communicable ways. I’ve talked about this a little bit, never in great detail, I have not really thought through it in all the philosophical rigor that it would require, but the idea is that there’s something special about humans, that they can reason about the future, and in that sense, understand the prospect of not being alive in a way that animals can’t. Animals can be afraid of being hurt or dying, but they can’t really sort of trade off in their minds an agreement, “Well, if this happened, then I would cease to exist, and I don’t want that to happen, and so thinking about that happening now makes me sad now even though it hasn’t happened yet.” Whereas human beings can really do that very easily, so you’d have to remove from human beings that ability, and then they would become like cows or any other animals.
0:46:06.8 SC: A Mandalorian says, “How does the Heisenberg uncertainty principle apply to the idea of fixed electron orbitals, if at all?” Now, it applies perfectly well. The electron orbital has a shape in space, and the Heisenberg uncertainty principles says that you cannot have… Well, that there’s a minimum combined uncertainty in position and momentum. And the position and momentum has a specific technical meaning in quantum mechanics, and you should think about the momentum not really as a velocity but as a wavelength, as a frequency of variation. So, if a wave is varying very, very rapidly, it’s high momentum; if it’s varying very slowly, it’s low momentum. So the Heisenberg uncertainty principle says that when we go to observe the value of the position or the momentum of an electron, there’s a necessary uncertainty no matter what the wave function is. And that is absolutely true for fixed electron orbitals. They do not violate the uncertainty principle in any way.
0:47:08.6 SC: And in fact, just to be a little more specific about that, the electron wave orbital is kinda spread out in space, it takes up the space, an angstrom or so, that defines the size of the atom. So, delta x is already pretty big, delta p doesn’t need to be that big to satisfy the uncertainty principle.
0:47:27.2 SC: Brian Owenson says, “Do you have a good explanation why the Lagrangian is kinetic energy minus potential energy and not something else? Leonard Susskind says it’s kinetic minus potential because that’s what works. Was there no better physical sort of intuitive way to get to kinetic minus potential?” Yeah, at the end of the day, Lenny is right, that is what works, and that’s all you ever need. When you look for these explanations, why are things one way rather than another way, you gotta bottom out somewhere, and you gotta say like, “That’s what the laws of physics are. There’s no further explanation behind it.” If you want, you can sort of think about what else the Lagrangian could be. The Lagrangian, by the way, if any of you don’t know, check out the video I did on force, energy, and action for The Biggest Ideas in the Universe videos where I explain what Lagrangians are. So it’s kinetic energy minus potential energy.
0:48:18.7 SC: Well, look, it certainly can’t be kinetic energy plus potential energy, or minus kinetic minus potential, because those quantities are constant. They’re not doing anything interesting. The total energy is constant. So roughly speaking, if you’re gonna build something linear out of the kinetic energy and the potential energy and you can’t add them together, the next obvious thing to do is to subtract them. And there’s no big difference between kinetic minus potential versus potential minus kinetic, that would have been just as good of a choice. In both cases, you’re looking for extremal points of the action, and the extremizing either one of those would give you the same answer.
0:48:55.6 SC: Andrege says, “You said that the Big Bang is similar to a white hole, that is a time-reversed version of a black hole. But doesn’t such time reversal require gravity to be repulsive rather than attractive?” So no, it doesn’t. Think about throwing a ball up in the air. It goes on a paraboloid trajectory. It goes up and then it comes down, and think about time reversing that. It’s exactly the same thing. The time reversal of that trajectory is it goes up and then it comes down. So you didn’t reverse gravity, you didn’t make gravity repulsive; you reversed the initial conditions of the thing, of the ball you were throwing in the air. Likewise, gravity is absolutely attractive, not repulsive, near the Big Bang, as far as anyone knows. But the stuff, the stuff that was banging, was moving apart at a really, really rapid rate. That’s the initial conditions that we have. It has nothing to do with gravity being attractive or repulsive.
0:49:50.6 SC: Richard Moster says, “Suppose we have three ideal clocks: The earth clock stays on Earth, my clock goes with me on a spaceship, and a smart clock also goes with me on the same spaceship. But the point of the smart clock is it is fed data regarding spaceship acceleration, location, et cetera, whatever it needs to calculate and display the current time of the Earth clock in the sense that, after zipping around the galaxy, I can return back to Earth and the Earth clock and the smart clock will display the same time. Is this possible? And if so, don’t I have a map at all time between my clock in the Earth’s smart clocks?” Yeah, it’s perfectly possible. And yes, there is such a map. In fact, it’s not a very weird map; it’s a very well-known map, namely, sitting back here on Earth, I can create a coordinate system throughout the universe based on my time. In fact, that’s what we do in cosmology. We slice all of space-time, and we say, “Here’s what time it is now according to… Everywhere in the universe, according to my clock here on Earth.”
0:50:51.5 SC: Relativity doesn’t say you can’t do that. Relativity just says you could have done it otherwise. You could have done many other ways of slicing the universe into moments of time equally well. So your smart clock is just keeping time with respect to this time coordinate corresponding to the [0:51:11.0] ____ of people on Earth. There’s no problem with that at all.
0:51:15.0 SC: Justin Walcott says, “If I understood correctly, you said in The Big Picture that free will is impossible from the perspective of an omniscient observer, because the current state of the university equals the prior state of the universe plus the laws of physics. What about fictional universes where magic and deities exist and don’t obey the laws of physics?” Well, it depends on what kind of fictional universe you’re talking about. The simple thing is, if there are deterministic laws of physics then to Laplace’s demon omniscient observer, there’s no free will because you know exactly what’s going to happen. But something that is equally true, although people seem to have a hard time grasping it, is there would also be no free will, if there were indeterministic laws of physics that everything obeyed. If you have, for example, quantum mechanics in some truly stochastic formulation, where wave functions collapse and you just can’t predict what’s going to happen, that doesn’t give you any room for free will, because that has nothing to do with will, that has nothing with human beings. It’s just a random number thrown there in the laws of physics.
0:52:18.7 SC: So, what matters for whether physics… For free will, for libertarian free will, the this sort of strong version of free will, can exist is, are there laws of physics that things obey, or are things like people or other conscious creatures just not subject to the laws of physics at all? And for people who would wonder whether or not that’s true, I always suggest jumping off of a building, and I predict that the laws of physics will say what their center of mass coordinate will do when they jump off the building, and if they think that they can use their free will to violate the laws of physics, then they should do that and fly away rather than hitting the ground.
0:52:58.8 SC: Stefan Berninger says, “How does Eric Weinstein’s geometric unity attempt to unify GR with QFT compare to other attempts like string theory?” I have no idea. I’m really completely in the dark about what geometric unity says, so I can’t really comment on it.
0:53:14.6 SC: Simon Carter says, “I recently read Lost in Math by Sabine Hossenfelder, where she argues that modern physicists’ obsession with beauty has given us wonderful math but bad science, stating that there is no law in nature that says that the universe must comply with fundamental physicists’ notion of mathematical beauty. Do you have sympathy for her concerns?” Well, it’s certainly technically true that there is no law in nature that says the universe must comply with fundamental physicists’ notions of mathematical beauty. I think that the implication that therefore, we should not get too obsessed with beauty is wrong. I do not follow that logic, because what we’re talking about is hypothetically inventing new laws of physics. There’s the kind of physics you do where you know what the laws are and you’re trying to tease out their implications. That’s most of physics, and that’s perfectly straightforward. But then there is theoretical physics, where you’re trying to go beyond the known laws and figure out what ones are next.
0:54:12.9 SC: There’s literally an infinite number of laws you could propose that are compatible with all the data we have right now. You need some guidance as to what possible future laws to take seriously and which not to take seriously. And personally, I think that mathematical elegance is as good and as important as any other ones, in part just because it has worked so far. Why would you ignore that lesson of history? This does not mean that you say, “Well, this is a beautiful hypothetical law, therefore, it’s true.” That’s not what science does. That’s just not how science works at all. What you say is, “This is a beautiful, elegant hypothetical law. Therefore, let’s give it a chance. Let’s take it seriously and try to test it, and to try to compare it to the world.” I think that’s a perfectly legitimate thing to do.
0:55:02.6 SC: Danielle Cortezi says, “In your podcast with David Chalmers and Philip Goff speaking about philosophical zombies, you claim that since zombies say they have an experience and are conscious, that none of us know that we’re not zombies because they think they’re conscious like we do. So are you saying that consciousness could be an illusion, or am I misunderstanding something?” So no, from that statement alone, it’s compatible with the idea that consciousness is an illusion, but that’s not my perspective. My perspective is, there are no such things as zombies. Zombies are not even conceivable, unless you already believe that consciousness is something that is not physical. So Chalmers and Goff think that consciousness is not physical, that it’s something over and above the physical motions of matter, and therefore, they think we can conceive of zombies, which are things that physically behave exactly like we do but don’t have that inner experience of consciousness. Since I think that the inner experience of consciousness is nothing more than a way of talking about the physical stuff going on in our bodies and brains, I don’t think that zombies are conceivable.
0:56:05.5 SC: Sam says, “What is one piece of advice you wish someone had given you before you entered grad school?” That’s always hard, ’cause everyone takes different pieces of advice in different ways, and needs different pieces of advice. I would say there’s two things, actually, even though you just asked for one, you get a bonus. The two things that come to mind are number one, take initiative. You’re sort of trained through years of schooling, elementary school, high school, undergraduate school, to follow instructions, right? “Here’s the homework, do the homework, get the grade, take this class,” whereas suddenly, in grad school, you’re supposed to be a working scientist, and working scientists are not supposed to take instructions; they’re supposed to come up with their own ideas, start their own projects, read papers, talk to people on their own. And maybe no one tells you that when you enter grad school, but don’t wait for it to come to you; go out and take it, is a good piece of advice.
0:57:00.0 SC: The other piece of advice is a little bit… So that’s like the first-year grad student advice. Then there’s the third or fourth-year grad student advice, which is, if all goes well in your late undergrad… Somewhere between your late undergrad and early graduate school days, you will start writing papers. You will start doing science at a professional level, writing papers, getting results, submitting the papers, getting them published. And this is an amazingly heady experience. It’s thrilling to think that you are contributing to the progress of science in some way. And you probably have spent years looking forward to this condition. You’ve been waiting for this moment when you can be a scientist and write papers. And therefore, I think it’s a little bit too easy to say, “Well, now I can write papers. Let’s just write papers. Let’s do research, let’s, like, you know, “I’m good at this thing now, I can just keep doing it.”
0:57:56.2 SC: Rather than being a little bit more reflective and saying, “Okay, now that I have an existence proof that I can write a paper, now I should really think carefully about what papers I want to write.” There’s a hurdle to overcome being a published scientist, but then it’s a much bigger hurdle being a published scientist publishing things that other people care about, that contribute to science, that are interesting. I think a lot of people just get so caught up in the grind of doing what they’re doing, they don’t take a step back and say, “Okay, now that I have these powers, how should I put them to best use?”
0:58:33.3 SC: Chris Fotache says, “You mentioned many times about how we see an object falling into a black hole, how it disappears past the event horizon, but can you describe what would happen to someone like me once I get past it?” Well, yeah, if it’s a big black hole, literally nothing at all happens to you. In fact, no matter what the size of the black hole is, there’s nothing special about the event horizon. You don’t even notice. You could potentially fall into a black hole and did not know it, if it was big enough. You might have to be little bit clueless about what was going on around you, but you’re not pulled or affected in any way. What eventually affects you is inside the black hole, there are tidal forces, they rip you apart because they pull very strongly in one direction, compared to the other two directions of space.
0:59:15.4 SC: So this is what it’s called the spaghettification process. You are elongated by tidal forces and eventually ripped apart, but that is not anything localized to the event horizon at all; it gets stronger and stronger as you pass through the event horizon and approach the singularity.
0:59:32.8 SC: Brent Demeeker says, “There have been proposals to use the sun as a lens of a telescope by the Einstein ring phenomenon. Of course, the sun is bright and you have to block out its visible light to make this work, but the sun is transparent to gravitational waves, and it has varying gravitational potential higher at the center. So could you use it as a Luneburg lens of a gravity wave telescope?” Yes, in principle, you could, but for both visible light and for gravitational waves, the sun is just not that strong a lens. It’s just not that strong a gravitational field, because it’s a huge object with relatively weak gravitational fields around it. That’s why black holes are good for these things ’cause they’re small. So they take all that mass and they condense into a small region with a very strong gravitational field. So the deflection you would get of a gravitational wave passing through the sun is really, really tiny. And if you’ve looked at LIGO and their ability to locate gravitational wave sources on the sky, localizing gravitational wave sources is intrinsically incredibly hard, so some tiny deviation of a fraction of a degree on the sky would be completely unnoticeable.
1:00:38.9 SC: Paul Hess says, “Is empty space scale-dependent? Would a cubit parsec of emptiness have 34 times more chance of random quantum stuff happening than a cubic light-year would?” Well, let’s unpack the question a little bit. I’m not quite sure what it means by a chance of random quantum stuff happening. If you’re truly empty, if you’re truly in the vacuum, there is zero chance of random quantum stuff happening. You’re in a stationary state, there’s no dynamics, there’s nothing changing over time. If you’re interested more about that, I wrote a paper with Kim Boddy and Jason Pollack on why Boltzmann brains do not pop into existence if you are truly in the vacuum state of de Sitter space. And I wrote blog posts on that, too, and you can look that up. But otherwise, yes, if you’re in some non-vacuum state where there are dynamics and things happening, then the more space, the more things can happen. That seems logical to me.
1:01:32.7 SC: Dan O’Neal says, “On New Year’s Eve, I found myself trying to explain time dilation in special relativity to my wife and came up with the following thought experiment. It ignores travel time for light signals to the observers, but is it otherwise valid? And then I’m editing out, but there’s a long description of an experiment, so, I have to apologize, Dan, this is not why we’re here for the AMAs. Special… I almost want to sort of just ban special relativity questions entirely, because we understand special relativity, there’s no mysteries in special relativity. You can cook up complicated thought experiments. But me analyzing these thought experiments is not the reason for the AMA. You can go online, look up the rules of special relativity, and figure them out. Sorry to be disappointing, I get that it would be nice if I did this, but that is not the point of the AMAs.
1:02:22.4 SC: Michael Bookfitch says, “What do you get when you chop an avocado into 6.022 x 10^23 pieces?” I’m sure that what is being looked for here is some kind of Avogadro’s number joke, but I don’t quite have it on the tip of my tongue.
1:02:36.4 SC: Brian Tidmore says, “Two twins in a spaceship accelerated one g from Earth toward Alpha Centauri. One twin resides in the center hull of the ship; the other twin resides in a chamber connected to an arm that rotates around the ship so that she experiences 1.5 g’s. The twins can see each other through the arm that connects the two locations. What do the twins see as they travel?” Man, I don’t know what [chuckle] they see. This sounds like a homework problem. Even if it’s not actually a homework problem, which may be it is, it certainly seems that way to me. Maybe you’re just trying to tease me with this kind of question, but I think, as I just said before, I’m gonna pass on this kind of question for AMA purposes.
1:03:14.3 SC: Redbeard says, “I’m curious to know what you think about the category of arguments that includes the simulation and doomsday arguments.” I think they’re hard to entertain for two main reasons. The arguments appear to require that we place no evidentiary weight on the input we get from our senses, and I think there is no pathway built into the arguments that would allow an observer that is either not part of the simulation or not an average observer in the history of our technological civilization to determine that he or she is unusual. Yeah, I’m not a fan of the simulation or doomsday arguments, as I’ve said before, I think it is just a conceptual mistake to start an argument by saying, “Assume we are typical in the universe or in some category of people within the universe.” We are not typical. We are who we are, right? I think that as good Bayesians, we should take into account all the data we already know. I’m a person, I’m a physicist, I live on Earth, I have a certain age, and all those things that’s not typical in the universe at all.
1:04:13.1 SC: So, I do think that there is a version of these arguments that makes sense, or let’s say not these arguments, ’cause I think that these particular simulation/doomsday arguments are not right, but there’s a version of anthropic arguments that makes sense where you say, “I’m comparing two different scenarios for the universe, and I want to weight them by the probability that someone just like me would appear in these universes. If it’s highly likely that a person like me appears in this universe, then that counts for that… For that scenario. If it’s unlikely a person like me appears, then that counts against it.” But that kind of reasoning, which I like, has nothing to do with assuming that you’re average or typical or mediocre in the universe.
1:05:00.1 SC: Pavlos Papageorgiou says, “How does the gravity of a black hole get out of the black hole if gravitational waves normally move at the speed of light?” Well, the gravitational field of a black hole is not a gravitational wave. A gravitational wave is a disturbance that is moving in the gravitational field. It’s a ripple. It’s the difference between an electric field and electromagnetic wave. The gravitational field of a black hole is static, it’s just sitting there. And so the question for a black hole is, what is the solution to Einstein’s equation for a static, spherically symmetric metric for a distribution of matter or anything else? That’s a completely different question than, what is the dynamical solution to the equations when you perturbed the gravitational field and let the perturbations move? The answer to that latter question is, they move at the speed of light. But the gravitational field is not like a thing that escapes from the black hole; it’s just a solution to the equations including the black hole and the world around it.
1:05:56.6 SC: Will Sue says, “Regarding the way people usually talk about consciousness, would you explain the phrase ‘what it’s like to be something’?” So this phrase, “what it’s like to be something” is used by people who talk about consciousness, usually by people who want to emphasize or lean toward the idea that consciousness is not purely physical. What they’re trying to do is to emphasize the first-person aspect of consciousness, that whatever consciousness is, they would tell you, it cannot be summed up in objective third-person outside view statements. So, this goes very hand-in-hand with the potential interest of the zombie argument, because the point of the zombie argument is you can talk to the zombie and from your outside perspective, they’re looking perfectly normal. They look like they act and behave like any other person, but there’s nothing interior that is experiencing things about the world. So the idea of these people is, there is something that is special and different in that first-person perspective of what it is like to be something.
1:07:00.0 SC: So a good example I heard just the other day is, you can imagine what it is like to be eight feet tall. You’re not eight feet tall, I presume, but you can imagine what it would be like. You’d have to duck a lot more, airplanes would be a lot less effective, maybe you could get a basketball career. You can imagine what it would be like. There is something called what it would be like to be eight feet tall. But there’s nothing called what it would like to be a rock, because rocks don’t experience the world. Rocks don’t go, “Oh, yes, I have to duck, or otherwise, I will hit myself and a chip will fall off,” or something like that. Rocks exist, and you can talk about their external properties, but they don’t have internal experiences.
1:07:42.2 SC: William E. Clark says, “Are you an advocate of eternalism, I.e. The block universe? If so, how do you reconcile this with our experience of the flow of time, the many-worlds interpretation of quantum mechanics, and special relativity?” I basically am an eternalist, yes, although to be fair, Steve Savitt, who is a philosopher at Vancouver, I think, UBC, he watched my Biggest Ideas video about time where I talked a little bit about presentism and eternalism, and he pointed to a paper that he wrote that says there’s really no difference [chuckle] between being a so-called presentist who thinks that only the present exists and being eternalist who thinks that all time moments exist equally, have an equal claim to reality. It’s just a semantic distinction; it’s not really an ontological distinction. I’m a little bit sympathetic to that, but then I think I told him, “I bet that eternalists are sympathetic to this, but presentists are not,” and he agreed, that presentists don’t like his point of view about that.
1:08:38.6 SC: So, I’m not quite sure what you mean, how do you reconcile this with many-worlds or special relativity. Many-worlds and special relativity are part of the reason to be an eternalist. They don’t treat the past and present and future any different from each other. How you reconcile it with the experience of the flow of time is a more psychological question, dealing with the fact that there is an arrow of time, that we are dynamical systems that interact with the universe, we’re open systems, and that we’re changing over time. We’re experiencing things, accumulating memories, and increasing the entropy of the universe. That’s easy to say. Actually translating this into a detailed theory of the experience of flow of time is harder. I did talk a little bit with Jenann Ismael about this question, if you wanna go back to that podcast.
1:09:23.9 SC: Miron Mizrahi says, “What is the challenge of figuring out AdS/CFT for a universe with positive cosmological constant?” So the AdS/CFT correspondence is this idea that you can take an anti-de Sitter space cosmology, which is the solution to Einstein’s equations with a negative cosmological constant, and you can imagine certain kinds of theory of quantum gravity in that cosmological background that are actually equivalent to a field theory without gravity in one lower dimension. So the first example that Juan Maldacena, who invented this, thought of was five-dimensional quantum gravity in anti-de Sitter space and four-dimensional quantum field theory. And the reason why it works is because actually, from an idea that Roger Penrose had years ago, in the ’60s, where he invented this idea called conformal transformations, where basically, you can use this to draw what is called a Penrose diagram, which takes an infinitely big space and squeezes it down to a finite subset of a piece of paper or whatever, with even the boundary being squeezed, even if the boundary is infinitely far away. So anti-de Sitter space, just like flat Minkowski space, doesn’t literally have a boundary. You can go off in any direction infinitely far and never hit an edge, but by doing a mathematical transformation, you can bring the whole space to a finite distance in this mathematically transformed way.
1:10:51.0 SC: So, if that doesn’t make sense, you have to take my word for it. Sorry about that. The point is, when you do this for anti-de Sitter space, the boundary that you bring in from infinity has the geometry of a space-time of one lower dimension. So the boundary of five-dimensional anti-de Sitter space looks like a four-dimensional space-time. And so it’s kind of straightforward to come up with a potential map between things going on in AdS and things going on on the boundary. Whereas if you don’t have that negative cosmological constant in anti-de Sitter boundary conditions, there’s still a boundary that you can bring in from infinity using Penrose’s trick, but it doesn’t look like a space-time. A space-time means you have space and you have time, and they’re different from each other. So the boundary for de Sitter space, for the cosmological solution with a positive cosmological constant, is purely spatial. It is not a space-time. It’s not to the left and to the right of you; it’s to the future and the past of you, and it’s a space-like surface. So there’s just no easy way to map things going on in space in the future to things going on in de Sitter space.
1:12:03.8 SC: And for Minkowski space, the boundary is actually null, it is light-like, if you want. It is a set of… It can be constructed from a set of rays that are all traveling at the speed of light. So it’s also not a space-time in any easy way. So at the very basic level of how the math could possibly work out, when you’re not in anti-de Sitter space, there’s no easy way to make it happen. People have certainly tried to make it happen in less easy ways, but with let’s just say mixed success, so far.
1:12:34.3 SC: Wes Clibran says, “What is your favorite species to have ever existed in the entire history of the planet excluding modern human?” I don’t have a favorite species, but I’m a big sucker for the charismatic megafauna, the dinosaurs. I love dinosaurs. If you dig back into the old days of my blog, you can find reports of going on dinosaur hunting expeditions. I found the fossils of an Edmontosaurus while we were digging up dinosaurs in Wyoming. And among modern things, I like cats, obviously, pictures of cats, but also falcons and other predatory birds. I seem to like apex predators, so I’m not sure what that says about my personality or not.
1:13:15.4 SC: Christopher Matthews says, “As an attorney, I can’t help but wonder about the implications the many-worlds interpretation could have on our legal system.” Well, as I’ve said before, I did write a book about this. I know that there’s a lot of questions we get about many-worlds, I wrote a book called Something Deeply Hidden, where I do talk about this stuff, and one of the stuff… I didn’t talk about the legal system specifically, but I did talk about ethical immoral issues, which I think port over fairly naturally into legal questions. And the answer is there are zero implications of many-worlds for ethical and moral questions, and I think also for our legal system. Many-worlds, for all intents and purposes of people living in one of the words, is exactly like a truly stochastic Copenhagen-type interpretation, where something happens with some probability and you don’t know what it’s gonna be, but all of the evolution of the physical system up to that point is perfectly deterministic. Many-worlds just says that the other alternatives also come true in other worlds, but who cares? I don’t see why that fact should, in any way, change our legal or moral or ethical implications.
1:14:25.7 SC: Ivor Trosief says, “What is your personal opinion of Gödel, Escher, Bach, and how it has aged science-wise?” I love Gödel, Escher, Bach. It was one of the… It was exactly rightly timed, to fall into my lap when I was in high school, when I was reading all the science books I could get my hands on that weren’t truly overly technical. And it was a little bit technical, if you read Gödel, Escher, Bach. Everyone reads the dialogues first, and then goes back and struggles with the regular chapters, but I think it’s a brilliant book. Structurally, it’s amazing. The fact that he could do those dialogues and have all of this intricate typography and everything like that and still be a best-seller and win the Pulitzer prize is amazing, but also, the explanation of Gödel’s theorem and all of the tying into Gödel’s theorem to recursion and self-awareness and art and music and so forth, I feel it was absolutely lovely and it completely holds up even today. So anyone who has not had a look at Gödel, Escher, Bach by Douglas Hofstadter, I would absolutely recommend it. Hofstadter, of course, was the PhD thesis advisor of Melanie Mitchell, who’s a previous Mindscape guest.
1:15:31.4 SC: Tim Kennedy says, “What are your thoughts on the social contract as we head into the next 50 years or so, maybe particularly your expectations for both high and low achievers?” So I’m sorry, but I have no idea what you mean by that last clause about high and low achievers, so I can’t answer that. The social contract, I think of as a metaphor. There’s no literal contract that we write down our names on to sign, but if we live together in a society in which we’re in some sense democratic, in other words, power or authority or legitimacy, in some sense, lies with the people, not with some boss at the top, then some kind of social contract is implicit, that we do things for the society and society does things for us, and society is just all the other people in society. Different people have been better or worse at articulating what this means, but one way of putting it is that almost nobody in the real world today is self-sufficient, is completely independent, right? Certainly no one who uses electronics, certainly no one who doesn’t build their own house with their own hands and catch their own food and so forth. You can imagine being self-sufficient, but very few people are. We need the rest of society to live the lives that we do, so some kind of contract is implicit as long as that ever happens.
1:16:53.8 SC: Jim Murphy says, “While reading Feynman’s lectures, I found it interesting that he essentially posits conservation of energy as an axiom of the universe. After thinking for a while, it occurred to me that the most reasonable explanation for this fact we based on the… ” You say anthropological principle, but I think you mean anthropic principle. “Universes where energy is not conserved would not be able to sustain processes like life in any meaningful way. What are your thoughts on conservation of energy as evidence for a cosmological multiverse?” Well, interestingly, I just wrote a paper that you can read on the archives saying that energy is not conserved to observers who do quantum measurements, so clearly, I don’t think it’s necessary. To think that something like conservation of energy is necessary, I think it depends on how badly it would be violated. People tend to think that unless you have this iron-clad rule that something is perfectly conserved, then anything goes, and I do think that life would be hard to sustain in an anything-goes universe. But what if energy was just a little bit not conserved? I think that would be entirely compatible with life existing. So I don’t think there’s a good reason to reach for anthropic explanations in that particular case.
1:18:00.0 SC: Dennis Gedard says, “Suppose for a moment we exclude supersymmetry, what alternative mechanisms do you think are most likely to explain the low observed value of the cosmological constant?” You know, we don’t even have to exclude supersymmetry, ’cause supersymmetry does not do the job of explaining the observed value of the cosmological constant. “I don’t know” is the short answer, but I do think following Tom Banks, especially, who is a very well-known physicist who has pushed the following idea, which I do buy into, that we shouldn’t be trying to explain the low observed value of the cosmological constant. Once you go to quantum gravity, and if you believe in the holographic principle, which I think some version of is right, then the positive cosmological constant puts us in de Sitter space in the far future and empty space with a positive cosmological constant, and that means there is a horizon around us with a finite area, and therefore, holography says, a finite number of degrees of freedom inside that horizon. And there’s a relationship between what the cosmological constant is and how many degrees of freedom are inside the horizon.
1:19:00.6 SC: So Banks says, the thing to be explained is not the low value of the cosmological constant; it’s the high value of the number of degrees of freedom in our observable universe. And I think that is a good attitude to take. It does not suggest an answer, it does not say, “Okay, therefore, here’s the answer,” but it changes the question to something that I think is gonna be more productive going forward.
1:19:25.1 SC: Kirkland Edward Hobbler says, “What is a good meta-mathematics or philosophical explanation for why the complex number system is necessary for contemporary science?” I have no idea, to be honest. I wouldn’t take it too seriously. There’s a lot of mathematics that is necessary for contemporary science. Why are Fourier transforms necessary for contemporary science? Why are partial differential equations necessary? I don’t know. Why are matrices necessary? I’m not sure that those kinds of questions ever have simple, satisfying answers.
1:19:55.8 SC: P Walder says,”Within the Popperian worldview, when knowledge is created, is it being invented, or discovered?” I think there’s gonna be a later question about this also. So, number one, I have no idea, because I do not hold the Popperian worldview, so, I’m not the expert. I’ve written about falsifiability, and despite my disclaimers, people think I’m writing about Popper. My disclaimers when I write about falsifiability say, “I’m writing about modern contemporary physicists’ cartoon view of falsifiability, not the actual view that Karl Popper had. I do not pretend to claim to know what view Karl Popper actually had.” So I think that knowledge is, by definition, invented. You don’t discover knowledge; you discover facts, and then that gives you knowledge. For knowledge, it’s a little bit trickier. The question later on in the AMA will be about theories, like theories are clearly invented. You never walk down the beach and stub your toe on a theory. You stub your toe on a rock or a shell or something like that, and then you invent a theory to explain it.
1:21:03.7 SC: So, knowledge appears. Knowledge is something you learn just by your senses and then you sort of interpret it in a theoretical framework, and that theoretical framework has to be invented. So I have no idea what Popper would say about this, honestly, but that’s my personal view.
1:21:18.7 SC: Gregory Eegan says, “I can only wrap my head around the many-worlds interpretation if I concede to infinity and at least one additional spatial dimension. Do you concur?” I have no idea about the infinity part. Certainly not, I do not concur about the one additional spatial dimension part. There’s no need for extra spatial dimensions in many-worlds, it works with whatever number of dimensions you end up having.
1:21:38.6 SC: Dragen Tubich says, “Although Wheeler himself denied retro causality in the delayed choice quantum eraser experiment, I’m still unconvinced. Could you please explain why the causality is not violated in this particular experiment?” I can’t explain this in real time, people have asked this AMA question before, I have a blog post exactly on this, the notorious delayed choice quantum eraser experiment, you should go check that out and I explain it in detail. The point is, all of the versions of the delayed choice experiment that make it seem like causality is violated are purposefully confusing. They’re not telling you what is really going on so as to make it seem more mysterious. It all has to do with what you mean by making a measurement or an observation. So if, like me, you’re an Everettian and you know what that means, this thing in the delayed choice experiment that is tricky was never really a measurement at all. If you can undo it, it’s not a measurement. So I explain that in much more detail in the blog post, I suggest you check it out.
1:22:39.5 SC: Ken Wolff says, “If the expansion of the universe driven by dark energy continues to accelerate and we are headed for The Big Rip, will there be any way to survive that?” No. It’s literally a singularity in the future, so there’s no way to survive that. I think The Big Rip is extremely, extremely, extremely unlikely, so I wouldn’t worry about it.
1:23:00.3 SC: Saraj Rajan says, “In the Time Travel episode, you mentioned that the movie versions of the watch of a time-traveling hero running fast is incorrect, and that the watch would actually be showing the passage of time as personally experienced by the time traveler. So, what would the time-traveling twin appear to have aged less with respect to? Does this aging process has something to do with the entropy of the twin’s bodies?” So no, it has nothing do with entropy. In the twin paradox, there are two twins, both of them have watches. Both of them have heartbeats and breathing and inner biological rhythms that are in synchrony with their watches and out of synchrony with the watches of their time-traveling twin. There is no unique, obvious way to compare what those watches say, unless the two twins are at the same point in the space. So they start at the same point in space, they can synchronize their watches, and they go out and do their things, and they come back to the same point in space, and they can compare how much time has elapsed for either one of them. But at no point did either one of them look down on their own wrists and see their watch behave weirdly.
1:24:07.3 SC: Anonymous says, “This question is about general advice for science fiction writers. Consider a setting like the one from Star Wars: That society has the technology, budget, and political will to build fleets of spaceships and so on. Surely the scientists of Star Wars and similar settings are building equally ambitious, even fantastical, physics projects, for example, using asteroid belt material to construct a massive circular particle accelerator in space. What kinds of projects would today’s physicists build if a Star Wars technology level and budget suddenly became available, and what are some possible discoveries that could change society?” That’s a very good question, actually, it’s a tricky one. Star Wars itself and almost Star Trek, almost any other futuristic TV/movie/space opera kind of movie, they don’t try very hard to invent realistic things that scientists would do in the future. I mean, Star Trek had recorders, or one of the devices you call when you talk to someone in Star Trek, I don’t know what they are, but basically, they’re iPhones, nothing more tricky than that. They were walkie-talkies, highly advanced walkie-talkies. It’s not very, very different.
1:25:16.5 SC: So they just take technology we already know, they advance it a little bit, so instead of have guns with bullets, you have ray guns, and then you just magically ignore things like the speed of light, so you say, “Oh, I have warp drive, so I don’t need to go to at the speed of light.” But there’s not any deep thought being put into truly society-changing variations, like human beings just mind melding with computers and not looking the same anymore that they usually do, or human beings living forever. There are individual stories written about these things, but most science fiction stories don’t systematically try to take those things into consideration. I do think that we are not going to see major changes in technology from new fundamental physics. I just wrote a paper saying that, that you can check out online, but I think that most technological change will come from the use of the ingredients we already know about in novel, creative ways: Atoms, molecules, electromagnetic fields, et cetera, that kind of thing.
1:26:19.7 SC: So I don’t think you need giant particle accelerators in space to do that. If you build a giant particle accelerator in space, you can discover new… It’ll be great, you can discover new particles, you could help particle physicists, but almost by construction, anything you discover there requires a giant particle accelerator in space to make it. This is the phase transition that we went around circa 1950 or so. Before then, the things we were discovering were already around us; we just hadn’t pinpointed them. Since then, the things that we’re discovering, the top quark, the Higgs boson, the W-Boson, there are no ambient Higgs bosons lying around because they decay away very, very rapidly. So that’s why fundamental physics doesn’t contribute to technological change anymore, and probably it won’t in the future, as far as I can tell.
1:27:08.4 SC: Aaron McBride says, “How is the quantum gravity research going?” It’s going fine, it’s going slowly. Everything is slow. I’m just slow because I do lots of different things at different points in time. But we’ve taken a step back and rather than trying to directly quantize gravity, I’m still trying to work out some fundamental features of quantum mechanics. I wrote a paper with Ashmit Singh a couple of months ago trying to see how… What are the rules by which you take an abstract quantum mechanical theory and divide it up into subsystems. This seems very down-to-earth in some way, but it might actually be really important for quantum gravity. And we’re trying to… The next step on that might be asking why certain kinds of classical theories appear as limits of quantum mechanical theories, and other kinds of classical theories don’t. And you can see why these are sort of good steps along the way to a full theory of quantum gravity. The thing I would really like to do and haven’t really made tangible progress on is take the work that we did on emergent space-time, which we did, this is with Charles Tau and Spiros Michalakis.
1:28:15.4 SC: We really only looked at weak field gravity in the solar system, things like that. When holography kicks in, we have a black hole or cosmology, things become very different. So I wanna sort of boot up… Not boot up. [chuckle] Up-level? Upload? No. I wanna improve our current way of thinking about it so that we can take those holographic, non-local effects into account in the kind of formalism that we built up, deriving space from Hilbert space. That’s the next obvious step.
1:28:44.5 SC: Michael Baker says, “What is the continuous rotation… Why is the continuous rotation of the galaxies not considered perpetual motion?” Well, you can have perpetual motion. There’s no problem with having perpetual motion. There is something called conservation of angular momentum, right? What you can’t do is have perpetual motion in a world with friction, in a world where there’s dissipation of energy, because there is energy conservation, to a good approximation anyway. So, galaxies don’t have friction, there’s nothing stopping them, so they’re just gonna continue to rotate.
1:29:12.4 SC: Alan White says, “What do you think of Stephen Wolfram’s physics project?” I don’t know much about it, honestly. They send me the papers, it’s just… It’s not at all built in a way that would make me get interested in it, because it just starts from a very, very different starting point and claims everything. And it’s just not in any way gonna motivate me to read 800 pages of stuff to get to what I’m interested in. If they really think that, for example, they can build quantum mechanics out of a discrete classical probabilistic cellular automaton, then write a 10-page paper saying that, then I’d be very interested. But I haven’t seen that yet, so I haven’t been following.
1:29:54.7 SC: Ash Wright says, “Is the unlikely state of entropy at The Big Bang the only difference between time and space dimensions? If one of the space dimensions had a similarly low entropy, would we perceived time-like effects along that dimension?” No, it’s not the only difference. As much as we talk about space-time as being one four-dimensional thing, there is a difference between time and space. There’s the simplest differences, there’s only one dimension of time, and there are three dimensions of space, which means in space, I can walk in circles. [chuckle] I can easily leave and come back to where I left. Whereas in time, I can’t do that. There are no closed time-like curves. That would still be true even if The Big Bang did not start with low entropy.
1:30:37.2 SC: David Baldwin says, “I watch videos by a gentleman on YouTube named Polosia who addresses ideas like the kalam cosmological argument, which I know you know from William Lane Craig. Paul’s response is typically that energy could have always existed. I’d prefer to phrase it that there could be a first moment in time or that there could be a larger system involved. Is either of us in the ballpark of saying something sensible?” I think your response is better, but they’re both kind of plausible. The kalam cosmological argument, for those of you who don’t know, says that the universe had a beginning; everything that has the beginning has a cause; therefore, the universe has a cause. And both of those first two premises, I argued in my debate with William Lane Craig, are not necessarily true. They might be true, the universe might have had a beginning, or it might be eternal, we just don’t know.
1:31:26.1 SC: And the fact that everything that exists or that has a beginning has a cause is just an outdated philosophical way of thinking about what you mean by cause. The correct question to ask is not, what is the cause of this, but is this happening compatible with the laws of physics? And again, we don’t know whether a universe with the beginning is compatible with laws of physics or not. We have no reason to think it’s not. We just don’t know what the laws physics are, so we can’t say so quite yet.
1:31:54.8 SC: Ashley Hyatt says, “How do cosmologists estimate the amount of matter and energy in the universe?” Well, there’s different ways to do it. You can’t even try… Well, you could try, you can’t successfully measure the amount of matter and energy in the whole universe. You can do it for the observable universe, but outside the observable universe, we can’t really say anything with any confidence. We can just guess, which is fine, but it’s not really very reliable. Inside, we use gravity. Gravity is the one thing that always couples to matter and energy. So you can have matter and energy that are invisible to photons or to neutrinos or to whatever, but everything both causes and is influenced by gravity. So, you can use that gravity in different ways: You can use it locally, like in a galaxy or clusters, just see how fast things are moving and therefore estimate the gravitational fields and therefore the amount of stuff; or you can look at the expansion and dynamics of the universe as a whole. That’s the most reliable thing to do, ’cause then you count everything. And that is what modern cosmologists do to get the answers that they get.
1:33:00.0 SC: K C says, “Please speculate on whether the Everett branching creates a new universe quantum field, or is there only one?” I don’t know what that means, a universe quantum field. Quantum fields exist within universes, okay? So within every universe, there are a bunch of quantum fields, and so every time you branch, you have a new space-time, new quantum fields, the whole bit.
1:33:20.3 SC: Linieu Misiaras says, “In Frank Wilczek’s wonderful book, Fundamentals, he says that the cosmic horizon is expanding. How is this possible if the expansion of the universe is accelerating?” Well, the cosmic horizon is roughly… Well, sorry, it’s exactly, exactly given… If you assume there was a Big Bang, okay, that’s where the “roughly” comes in, assume there is a Big Bang 14 billion years ago, which is a boundary past which we can’t see, the cosmic horizon is what you get if you take our current position in space and trace the light cone backwards until you hit The Big Bang. So the light cone going into the past is a big spherical thing expanding and expanding until it hits The Big Bang. So, the cosmic horizon is getting bigger just because we’re getting older, just because we’re aging more, and therefore, the past light cone is enclosing more and more stuff. That’s gonna be true whether or not the universe is accelerating.
1:34:13.9 SC: Enrique Areola says, “If black holes are formed in part by the presence of mass and most of the mass is a product of the strong forces holding quarks together, what happens to the strong force once the black hole is formed?” So, what you say is almost true, but I wouldn’t say most of the mass is a product of the strong force holding quarks together. There’s the mass, or the energy, equivalently, in some object, is inherent in whatever the fields in that object are doing. If you have a thing made of protons and neutrons, then the energy comes from or is described by a configuration of those quarks and… Quarks coupling to the gluons, namely, the strong force. Now, that can convert into other forms of energy. The energy can be conserved even if the strong force fields convert into other kinds of fields, like the neutral pion. A pion, neutral particle, one quark, one anti-quark, it can decay into two photons, keeping the total energy the same. So you start with two quarks, or a quark and an anti-quark, bound together by gluons, they convert into two photons. So there was strong force there, there no longer is, but the total energy is conserved.
1:35:28.9 SC: So when you make a black hole out of things that have the strong force in them, there’s nothing to prevent that particular strong force field configuration from turning into something very different.
1:35:36.7 SC: Mike Briggs says, “I’ve heard you say that you believe in many-worlds. Wouldn’t it be better to say ‘convinced by’?” I have no problem using the word “believe” to represent my beliefs. [chuckle] I think a lot of people confuse the word “belief” to mean belief without evidence, or believe against evidence, or belief based on faith, or something like that. You can have belief based on evidence, that’s what I have, and so that’s why I’m happy to use the word.
1:36:04.6 SC: Jason says, “To what extent is the spin of a particle, such as an electron, analogous to the spin of a macroscopic object, such as a basketball?” It’s largely analogous. The real difference is that when you measure the spin, you get these quantized answers. Also, there’s a lot of confusion in books, in textbooks, written about this. If you were to think of electrons as little, literally point particles, or even as little spheres of constant density, and everyone knows they’re not spheres of constant density, but imagine that they were, okay? Neither in the point particle idea nor the sphere idea could you literally have an electron spinning at the radius the electron is supposed to have at a velocity less than the speed of light to account for its spin. And therefore, people have said that the spin of an electron is not really spin, it’s not really angular momentum. But we know that what electrons really are are not point particles or little spheres; they’re field configurations.
1:37:04.6 SC: So my Caltech friend, Chip Sabins, who I wrote the paper with on the probability rule in many-worlds quantum mechanics, he has been working on trying to understand things like energy and spin in a real quantum field theory context. And what he says is, the spin is just the angular momentum of the field. That’s all it is. It’s just angular momentum, just like you always thought it was. I’m not a super expert on that, but his argument seemed convincing to me.
1:37:32.6 SC: Keith says, “In Everett and quantum mechanics, can split wave function branches evolve to an almost identical or identical state, essentially giving that state multiple possible histories within the universal wave function, at least on very small time scales?” I’m not exactly sure what is going on in this question, sorry about that. I mean, split wave functions can evolve to an almost identical or identical state. Imagine… Well, it’s always hard, because we always have assumptions in the back of our minds that we don’t always explicitly lay out. What I was going to say is, imagine measuring two spins, and you get one spin up and one spin down. If you get spin up and then spin down, you end up in the same position as if you got spin down and then spin down. If you did the same spin, I should say the same spin, you just measure it twice. But of course, if you recorded the answer, then somewhere in your brain, you know what happened in the past, so it’s not really the same kind of thing.
1:38:30.3 SC: If what you mean is it… What I’m trying to say is two branches of the wave function can look very, very, very similar, essentially as similar as you want them to look. But then you say, giving that state multiple possible histories within the universal wave function, I’m not sure what exactly that means. Of course, a state can have multiple possible histories ’cause it can branch, so I think that you’re assuming something that is not quite clear to me. Sorry about that.
1:38:56.3 SC: Jim Sicilian says, “Have you considered inviting Colin Woodard, author of American Nations, as a guest?” I have not, but I always like to get suggestions.
1:39:06.3 SC: Duncan Palmer says, “Do you believe it is possible or even inevitable that computational algorithms will develop some of the next important breakthroughs in theoretical physics, and if so, does it matter if the human mind understands how the algorithm connects input data to observed output, or simply the algorithm’s work in fitting the data?” I can certainly imagine that algorithms will develop some of the next breakthroughs in theoretical physics. I don’t think we should be too parochial about our own human abilities. Why shouldn’t computers be able to do it? Whether or not we can understand what they’ve been done, I think it’s useful either way. It’s more useful if you can understand it. So, if the computer comes up with a theory of theoretical physics, but the computer tells us, “The theory’s just too complicated, you’ll never understand it, but if you ask me a question, I can give you the answer,” that’s useful in certain obvious ways, but it’s not nearly as useful as being told the theory in a way you can understand it. So either one is interesting, but there’s definitely one better than the other.
1:40:08.3 SC: Elias Chunchula says, “If you could experience a historical event in person, what would it be?” I don’t know, I mean, I thought about this. I don’t know. Presumably, you’re talking about past historical events, right? I’m not a big fan of the idea of going to the past. The past was a terrible place. They didn’t have good sewage, they didn’t have good hygiene, or they didn’t have good safety in all sorts of different ways, it was a hard life in any kinds of ways. And I think I would rather just be now and still age into the future and read about the past in history books, so sorry, I’m not giving your answer a good… Your question a good answer. But this is… I’m more of a future-oriented person than a past-oriented person.
1:40:53.0 SC: Helen Gebhard says, “This relates to the hard question of consciousness as presented by David Chalmers. Assuming that sensory input increases with the complexity of organisms, and that a self-reflective consciousness provides significant survival advantages, how is it a problem to posit that such a deliberative executive functionality is a natural development of Darwinian adaptations across species?” So it’s not. And David Chalmers’d be the first to agree that it’s not a problem at all, but that’s what he calls the easy problem. That’s not the hard problem. The hard problem of consciousness is not a deliberative executive functionality, it’s explicitly not. That’s the easy problem. The hard problem is the inner first-person subjective experience, what it is like to be eight feet tall, as we were saying before. That’s the thing the Chalmers says that’s hard to explain, harder than the simple Darwinian explanation of functionality in any sense.
1:41:46.6 SC: Jorge says, “Watching the Biggest Ideas, I understood that at least in the context of effective field theory, the constants like the fine-structure constant alpha are not constant, but functions of the cut-off energy. In that case, when you say that, for example, alpha is 1/137, which cut-off energy is being considered and why?” So, yeah, that’s a very good question, and the answer is zero energy. [chuckle] So in other words, we measure alpha, the fine-structure constant, by scattering particles off of each other with some mutual energy between them, and we take the limit as that energy goes to zero. And that is what we quote as the value of alpha being 1/137. At higher energies, alpha is a slightly bigger number. If I’m getting the sine right, I think that’s right. For the strong interactions, for example, you can’t do that. The zero energy limit blows up, it’s infinite, the coupling constants. So, for the strong interactions, you have to define the strong coupling constant at some other energy other than zero, usually defined to be the QCD scale around 300 million electron volts or something like that.
1:42:49.9 SC: Christian Dobo says, “It seems to me governments and organizations sometimes choose to distort or simplify facts for the public’s own good. When there are not enough PPEs, they might say it’s not effective anyway. When arguing with anti-vaxxers, they say no vaccine ever cause serious side effects, et cetera. Nothing is ever black or white, but what do you think in general of society’s right and capacity to know and handle the truth? Where will you draw the line?” I think the society has the right and capacity to know and handle the truth. I think you should tell the truth. And I think that it doesn’t do much… It’s not very conceptually useful to blame “governments and organizations.” There are people who make up these governments and organizations, and these people make these choices, and different people might make different choices. So I think that you should be honest with the public, for various reasons. There are exceptions to that. There are sort of self-interest, dangerous situations where you don’t want to give away your battle plans in public if you’re in the middle of a war or something like that. But when it comes to the safety and things that the public themselves should be doing, just tell them the truth.
1:43:56.1 SC: Tom says, “What do you think of the name The Second Law of Thermodynamics? I studied physics and philosophy in university, and we had a short course on course on entropy, and I never understood it because they kept calling it a law, and it seemed obvious that it wasn’t a law; it was just a likelihood. How can you get anything profound out of a likelihood?” Well, if something has a likelihood that is 99.99999%, you can get some very useful things out of that. If someone’s willing to bet you even money on a coin flip, and you know the coin is not fair, and it’s a 60% chance of being heads, you can make a lot of money out of that. There’s all sorts of likelihoods that are very, very profound. So, I think that’s my answer.
1:44:37.1 SC: Gary Miller says, “In your Mindscape episode with Max Tegmark, you mentioned that the high resolution of our universe may signal that we are in the real thing. However, could there be a selection pressure for high-quality simulations like the creator of the simulation prefers high-quality simulations?” Well, two answers to that. One is, sure. There could be, who knows? That’s why you see very quickly how this kind of speculation becomes ungrounded from things we can actually check against any experiment. But the other thing is, my argument was not just, “It could be lower resolution”; it’s that, we will make simulations ourselves. Our civilization will simulate others, if you believe this kind of reasoning. And ours will necessarily be lower resolution than the simulation that we are supposedly in. And therefore, it’s just inevitable that most simulations are the low-resolution ones, regardless of selection pressure. Again, I don’t think that the original argument for the simulation hypothesis really works, but I think that’s one of the possible objections to it.
1:45:40.7 SC: Anonymous says, “There are about as many human thought moments, 0.1 seconds, in a human lifespan as there are human lifespans in the age of the universe. Is that some sort of anthropic maximum likelihood thing?” As far as I know, no. I think that it’s numerology, as far as I can tell. Maybe if you come up with some explanation, I’d be willing to think about it, but as far as I know, no.
1:46:05.4 SC: Christopher Gustafson says, “Is there a globally agreed-upon map structure for all the known stars, planets, galaxies among people working with astronomy?” Well, there are two things. When you talk about something in the sky, whether it’s galaxies or stars or whatever, you talk about its position on the sky, the faraway stars are almost stationary over human lifetime, they don’t move very much. There is a reference frame with respect to which you can locate things on the sky. There’s a universal map structure for that called right ascension and declination. And the third dimension is just the distance to the thing. And the problem with the distance is we usually don’t know it, right? It’s very, very hard to measure distances of things in astronomy, as we found out talking to Lena Naseeb and also Adam Reese on the podcast in 2020. So, that’s often not listed when you talk about positions of things in the sky; it’s just much harder to map distances than angular positions on the sky.
1:47:04.6 SC: Elon G says, “It seems logical that if there is any place in the universe where information is not preserved, it is inside a singularity. Why not accept this and face the consequences accordingly?” So I presume you’re talking about the black hole information loss puzzle, and the point is, the black hole evaporates away. [chuckle] So, the point is, you go from a situation where there is no black hole, and then you evolve through a situation where there’s a black hole, but then in the future, again, you have no black hole. So there are laws of physics, quantum mechanical laws of physics, that evolve you from no black hole to black hole, and quantum mechanics preserves information. So, the options are either that there is something that… That that picture is just entirely wrong, that you don’t have an evaporating black hole in some sense or another, or that quantum mechanics is wrong in some way, or the information somehow gets out. And of those three options, the information somehow getting out seems to be the mildest. Sure, maybe information could not be preserved in black hole evaporation, but that leads to all sorts of other bad things, like you can have virtual black holes and Feynman diagrams all the time.
1:48:19.3 SC: So you would have information being lost all the time in ways that we don’t seem to actually observe it. So, none of these are airtight arguments, but that’s why people have the opinions that they do.
1:48:29.3 SC: Samuel Valle says, “Based on your experience as a student and professor of physics, are there any issues or changes you would make to this post-graduate process of a PhD in physics?” I guess there’s one change that I would take very seriously, which is, I don’t know whether current post-doctoral fellows would like this change, but I think we should have fewer post-docs. So right now in physics, you get a PhD, it takes five, six years or whatever. Then you get a post-doc, and so, that lasts for three years, and you just do research and you build up your CV, your experience. And many people do two or sometimes three or more post-docs, then eventually, you either get a faculty job or you get some other job. You leave the field or you go into industry or whatever. And the number of faculty jobs… Everyone who goes into a PhD program… Not everyone, but the vast majority of people, would like to get a faculty job doing physics. But the number of faculty jobs is much smaller than the number of graduate students. So, there is a bottleneck, and you have to choose where to put the bottleneck.
1:49:35.5 SC: You can’t get rid of the bottleneck, except by just dissuading people from wanting to be physicists, which I don’t think is the right strategy. So right now, there’s a huge bottleneck in going from post-doc to faculty member. Most post-docs do not become faculty members. And I think that’s too late in life. You get your PhD, you’re 26, 27, 28 years old, and then you get another six, eight years of post-docking, you’re in your mid-30s before suddenly, you have to change your career. And that’s hard to do at that point in life. I think that getting a PhD in theoretical physics, probably also in experimental physics, though I’m just admitting that my experience there is not as big, is intrinsically valuable no matter what you go on to do. It’s good training. Whereas being a post-doc in physics is just more of the same, it’s better training, but it’s in the same kind of vein as your graduate student training. So, I think where the bottleneck should be is closer to after being a graduate student then after being a post-doc. So I think there should be fewer post-docs, to be honest.
1:50:42.7 SC: Eric Chen says, “What’s the argument for why humans should try to reason like Bayesians? Even if we accept something like Solomonoff induction as the correct account of ideal reasoning, what’s the further justification that boundedly rational agents like us should try to approximate this ideal when reasoning in practice?” Well, I think the phrase “Bayesian reasoning” is a little bit vague. It’s a little bit of a catch-all. There’s different aspects of the story that go into what we call Bayesian reasoning, and so you could question different ones of them. So one aspect is just accepting Bayes’ rule, accepting that if you have some priors on different propositions and you get new data, you can update those credences that you have in those propositions by Bayes’ rule. Bayes’ was just true, you don’t have a choice, it’s a theorem, you can derive it, it’s inherent in the meaning of what you mean by conditional probabilities. So there’s really no option there.
1:51:39.2 SC: There is a question of practice, like when you actually update your credences and different beliefs. Very few people in the everyday world literally write down their priors and then calculate likelihood functions and multiply them and all that stuff. So maybe that’s where the bounded rationality comes in. You wanna approximate that procedure by a way that is easier than actually plugging in Bayes’ rule. That’s fine, but you wanna approximate it as well as you can, so I would still say that you’re trying your best to be Bayesian. But then there are other aspects. One aspect is the very idea that probability should be subjective. There are people who just reject the Bayesian philosophy that the way to think about probability is a set of credences which you then update when new information comes in. I just think those people are wrong. I’m a favor of the subjective view of probability, so, I think that’s a sensible way to treat credences and how to manipulate them.
1:52:37.9 SC: Peter Bamber says, “Is a wormhole linking space inside the event horizon of a black hole to space outside the event horizon a valid solution to the equations of general relativity?” No. It’s not. [chuckle] At least you can always start with a wormhole linking them, but like I said earlier in the AMA, the wormhole just collapses. A traversable wormhole with positive energy sources is not a good solution to general relativity, sorry.
1:53:05.4 SC: Chris Figorido says, “How would you describe the difference between an ether and a quantum field? On a superficial level, they seem to be similar in concept.” Well, the big difference is the entire point of the ether was that it is supposed to define a rest frame for the universe. You could measure your speed with respect to it. And quantum fields don’t do that. Quantum fields exist in space-time, but they look the same to all observers, no matter how they’re moving, therefore, it’s quite different from the ether.
1:53:29.0 SC: Jaljel Gooberman says, “Regarding life outside Earth, you say that we have a sample size of one so life could be ubiquitous or rare. Life appeared on Earth fairly early on, and we have no evidence that life evolved independently more than once on Earth. Doesn’t that count as evidence against ubiquity?” Well, the fact that life appeared on Earth fairly early on might count as evidence for ubiquity, but I think that either evidence is very, very weak. Maybe life would have evolved on Earth many times, but the life that already existed sort of ate it up, I destroyed the competitor of life. We just don’t know any of these things. So I think that still, we should just be open-minded about this.
1:54:09.0 SC: Damian Alexeyev says, “What are your thoughts on Hermann Minkowsksi’s statement that space by itself and time by itself are doomed to fade away into mere shadows and only a kind of union of the two will preserve an independent reality?” I think that statement is very, very accurate as far as the philosophy of relativity is concerned, either special relativity or general relativity. Since we don’t yet have a good quantum theory of gravity, we can’t be sure that that philosophy will continue to be true, but it’s certainly been very successful over the past 100 years.
1:54:40.3 SC: David Grimes says, “It was recently proposed that black holes could have formed orders of magnitude larger than supermassive black holes. There could be stupendously, large black holes theorized to mass 100 billion solar masses. However, reports also state that black holes could not grow larger than 50 billion solar masses due to the way accretion works. What is it about the supermassive black hole accretion process and the special areas of space-time surrounding such monsters that keep them from growing further?” So I’m not a super expert on this, but my superficial understanding is it’s not anything specific with black holes or weird general relativity effects; it’s the accretion process. If you have a very big black hole, anything can fall into it one particle at a time, you can absolutely pour more mass on to it, but the point is that if you try to accumulate a lot of matter in a giant accretion disc, it heats up and then pushes the rest of the matter away. It becomes harder and harder to sustain a sort of quasi-equilibrium configuration where matter is constantly accreting onto the black hole. I really don’t know anything about these stupendously large black holes, but that is the little bit of knowledge that I have.
1:55:50.7 SC: Alan Rasmussen says, “Do you see the number of spatial dimensions experienced by humans and used in Newtonian and quantum mechanics as three as an emergent or fundamental property?” My guess is that it’s an emergent property. I think that all space-time is probably emergent, and that includes the dimensionality. But again, we don’t know for sure.
1:56:07.9 SC: Anton Hawthorne says, “I’ve heard talk about a multiverse in the sense of inflation causing the separation of bubble universes where physical constants can vary between them. My question is, what theory predicts these constants to not be constant? Why would we expect them to vary over cosmic distances?” So there’s a few different theories that predict that. By the way, they’re not necessarily true, these theories, so, it’s absolutely possible to imagine a multiverse in the sense of eternal inflation with bubble universes where all the constants are the same. It’s somehow more fun to imagine them being different, so that’s often what people do, and the anthropic principle doesn’t work unless they’re different, so, that’s another motivation. So the simplest motivation is actually extra dimensions of space, which is predicted by string theory and might even be true if string theory is false.
1:56:56.8 SC: Once you have extra dimensions of space, you need to hide them somehow. Generally, the simplest way to do that is to curl them up into some small topological geometric manifold, and there are many, many ways to curl up the extra dimensions. And it turns out that once those extra dimensions are curled up, you can’t see them any more microscopically from the big three dimensions, but they sit there setting the background values of various constants in your low energy effective theory, including the fine-structure constant, the masses of particles, the Higgs boson, expectation value, all of these different things that go into particle physics as we know them could be different, depending on the topology and the geometry of the extra dimensions. So, it’s very possible that in the inflationary multiverse, that is actually what happens. Again, we don’t know for sure.
1:57:45.0 SC: Alan Mrochek says, “Do you think there’s something that we can think of as a cognitive wall? By ‘cognitive wall,’ I mean a limit to what can be learned, processed, and understood within one human lifespan, or that helps certain topics… That certain topics have too many variables to consider.” Sure. I think that there’s absolutely a finite calculational capacity of the human mind, and the effective capacity is probably much less than the in-principle capacity if we treated every neuron as a perfect computer. So, if you take two numbers, both of which you have 10 to the 100 digits and ask me to multiply them in my head, I’m not gonna be able to do it. [chuckle] But I have no idea what that cognitive wall is in practice, and I’m suspicious of any idea that we’re close to it now in terms of developing new theories of physics or anything like that. I think there’s a long way to go before we reach the levels of our understanding in those regimes.
1:58:38.8 SC: Alexander Cordova says, “How is it that some dimensions are smaller than other dimensions in the context of possible extra dimensions existing?” Well, as I just said, you could imagine curling them up. So, the usual metaphor which I can’t do any better than is to imagine a straw, like a sipping straw or a rubber hose or something like that, where one dimension is very, very long, the length of the straw, and the other is curled up into a little circle. So this is topologically aligned. If you imagine an infinitely long straw, that’s a line in a long direction, in a circle in the short direction, and as you sort of move further and further away from the straw, that extra curled-up dimension look smaller and smaller, and the straw looks more and more like a literal one-dimensional line. So in particular, in our universe, where you have quantum mechanics and particles and fields have wavelengths, if the wavelengths of the particles and fields that we know about are all much larger than the sizes of the extra dimensions, then those extra dimensions would be invisible to us.
1:59:41.7 SC: Craig Gordon says, “Are all the force carrier particles virtual? If not, what is the difference between a real photon, gluon, W and Z boson compared to a virtual one?” So no, they’re certainly not a virtual. The photons that are hitting your eyeballs right now are not virtual photons; they’re real photons. The word “virtual” in this sense only and explicitly means particles that are in the interior of Feynman diagrams. So we use Feynman diagrams to think about scattering processes. Two electrons scatter off of each other by exchanging a virtual photon. But if any particle is at the edges of the Feynman diagram, not on the interior, one either comes in from the outside or goes out at the end of it, ingoing or outgoing particles, those are all real. So, photons, gluons, W and Z bosons can all be real.
2:00:34.4 SC: John Eastman says, “Have you seen the Netflix series Surviving Death episode six featuring Jim Tucker’s fascinating research on children with memories of past lives?” Nope, I’ve not seen that.
2:00:46.5 SC: Marco Faela says, “Regardless of current technological limitations or from the point of view of an extremely advanced civilization, what would be the pros and cons of communicating via gravitational waves?” Well, they would almost all be cons because gravity is really, really weak, which means it’s incredibly hard to both create a gravitational wave and to detect it. It’s much, much easier to create or detect photons, or even neutrinos, compared to gravitational waves. So, unless you were really locked in some completely opaque region of space and wanted to get signals out, there’s almost no circumstances under which I could imagine gravity waves being the way to go.
2:01:24.9 SC: Ian Smitt says, “Assuming the many-worlds interpretation is a correct interpretation of quantum mechanics, here is my question… ” It’s a sort of nonsensical phrase. “I assume in one of these many worlds, you’ll be able to understand this gibberish and answer my question. Let’s hope it is in this world. I don’t wanna waste your time.” So nope, it’s not. This world is just gibberish, sorry about that.
2:01:46.4 SC: Anon says, “Why do you assume that the entire opposition to Democrats is anti-democratic?” I don’t. Not at all. Not in any possible sense.
2:01:54.3 SC: Andre Dinu says, “Would you agree to the following description of reality: Ignoring complications related to particle spin, a system of n particles is defined by a complex valued way function of 3 x n variables. If the wave function is real, then the 3n dimensional space is real, too, and this means the answer to the question about where Everett’s world actually exists is simple: There are nothing but peaks of a wave function in the 3n dimensional configuration space.” Well, I edited that down, so I hope I get the essence of the question there. The point is that the world is not a system of n particles [chuckle] as far as we know. Our best models of the world are as quantum fields, which have a continuum of possible values. It might be, in fact, I think it probably is, the quantum field theory is just an approximation and there’s some underlying finite dimensional structure, but it’s not a set of n particles. So, I think that the thing that exists is Hilbert space, or the wave function as an element of Hilbert space evolving through Hilbert space. That seems to me to be the right way to think about reality.
2:03:00.6 SC: Avi Chain says, “I’m an undergrad physics major. Last semester, I took quantum mechanics, I’m trying to understand spin of elementary particles. How should I imagine it? Is it similar to a classically spinning object?” So yes, as I said earlier, once you go to quantum field theory, it becomes easier to see how to think of elementary particles with spin as classically spinning objects, as objects with angular momentum. Even at the level of just point particle quantum mechanics where that doesn’t work, you have to just attribute some quantity called spin to this point particle called the electron. It’s a kind of an angular momentum, it’s an intrinsic angular momentum. It can’t change; the electron always has been a half, there are no electrons with spin three halves. It can’t rotate faster. But it’s an intrinsic amount of angular momentum. That’s what it is. I don’t see why that’s hard to think about. I never really struggled with that one.
2:03:51.7 SC: Alexis Ablatsky says, “I’ve watched Max Tegmark’s mathematical universe thing and quickly became convinced, but there’s a problem. There are more complex mathematical structures than simple ones, and our actual laws of physics seems simple. So imagine a sequence of random bits with length l and some sub-sequence around it, and then interpret it in some simple formal language, breathing fire into the equations,” and then it goes on longer than that. So there’s two things to say about the scenario that you’re sketching out.
2:04:23.7 SC: One is, I’m really not sure what to make of the idea of being interpreted in some simple formal language, therefore breathing fire into the equations. I think the metaphor “breathing fire into the equations” is a vivid one, but I have no idea what it means. I have no idea to operationalize that or to understand what it means. I understand the universe as stuff evolving according to patterns. So, when you say the mathematical universe, I interpret it as, there’s some stuff and there’s some patterns, that’s what you give me. I agree with you that in the world of all complex mathematical structures, way more of them are complicated and messy than simple. So, unless there is some principled reason to pick out the simple ones, that’s why I don’t think that Tegmark’ mathematical universe is very convincing. I would expect, I would make a prediction, on the basis of that model, that our universe looks very, very different than it does. We don’t understand enough about the space of all mathematical structures to say that definitively, but that’s why I don’t think it’s promising as of our current level of understanding.
2:05:26.4 SC: Jonathan Gordon says, “Could you do an interview with Judea Pearl on his book about the science of cause and effect called The Book of Why?” Always happy to get suggestions, as I always like to say.
2:05:35.1 SC: Tim Olmond says, “One of the first things we all learned about quantum mechanics is that matter and anti-matter will annihilate each other in a flash of gamma rays should they can contact each other. However, mesons, which consist of a quark and an anti-quark, exist, even if only for a short time, and when they decay, they produce particles such as electrons rather than photons that one might expect. Why is that?” So, number one, mesons can decay into photons, as I already said, that’s what neutral pions, for example, do, they decay into pions. But it never was true that quantum mechanics taught that matter and matter annihilate into photons as some exclusive thing. Particles and anti-particles annihilate into other particles. It could be all sorts of other particles, as long as the particles they annihilate into are not heavier than the total energy of the system that did the annihilating in the first place.
2:06:23.9 SC: So that’s why, when you collide an electron and a positron, there, roughly speaking, aren’t any particles lighter than them to annihilate into other than photons. There are exceptions. There could be graviton or neutrinos. But both gravitons and neutrinos interact really, really, really weakly, so you will make them sometimes when electrons annihilate, it’s just really, really unlikely. But if other particles annihilate, like if you have a W boson and an anti-W boson, they can annihilate in all sorts of things, not just photons.
2:06:56.8 SC: Jason Levy says, “We assume that our senses and our level of intelligence are enough to comprehend any truth about the nature of nature given time and the proper thinking. Is it possible that that is a misperception? Do you think it’s possible the most fundamental questions of physics are beyond the reach of human understanding?” Well, it’s related to the question I just talked about. In some sense, some strict sense, sure. It’s absolutely possible that there are questions that are beyond the reach of human understanding. It would be foolish to assume otherwise. On the other hand, don’t leap from there to “Therefore, I expect it’s true” that there are questions beyond the reach of human standing. It’s possible; that doesn’t mean it’s likely. My personal view is that human understanding, human ability to think about the world cognitively, rationally, and abstractly has undergone some kind of phase transition. We can model formal systems in our minds in ways that other animals cannot. And that gives us a certain ability, which might be all you need to answer the fundamental questions of physics. We’re basically Turing machines, right? You might not need more ability than that. So, it’s up in the air, but I would be skeptical of worries that there are fundamental questions we can’t understand.
2:08:10.5 SC: Anders says, “If a mouse made it into your house, who would catch it first? Ariel, Caliban, you, or your wife?” Well, it would not be me or Jennifer; it would be between Ariel and Caliban. Ariel is much faster and more energetic, so if she moves quickly enough, she has a chance. But Caliban is stronger, he’s a big 15-pound cat and he’s all muscle, and he’s also very, very patient, so if the mouse hid and some nook or cranny, Caliban would wait for it to come out. So he’d probably be the most likely.
2:08:40.0 SC: Mark Eimmel says, “In your recent podcast with Avi Loeb, he mentioned that ‘Oumuamua is more or less at rest in the local galactic frame. Are their cosmological models that would predict rocks being at rest to the local galactic rest frame?” Sure. Every cosmological model. The point is that the galaxy is full of stuff, full of stars, full of rogue planets and comets and pieces of riff-raff and whatever, with various velocities, so some of them are going to be at rest in the galactic rest frame just by chance. Avi was trying to make the point that it’s weird that most things are not at rest or close to it, but some things are going to be. That’s not so surprising all by itself.
2:09:21.8 SC: Eric Coker says, “What inspired the term ‘Mindscape’ for your podcast? I asked because I just picked up Rudy Rucker’s Infinity and the Mind, and the term figures predominantly in the first chapter. Is there any relationship?” So there’s no relationship between Rudy Rucker’s book, sorry, I didn’t know about that. It was just a word we came up with, bouncing ideas around. I had lots of ideas early on, most of them were pretty bad, and I Googled it, and the word “mindscape” certainly exists out there in the world, but there was no really famous book or other podcast called that. So we went with it.
2:09:53.5 SC: Beth Mauri says, “Re: The idea of alternate versions of ourselves existing in parallel universes, do these versions split off every time a choice is made?” So no, choices have nothing to do with it. This is, again, something I discuss in detail in Something Deeply Hidden. Versions split off when the universe branches, and the universe branches when a quantum mechanical system becomes entangled with a wider macroscopic world, in particular with its environment, one way or the other. Those words had nothing to do with human beings making choices. Now, it can happen the other way around. It can happen that a quantum event can affect a choice you make, and therefore, a single quantum splitting can have an effect so that in one universe, you make one choice; in another universe, you make another one. But it’s the event that causes the difference in your choice-making, not the other way around.
2:10:45.1 SC: Jack Lyle says, “Why is the charge on an electron exactly equal to minus the charge on a proton? In 1911, this was a just-so story, but is it still one today?” Well, as far as we know, yes, it is just a just-so story that just happens to be the truth. But we have ideas as to why it might be true. The simplest, most compelling one is Grand Unification. So we have these forces of nature, the strong, the weak, and the electromagnetic, SU three cross SU two cross SU one, and since the ’70s, people have tried unify these three forces into a single, simple gauge group like SU five or something like that. And it turns out that if you do that, then you predict quantization of the different charges, and part of that is that all electrons have the same charge, and it is minus the charge of a proton. That is a prediction of Grand Unification, but we don’t know if Grand Unification is true, so we have to be a little bit open-minded about that.
2:11:44.3 SC: David Whitmarsh says, “How might someone experimentally test whether the Everett interpretation of quantum mechanics is correct?” Well, you can do any experiment at all in quantum mechanics and Everett will make a prediction and you can see whether that prediction is right. Probably what you’re asking is, how can you experimentally test whether it’s incorrect? Could you falsify it somehow? And there, yes, just see an isolated quantum system violate the Schrödinger equation. That’s all you need. And indeed, experiments are ongoing to test exactly that, but so far, they haven’t seen any violations probably because Everett is correct.
2:12:18.4 SC: Richard Taylor says, “Imagine that as an undergraduate, you were abruptly transported to the year 2021. What would impress you the most about this world of the future and what would be your greatest disappointment?” So here, again, I think that all of my answers are extremely conventional. I don’t have anything clever to say to this question. The most impressive thing is in various versions, the Internet and electronic connections that we have between us. From my undergraduate days, we were regularly flying the space shuttle and the Concord, so, there’s no rule that says the technology has to advance. We can have technologies in regular use that we lose use of. But obviously, the ability to do podcasts on the Internet, read them on my computer or listen to them on my cellphone, none of those words would have made any sense when I was an undergraduate. So that’s impressive. Again, not very surprising that that’s the answer, but it’s an impressive one.
2:13:19.9 SC: And the disappointing things are things like climate change, which we already knew about in the ’80s but was something that seemed a little bit less obviously disastrous than it does now. And threats to democracy, which are always there in the background, but they were way in the background in the 1980s. We still have the Cold War going on when I was an undergraduate, but it was kind of becoming clear that the Soviet Union was not robust. I think that people tell themselves they saw the collapse of the Soviet Union coming, even though maybe they didn’t. I certainly didn’t predict it was gonna happen nearly as quickly and dramatically as it did. So that was, at the time, the threat of nuclear war was very real when I was an undergraduate, and it’s not now. The threat of a rogue nuclear explosion is bigger now than it was then, but a full-scale nuclear war seems less likely. But we have things like rising authoritarianism and climate change that I’m very worried about. Those are my greatest disappointments.
2:14:17.9 SC: Josh says, “In Something Deeply Hidden, you mentioned that the many worlds don’t really exist and are instead just a useful tool for human thought. Can you elaborate on this point?” You’re gonna have to bring up a quote on that, ’cause I never remember saying that. I usually say exactly the opposite. I think the other worlds do really exist, that is my view.
2:14:34.2 SC: Siddhartha says, “Even if Hilbert space is sufficiently big to contain all branches of the wave function of our universe until now, what would it mean to have run out of room when it eventually happens? Say our calculations are wrong in the universe runs out of room tomorrow, what would be our subjective experience?” Well, I think that the upper limit of the number of splittings you can do of the wave function of the universe is tied to the approach to thermal equilibrium. The maximum number of splittings happens in the maximum entropy state that the universe can be in. So we’re very confident that our calculations are not wrong and we’re not gonna run out of room tomorrow, because we’re clearly not in a maximum entropy state. All of the energy in the atoms that make up you and me could be converted to photons, and that would dramatically increase both the entropy of the universe and the number of branches of the wave function. So, we have a long way to go.
2:15:30.2 SC: George Sarbidza says, “How does Zeno’s dichotomy paradox affect many-worlds theory?” So Zeno’s dichotomy paradox is just the classic one, that if you want to run a mile first, you have to run half a mile, and then half of the remaining distance, so another quarter of mile, then another half of that, et cetera, and therefore, you have to traverse an infinite number of segments and you can’t do that, so you can never run a mile. So, Zeno’s paradox has zero effect on many-worlds theory, because Zeno’s argument was wrong. [chuckle] I hope that everyone knows this. You can run a mile. It is possible to run a mile. Why? Because it is possible to traverse an infinite number of segments, and that we sort of figured this out in the most correct way when we got calculus right. You sum an infinite series and get a finite answer. So, Zeno, you’ll put his finger on puzzles of the concept of continuity, but we resolved those puzzles. They’re no longer puzzling to us. And the same thing is true of many-worlds. There’s nothing special about many-worlds.
2:16:35.1 SC: Dan Inch says, “As a non-American, the concept of a legacy admission to university seems very odd and not in keeping with the goal of excellence and learning or research. I understand that Caltech does not adopt this approach, but wonder what your views are about the practice.” I’m not a super expert. Legacy admissions, for those who don’t know, are when universities give preferences to applicants whose family, whose parents or whatever, were graduates of the university. It absolutely happens. My impression is it happens more than it should. But look, you know why it happens, because families that sort of keep their people, their parents and children and grandchildren, going to the same university, are more likely to donate money to the university. And the thing about universities having money is it’s not just used for lavish parties for the university president. A lot of that money is used for research and teaching. So, universities having money is something I’m in favor of, and how to best do that is a difficult question. There’s a mixture of different strategies one has: Government grants, private donations, legacy admissions, inspiring private donations, tuition and fees, and all these things.
2:17:48.2 SC: So, I’m not against universities making money, because eventually, most of them are non-profit organizations. Actually, that money goes back into the university; it’s not just used for frivolities. Having said that, you’re right, you would get a better student body if you didn’t have legacy admissions, but you would also look at less donations, so maybe you could give that student body less of a good education. That’s the trade-off you need to think about.
2:18:13.2 SC: Arthur Edelberg says, “Does the kind of timepiece being used affect whether it would register time dilation?” No, not at all. Every timepiece would be affected exactly the same way.
2:18:24.1 SC: Herb Berkowitz says, “If general relativity is the physics of the big world and quantum mechanics is the physics of the tiny world, where is the border between the two where one applies on one side and the other applies on the other side?” Well, honestly, it’s completely untrue that quantum mechanics is theory of the tiny world and general relativity is the theory of the big world. Both quantum mechanics and general relativity are true on all scales, roughly speaking. The better way to say it is that the intrinsically general relativistic effects as opposed to those that you could describe using Newtonian gravity only become noticeable in the big world, in fact, in the big world, where gravity is important, and the intrinsically quantum mechanical effects as opposed to classical mechanics only become noticeable in the tiny world. But quantum mechanics has classical mechanics as a limit. It’s always true. Likewise, general relativity is true even in the room where you’re sitting right now.
2:19:17.5 SC: But there’s no hard and fast boundary where the approximation of approximating general relativity with Newtonian gravity or approximating quantum mechanics with Newtonian mechanics becomes correct or incorrect. They just become more and more accurate. It’s a smooth transition; it’s not a hard dividing line.
2:19:38.3 SC: Robert Merley says, “I frequently heard it said that the vacuum energy of the universe is about 120 orders of magnitude lower than it should be. But I’ve never read why physicists think it should be that high. Could you oblige?” Yes, I did also talk about this in the Biggest Ideas videos, maybe in the one on re-normalization. I’m not sure. The point is that when you take a classical theory and quantize it, there are quantities, like the vacuum energy, that have a classical part, and then they are corrected by quantum mechanics. So, if you start with any classical amount of cosmological constant vacuum energy at all, you can separately calculate the corrections that you expect from quantum mechanics. And it’s those quantum corrections that are 120 orders of magnitude bigger than the actual cosmological constant we observe.
2:20:32.4 SC: So it’s possible that the right way to think about it is the classical contribution to the cosmological constant is exactly minus the quantum correction except for the tiny little bit that we observe. But no one sees why that should be the case, so it’s just really weird. Since we have no idea and no expectation for what the classical contribution should be, people say, “Well, the best guess for what the total cosmological constant should be is approximately the quantum part of it,” and that is in the effective field theory approach given by the Planck scale, it’s a dimension of this quantity, and the Planck scale energy density is a 120 orders of magnitude bigger than the observed cosmological constant. That’s where the number comes from.
2:21:18.0 SC: Robert Grenise says, “I’ve often heard you say that the universe has no center. How is this possible?” Well, imagine a plane, a two-dimensional plane. The idea behind the mathematical two-dimensional plane is, it literally goes off in all directions, infinitely far. There’s no edge to it. So because there is no edge, there is no center to a plane. You can put coordinates on it where the coordinates have a center, x = 0, y = 0, but I could put coordinates where the coordinates are different, and x = 0, y = 0 is a million light-years away from that. There’s no preferred center at all in the two-dimensional plane. The universe, as far as we know, is like that. It’s three-dimensional, three-spatial-dimensional, but no matter what its global geometry is, there’s no reason to think it has an edge. The edge is either infinitely far away or just doesn’t exist at all, so therefore, analogously, there’s no center to it.
2:22:13.3 SC: Rosemary Fillner says, “What do you think are the most interesting frontiers for the intersection between philosophy and science today?” Well, there are many, but there’s two broad classes there. One broad class is philosophy about science, and I think that if… This is what I was exposed to when I was an undergraduate, and I thought, “It’s cool, but it’s nothing I wanna do for a living.” Things like Thomas Kuhn, Paul Feyerabend, Imre Lakatos, Karl Popper, for that matter, philosophers trying to understand how science works. I really don’t know a lot. That’s still a very, very active area and I don’t know a lot about it. There’s another area of intersection between philosophy and science, which is the foundations of science.
2:22:58.2 SC: So, like I said before, it’s really just science, but it’s done in a philosophical way, in a way that is more amenable to being in a philosophy department. And obviously, there’s physics questions there, like the measurement problem in quantum mechanics, the origin and nature of the arrow of time, the anthropic principle. These are all very interesting frontiers for the intersection of philosophy and science, but it’s also very, very active in biology and complex systems. When do you call something an organism? What are the levels of selection on which evolution happens? Where does information seep into the evolution of non-organic materials in their transformation into living beings? These are philosophical questions at the philosophy of biology. So, I just think there’s sort of too many interesting frontiers to actually address that in any simple way.
2:23:50.4 SC: Phil Johnson says, “I would love to know your thoughts on if you think we’re living in the simulation and what it would take to create a universe-sized simulation. Are there any clues at the quantum level that could suggest an underlying code structure?” So no, I don’t think we’re living a simulation. I already talked about that. Sorry, Phil, it’s earlier in the AMA, if you wanna find it. What would it create to take one? Well, it depends on how good you want your simulation to be. Many people have simulated universes with a few particles in them on their computers already. Are there any clues at the quantum level? Not that I know of. There’s just laws of physics, just like there are at any other levels, there’s nothing special about quantum mechanics that make it look more simulation-like than classical mechanics ever did.
2:24:31.5 SC: Maddox McGray says, “If you had to choose, which do you think contributes most to the dark matter puzzle: WIMPs, axions, or other light particles, MACHOs, MOND, or something else?” So I think that WIMPs are still the most likely candidate, weakly interacting massive particles. There is an ongoing mystery in physics about this energy and mass scale given to us by the electroweak interactions. It’s an energy scale about 100-1000 GEV, 100-1000 times the mass of the proton. That’s the mass of the Higgs boson, close to the W and Z bosons, it’s the area where the top quark mass is. It would be very natural for a new neutral particle to be found at that energy scale, that mass scale, with the weak interactions of particle physics, and if so, it will be a great dark matter candidate. The only reason to be skeptical of that is that we’ve looked, and we haven’t found it yet.
2:25:26.3 SC: Now, it’s hard to look, and it depends on exactly the details of how the thing interacts. These days, a successful model of dark matter would have to have the dark matter candidate WIMP interacting with ordinary matter via the exchange of the Higgs boson, and that gives a lot of freedom for how big that could be. So it’s kind of an open question there. But we’ve looked, and I would say, very, very, very roughly, we’ve ruled out half of the feasible parameter space. So we could have found it already, but we’re nowhere close to ruling it out in any sense. Axions are another great candidate. Other light particles could constitute the supermassive… Sorry, superfluid dark matter we talked about earlier. So those have risen a little bit in the credence you should attach to them, as we’ve not found WIMPs. Axions are also things we can search for, but they’re harder to search for, so we haven’t ruled out as much of the parameter space.
2:26:20.6 SC: MACHOs are basically ruled out. MACHOs are just collections of baryons, but we know there’s good reasons that baryons do not make up the dark matter. And MOND is basically ruled out as a replacement for dark matter. It could be true that dark matter… That gravity is modified in some interesting way, but things like the Bullet Cluster in the cosmic microwave background give very clear indications that there is a gravitational pull other than in the direction of where the ordinary matter is. You need something to be generating that gravitational pull.
2:26:51.1 SC: Gerald Swan says, “In a recent podcast, you expressed some sympathy for Jacques Derrida. What are some of his ideas that you think are genuinely original or that actually benefit from his style of writing?” I’m by no means a Derrida scholar, so, anything that I might think is interesting in what he said, I am not gonna promise you is genuinely original or wasn’t said better by someone else. That’s just not something I’m qualified to talk about. To me, the interesting aspect of what Derrida is on about is his insistence that it’s not nearly as easy as you think it is to say something that is absolutely clear and definitive and univalent and impossible to wriggle out of. This is a linguistic statement, but it goes to ontological statements as well, that this idea, there are categories out there in nature that we can just definitively pin down once and for all, sort of metaphysical absolutes, is something that I find it very healthy to be skeptical about.
2:27:53.2 SC: I think there is a real world. I think the world exists. I think that Derrida would agree with that, would have agreed with that. His brother was a very famous physicist, he was a fan of physics himself, Derrida. But other than the world existing, our human-scale world, the world of people and animals and concepts that we see in our everyday world, that’s where Derrida lived ’cause he was a philosopher and literary critic, literary theorist. So, those kind of categories are all approximate and emergent. And I am very much in favor of being skeptical of their definitiveness. That’s what I take from him. That’s obviously a tiny, tiny sliver of what he said, and it’s oversimplification, but that if you wanna know why I’m interested, it’s that.
2:28:38.6 SC: And also, I have to say that physicists, my friends, my colleagues, my pals, just like almost any other set of academics, can be really anti-intellectual sometimes, in the sense that if they come across a different field of academia that they’re not experts in, and it seems mysterious and they use different jargon that they’re used to, they just dismiss it. And I think that in some sense, I stand up for Derrida just because a lot of other people, who have no idea what he’s talking about, make fun of them without knowing what they’re talking about. So, what I’m actually standing up for secretly is Jacques Derrida in particular as much as it is the idea that we should be extraordinarily charitable and open-minded and humble when it comes to people in one field of academia passing judgment on other fields of academia.
2:29:33.7 SC: Sahd Ahmed says, “As I have started using Twitter more recently, I’ve realized that there is no shortage of doctors, scientists, and academics who question the effectiveness of the lockdown, school closings, and the general narrative around the pandemic. How do you generally approach people or ideas that seem to contradict the mainstream views?” Well, it’s a good question, it’s an important question, because there are no views [chuckle] on which you will find every person in the world, even every expert in the world, in agreement. There are plenty of very famous, accomplished physicists who are crackpots about one idea or another. There are funny people out there who are flat-Earthers and you can find them on the Internet. But they generally are in the minority.
2:30:09.8 SC: And also, there are various ways of telling that someone is a little bit less credible. Look at what sources they refer to, look at how open-minded they are, look at how much they spend time ranting against the establishment instead of actually providing evidence for their claims, et cetera. In the case of lockdowns, school closings, et cetera, there are hard questions. School closings, for example, not at all obvious when we should and should not close the schools, because different populations spread the virus differently. If you do close schools, other things happen to bring people into contact more often, and so that’s a very difficult sociological problem as well as a virology problem. But overall, the idea of getting vaccinated, socially distancing, closing down big, public indoor events, these are all very well-supported, wearing masks, very well-supported by almost all of the respectable part of the establishment.
2:31:13.9 SC: And so, even if you are not comfortable judging individuals on the basis of their rhetoric or their evidence or whatever, you can just look at what most people who are smart are trying to say. And if you’re not an expert yourself, it makes sense to go along with what most of the experts are saying, until you have good reason not to.
2:31:33.2 SC: Ferry says, “In the case of black holes, at what point does our current understanding of physics break down?” Well, we don’t know. We haven’t seen a black hole up close and personal. Nothing about the black holes that we’ve observed, either through electromagnetism or through gravitational waves, is in conflict with any of our current understanding of physics. We predict that, of course, our current understanding of physics will break down near the singularity, when curvature of space-time becomes really, really large. That’s a pretty safe prediction. It is also a prediction that if you want to get information out of the black hole, then our current understanding of physics has to break down earlier than that. This is why the black hole information loss puzzle is so puzzling, because it’s not enough to hide all of the problem at the singularity. You need something that is outside our current understanding of physics to be going on at the event horizon, which is not close to the singularity.
2:32:31.2 SC: But putting our finger precisely on what that is is hard to do. Listen to the podcast I did with Anneta Engelhardt, we talked about the black hole information loss puzzle, and their modern ideas have virtual Euclidean wormholes connecting the matter that is outside the black hole to the quantum state inside the black hole also. That is, in some sense, a violation of our current understanding of physics, but on the other hand, it fits in, broadly speaking, with the way that we think about physics, so it’s not completely weird to us.
2:33:04.3 SC: Ross Hastings says, “Your recent chat with Avi Loeb got me thinking about aliens and the Fermi paradox. What if dark matter was an alien thing? Could dark matter be aliens?” So roughly speaking, probably not, and that is because we know a little bit about dark matter. Dark matter doesn’t interact with itself or with ordinary matter as strongly as ordinary matter interacts with itself. The reason why you and I can exist is because we’re made of particles, protons, and electrons, and neutrons, that interact with each other strongly, number one. And number two, they can lose energy by giving off photons, so when two particles bump into each other, they can cool down and stick to each other by losing energy by giving all photons. Dark manner can’t do that, as far as we know. Dark matter has a distribution in space which is almost exactly what you would expect if dark matter didn’t interact with itself at all.
2:34:02.3 SC: Now, I say almost exactly, ’cause who knows, and there are tiny, little anomalies that we have trouble pinning down. I actually wrote a paper that proposed that dark matter could interact with a dark photon. That you have dark matter particles and you have a copy of the electromagnetic force that only interacts with dark matter. And the most naive version of that is probably ruled out because you get dark magnetic fields that would soak up a lot of the energy, and it doesn’t quite fit the data, but then what we proposed at the end of our paper, and other people have since gone on to develop this idea in greater detail, you could have dark atoms. You could have two different species of dark matter particles that interacted with dark electromagnetism and made bound states. And they would still have to interact more weakly than ordinary matter, but maybe if something like that happened, you could get them to stick closely enough to make dark molecules, chemistry, aliens, whatever. They don’t stick nearly as well as ordinary matter does, so it’s hard to do, but I don’t think it’s completely ruled out.
2:35:09.8 SC: Bill Seltzer says, “Might an extension of quantum theory which enabled the derivation of space and time also show that the Schrödinger equation is not fundamental?” Maybe. Completely possible. Like I said earlier, at least I alluded to it, there are formulations of quantum theory where the Schrödinger equation is not the only part of quantum evolution. Objective collapse models say that the Schrödinger equation is usually true, but it’s not always true. So that’s one possibility. There are ideas, Steven Weinberg proposed a theory where the Schrödinger equation is never exactly true. There are non-linear corrections to it. But what you instantly discover is that a lot goes wrong as soon as you do that. You can get superluminal communication, or you get communication between different branches of the wave function, there’s all sorts of problems that immediately crop up. So it’s hard to modify the Schrödinger equation, but you could imagine doing it. I would just say since there’s zero evidence we have to, it is not a high priority thing for me to start thinking about.
2:36:12.2 SC: Kirk Briggs says, “I’ve read through your reading list on the PDF download from The Great Courses Plus,” good plug for the advertiser there, Kirk, good for doing that, “and all of the books were great, except John Bell’s book was over my head.” Well, sorry about that. “I’d like to learn more about eternalism and how it’s mapped to space-time.” Right, I’m not sure if there’s a great book to learn about eternalism. I wrote a book about time called From Eternity to Here where I talk a little bit about eternalism. Maybe that’s something you want. But a book specifically about eternalism, I’m not really aware of. Most books about the philosophy of time are either about relativity in one way or the other, or they’re about entropy and the arrow of time. Once you make the statement of eternalism, there’s not that much to do with it. All moments of time are equally real good. [chuckle] What are you gonna keep saying about that? I’m not quite sure what there is to say. But maybe there is a book out there talking about it, I just don’t know about it.
2:37:11.4 SC: David Lange says, “I know the universe is probably not anti-de Sitter, but what do you think are the chances of string theory in a holographic cosmos leading to a correct formalism?” Oh, I still think that string theory is the leading candidate for a good theory of quantum gravity out there, or at least maybe some successor to string theory. I am convinced… Not convinced, but I am moved by the fact that string theory is an enormously fruitful theory. Most attempts to go beyond known physics, you pose something new, and then you say, “Okay, well, what does that imply? ,” and you chug forward with your equations and you try to make it imply things, and you get stuck, because it doesn’t really tell you that much. Whereas string theory just leads you to all these cool places like the AdS/CFT correspondence. So I’m pro-string theory, tentatively, in that sense. It’s too bad that we haven’t had a lot of great progress in recent years connecting string theory to empirical data, and I do think that that is a problem.
2:38:15.5 SC: So I think… But AdS/CFT, I think of as having a problematic status. On the one hand, it’s a wonderful playground to learn about quantum gravity, all the new stuff that I talked about with Anneta Engelhardt was all inspired by AdS/CFT. You can ask questions about quantum gravity and get relatively well-defined answers, but it’s not the real world. And the fact that it’s not the real world may or may not just be a technical complication. It may be that the right answer for the real world is inspired by AdS/CFT, that it’s not that far away from it, or it might be that to get the real world, you need to think in dramatically different ways than what we do in AdS/CFT. So, I would remain unconvinced by either arguments on either side of that, I think it’s an open question.
2:39:03.8 SC: Ray Walden says, “The recently published paper testing the strong equivalence principle detection of the external field effect in rotationally spiral galaxies claims to have detected the external field effect in over 150 galaxies. If this result can be verified and be reproduced independently, will dark matter no longer be required to explain the observed rotation curve of spiral galaxies, and is there any other reason to continue looking for it?” So I don’t know anything about that paper, so I can’t comment on it, but what I can tell you is this: If anyone claims that they can get rid of dark matter with a new kind of theory and you ask them, “Well, what is the evidence that your theory works?”, and the only thing they talk about are the rotation curves of spiral galaxies, that person can be instantly dismissed. You do not need to take them seriously. The rotation curves of spiral galaxies were the best evidence for dark matter in the 1980s. That was a long time ago. Now, those are not the best evidences for dark matter. The cosmic microwave background, large-scale structures, baryonic acoustic oscillations, clusters of galaxies, gravitational lensing, we have a whole suite of evidence for dark matter that goes way beyond the spiral galaxies.
2:40:15.9 SC: So, the very short response you should give to anyone who says anything like that is, “Where is your prediction for the perturbations in the microwave background? Where is your power spectrum?” If they can’t do that, don’t take them seriously.
2:40:27.3 SC: Riverside says, “Let me once again rephrase a question on the moral implications of the many worlds-theory of quantum mechanics,” and then there’s a long question, so, Riverside, I’m sorry about this. I am not gonna let you once again rephrase that question. Part of the rules of the AMA is we do not have ongoing debates, so if my original answer to the question wasn’t very good, I apologize for that, but I don’t want the AMAs to be back-and-forths that continue from month to month. Sorry about that.
2:40:54.3 SC: Joram Ramberg says, “It is my understanding that the purpose of empirical science is to find out the truth about nature and how it works. This would suggest that progress and science involves the discovery of facts about nature. Yet, when talking about scientific theories, you’re always using the language of invention. Why are you using the term ‘invention’ and not ‘discovery’?” So yeah, I started to answer this with a previous answer, but let me restate it here. It’s the difference between a fact about the universe and a theory about the universe. Even if the theory is true, your theory is correct, like general relativity is correct in a certain regime of applicability, we still invent the theory. We don’t invent the facts that the theory is judged by; we take observations, we find out hopefully true facts about the universe, but then putting those facts into a structure that explains and accounts for the facts is a human invention. Like I said, you do not bump into theories lying on the beach or hiding in a desk drawer; you invent them in your brain. And honestly, that’s what I think about it. I do not think about any of this too strongly, I don’t think it matters whether I call it an invention or a discovery, honestly.
2:42:03.8 SC: Douglas Albrecht says, “If I understand, the many-worlds hypothesis flows from an ontological starting point that reality is the wave function, and that’s all there is, what’s the evidence for this being a complete description of reality, and why are you so certain of this assumption?” So I’m not certain of this assumption, I’m certainly not certain about any theory of physics whatsoever, certainly not many-worlds. I think it’s the most likely thing, but certainty is not the right way to think about it. The evidence is that every theory of quantum mechanics needs wave functions, and I don’t need anything else to fit the data, so that’s all the evidence that I need. I’m gonna look for the simplest theory that fits the data, and many-worlds is it. I could be wrong about that, and if there is more evidence that comes in that is not accounted for by the wave function being real, I will certainly consider it and change my mind.
2:42:53.9 SC: Jonathan Tucker says, “Can you invite David Albert back on? Pretty sure I’d be enthralled just listening to you two discussing toasters.” I have a bunch of people who we’ve had on the podcast already who certainly we’ve not exhausted all the interesting things there is to talk about. However, so far, there’s also a whole bunch of people that I haven’t talked to that have interesting things to say. And I’m open to changing my mind about this, but my tentative belief is that I would rather not have repeat guests on Mindscape. Like I said, I agree, there’s certainly many things interesting to talk about, but talking to new people is more interesting to me than talking to people I’ve already talked to. I’m probably gonna regret saying that, because I’ll probably have people later on, but that is my current feeling right now.
2:43:45.7 SC: Boris Petrovich says, “Regarding multiverse in many-worlds hypothesis, which simply can’t be in any way experimentally verified, are they science or are they religions?” Nope, they’re science. I recommend that you read my paper, Beyond Falsifiability, to find out why.
2:44:03.3 SC: Mark Cark says, “I’d like to hear musings as if you were talking to a small child. How can the universe be either finite or infinite? Neither makes any sense.” So, number one, I’m not very good at talking to small children about science. My specialty is talking to people who are a little bit beyond the small-child stage and bringing them into an even higher stage, but I can try. But this kind of question, I falter at, because both of them make perfect sense to me, that the universe could be finite or infinite. The phrase, the statement, “neither makes sense” just kind of stops me in my track. I don’t know what to do with that. Why don’t they make sense? Especially when you… I would have a little bit more sympathy or understanding if you said, “How could the universe be finite?”, or you said, “How could the universe be infinite? It doesn’t make sense,” but if you say it doesn’t make sense that it’s finite or infinite, then I don’t know what to do, ’cause those are the only two options. Finite means effectively that if you walk off in some direction and you just keep walking, eventually, like it or not, you will come back to where you started. Infinite means you start walking in some direction, and then even if you could walk forever, you would never come back to where you started. That’s the way I would try to explain it.
2:45:21.7 SC: Dominique da Kahlua says, “Why does a photon reflect from a mirror under the same angle if it’s re-admitted by an angle… By an atom,” sorry, “Why in this specific direction?” This is a good question. It’s a fun question. I’ve actually never thought about this, and it’s a sort of a condensed matter-y real-world question that we rarely get here. I don’t encourage more questions along these lines, because I would not be able to answer most of them, but in this one, I think I know the answer. It’s because you shouldn’t think of the reflection of a photon off a mirror as a single photon particle being absorbed by a single atom and then being reflected. There are collective effects in the mirrors. That’s why some… Well, let’s put it this way: I can take the atoms that make up a mirror and re-arrange them into a not-mirror, right? I can make it not polished, et cetera, and then it would not reflect under the same angle.
2:46:16.7 SC: So it’s clearly not a feature of the individual atoms; it’s a feature of the structure into which those atoms are put. And what happens is, and it’s just a cool fact about quantum mechanics, is that when you have a metal, like a really shiny metal, for example, I’ll just take that as an example, the electrons in a metal move relatively freely from atom to atom. They’re not tied very strongly to individual atoms. This is why metals are good conductors of electricity. And what happens, very roughly, to the best of my very meager understanding, when a photon or an electromagnetic wave is incident on a reflective surface of a metal, it’s the collective response of a bunch of electrons sort of moving in one direction and then bouncing back. And it’s almost like a trampoline where the electrons catch the photon and then push it back in the opposite direction… Not the opposite direction, but you know what I mean, its transverse momentum is conserved and its incident momentum is reflected backwards. That’s the best I can do for an answer to that. For more than that, you’d have to go to Wikipedia or something, I guess.
2:47:23.6 SC: Steve M says, “I seem to be at an impasse. I know my finger will not pass through this table because fermions don’t do well moving other fermions, but wait, those are only waves, perturbations and fields, lots of fields intersecting at my fingertip. Why do I perceive the table’s existence when all of those fermions are in superpositions of being and not being perturbations at this moment?” So I was with you, Steve, up until that last sentence. A real fermion particle, like a real electron, that is part of the table or whatever, is not in a superposition of being and not being a perturbation. It is a perturbation, it is a superposition of different locations in space, but that just means it has a sort of spread-out wave function. But it exists, there is some non-zero feature. And one way of thinking about this is that fermion number is conserved. So you can change fermions and into bosons, but only by interacting them with anti-fermions. So if you have an electron and a positron, they can annihilate away into photons, so those two fermions can turn into bosons. But if you just have one electron, it cannot turn into a collection of bosons, its fermion-ness is real and it’s there and it’s not going away. And all the particles making up your table are fermions, not anti-fermion. So they’re all real. Your table has a real conserved fermion number. There’s no trouble in understanding why it would resist your fingertip when you press into it.
2:48:56.3 SC: Alan Poxmire says, “Entanglement is one of the few topics that Google and YouTube haven’t given me the understanding I want. Even your Greatest Ideas videos leave me with a lot of questions. Would you consider doing another video on that?” So two answers to that. One is, I wrote a book [chuckle] about quantum mechanics and entanglement is explained, so, YouTube and Google are not the only places where you could go for better information. Sometimes you just need a deeper understanding and neither Google or YouTube are the place to go. Books still have a place in one’s intellectual life, I would say. The other answer is, probably not gonna do more videos on anything, I will do more videos in my life maybe, but I don’t have any plans to either extend the Biggest Ideas series or to start up a new series on something else. If I’m gonna do anything with that, I would rather turn the Biggest Ideas video into some kind of book themselves where I could fit in more details and do things even better. Thinking about the Biggest Ideas videos is… I said it, and it was true. They were informal and off the cuff. I would look up numbers and things like that, but I did very little research to do any of those videos, and as a result, there’re tiny, little mistakes that sneaked in, and I try to correct in the comments and so forth, but I hope maybe I could do those in a book form and get everything right the first time.
2:50:14.1 SC: Alright, Ramone Dorman says, “If you can imagine for a moment that you were attached to a weather balloon and that you’re leaving Earth’s atmosphere and you’re further into space beyond our system and the Milky Way,” so this is a very good weather balloon. “As you continue to drift in the same direction, you pass the galaxies furthest away from Earth until you reach the very edge of the universe. What do you see?” So even if, Ramone, I let you have the thought experiment of the weather balloon, you would still never reach the edge of the universe, ’cause there’s no such thing that’s the edge of the universe. At least there is no grown-up, respectable theory of cosmology according to which the universe has an edge. So there’s nothing to see when you were there, ’cause it doesn’t exist. Sorry about that.
2:50:57.1 SC: Alright, it was a long AMA, but we’ve reached the last question. Michael Lesniak gets the honors: “Why do the positively charged quarks have double the electric charge as the negative ones? Is this an indication we don’t have the entire picture, and the positive quarks are actually composite particles composed of two each, perhaps?” It’s absolutely possible that we don’t have the entire picture. There are reasons for the charges of the quarks to be related to each other, dealing with anomaly cancellations and other tricky things in quantum field theory. And like we said before about the charge of the electron, if you unify the forces of nature, then there are very good reasons, there are predictions about the charges of the quarks that fit into this pattern. The idea that the positive quarks, the up quark, for example, plus two-thirds, could be a composite particle, is one that people have thought about. There are theories called preon theories, P R E O N, which you could look up on the Internet for that, you’re welcome to Google it.
2:52:00.6 SC: But they’re not very promising these days, and the reason is they make predictions, and the prediction’s mostly ruled out. There’s no evidence that says that positively charged quarks are composite, and there are predictions for tiny, little effects you should be able to see if they were, and we don’t see any of them. So, probably, that’s just what it is. The charge assignments of quarks and electrons and all these things, they’re complicated, they’re nuanced, they come from a long series of symmetry-breakings and the Higgs boson and things like that. The desire to sort of break them down to saying, “Well, this particle has twice the magnitude of charge of the other one, therefore, it’s made of two,” is almost never gonna work. Things are more complicated than that. That doesn’t mean you shouldn’t look for deeper explanations, but the only way to really have a chance of getting a deeper explanation that is on the right track is to learn a whole bunch of quantum field theory and particle physics and group theory and then hope that you can come up with a theory that turns out to be correct, that’s what we’re all hoping. Isn’t that what we’re all hoping, to come up with a theory that turns out to be correct? That would be a good thing to do in our lives.
2:53:10.9 SC: And with that, me and my voice have reached the end of the AMA and we’re out of the questions that I have in this file, so, thanks for playing along. See you next month. Bye-bye.[/accordion-item][/accordion]
Choose 50-100 questions with you choosing the most interesting questions and the number of questions each month.
Hello,
Love your podcast. Question: does the fact that space is expanding apply on all scales? Just as the universe expands, does matter–including our local physical reality, out bodies–also expand?