For this installment of The Biggest Ideas in the Universe, we turn to one of my favorite topics, Time. (It’s a natural followup to Space, which we looked at last week.) There is so much to say about time that we have to judiciously choose just a few aspects: are the past and future as real as the present, how do we measure time, and why does it have an arrow? But I suspect we’ll be diving more deeply into the mysteries of time as the series progresses.
The Biggest Ideas in the Universe | 5. Time
And here is the associated Q&A video, where I sneak in a discussion of Newcomb’s Paradox:
The Biggest Ideas in the Universe | Q&A 5 - Time
Space, time and Quantum mechanics. Discrete or not? I have read that QM is discrete and that to help unify QM and relativity it helps for space to be discrete (resolves certain infinities). So some projects try to discretize space of which Loop Quantum gravity is one. I would like to ask for some clarifications.
Is QM really discrete? Sure certain properties such as energy and spin are discrete but it seems things like position is not discrete so in QM which properties are thought of as waves and if thought of as waves are not discrete?
Second i understand that measurements of Lorentz invariance are able to probe the discreteness of space down below the plank length and this fact should lead is to discount theories that try to discrete space.
Which theories of space are discrete?
Loop QM -> discrete?
space emerging from geometry you talk of here and your recent video -> not discrete?
https://www.preposterousuniverse.com/blog/2016/07/18/space-emerging-from-quantum-mechanics/
cellular automata ie Wolfram -> discrete?
Dear Sean,
Thank you so much for stating the time topic. I’m excited about learning more about time in next videos.
I like the eternalism theory. I was wondering if someone could travel into the past (through a wormhole, for example), could this person change the path of future by different means, such as preventing the second world war from happening, and so forth?
Kind regards,
Ali
@ Iveno Thaim: If you liked this talk as much as I do, you will looove From Eternity to Here, which yes, goes deeper on these themes.
Hi Sean, I had never heard about Sidereal days- and you briefly mentioned it on the episode- any chance in explaining it in more detail- especially the piece around “fixed stars”?
Thanks for triggering the desire to learn more about our (not the) universe.
I’ve been thinking about a civilization living in three dimensions of space. They are free to move in two dimensions but are restricted to move in one direction (at a constant velocity) for the third dimension. I imagine the floor of an elevator moving up an infinite flight of stairs (with an infinite set below).
Would the civilization perceive the fixed third dimension in a temporal fashion? “Let’s meet at coordinates x and y when we are at level 78” or “It’s been thirteen levels since we last met”.
I feel like my assumption is wrong but I can’t seem to pinpoint what is the flaw. Am I invoking the time dimension within this perception? Almost how we use pulsars or atom clocks to derive our collective perspective of time?
My comment is more on the philosophical side, but here it goes.
If the universe has no beginning and no end, it’s just always been there. And the universe is infinite, you can’t just bump into non-existence or non-universe.
Then finding what’s the fundamental particle, or wave, or energy, or law, or equation, or number, or constant, etc… doesn’t matter. Because there will always be something more quantum than quantum, and something more macro than macro.
I think reality and the universe are the same, and they are infinite in all ways, sizes, and directions. Time is definitely real and part of the universe, however I think it’s not fundamental. Because only finite things have a foundation, but something infinite like the universe or time, does not… it just is.
Thank you for these videos, Sean!
isn’t the fact that you can’t time travel to your past a solid indication that the your future doesn’t exist(in the sense that it’s already “there”)?
I like this old saying about “The Flow of Time”:
Time came out of the Future which did not exist yet into the Present which had no duration and went into the Past which had ceased to exist.
I found the following well-founded critique of the the “Eternalism” Block Universe on Physics Forums by Peter Donis:
https://www.physicsforums.com/insights/block-universe-refuting-common-argument/
The conclusion is that part of the future of the spacetime in the Block Universe is not complete: If the past is fixed, the future is not necessary real.
A way out of this inconsistency is to adopt the “Possibilism” approach of George F R Ellis, his Evolving Block Universe (EBU) which you referred to as “The Growing Block Universe” in your video. Adopting this view also allows for free will.
It’s not directly about Time but it seems to me that the future doesn’t have to be precisely determined in the manner of Lagrange’s Demon. There’s a space of possibilities which largely overlap in character, which one actually manifests is not predictable, what is predictable, up to a point, is that it will have certain limits on it.
It’s not that Lagrange’s Demon can deduce your exact future, more that it can make probabilistic statements about it.
Free Will/Determinism seems like a false dichotomy.
Here’s a thought. Time is an Illusion (like everything else).
Humans, and more to the point, perhaps, human brains, are made up of particles, which, if I understand it correctly, are, for now at least, thought of as fluctuations in some field, or some set of fields.
Incidentally shouldn’t this perspective strip us of any notion of, for want of a better word, privilege.
We’re not external observers, trying to grasp the nature of objective reality which we only get to see as either shadows on a cave wall or blurred silhouettes through nearly opaque glass. We’re not on the outside looking in. We’re part of the system.
That system changes. It’s very tempting to say that system changes over time, but isn’t it possible to have change without time. What about, say, a sine wave. A sine wave has a cyclical nature, very similar to what you could think of as humans first dimly conscious encounter with time, the seasons.
What seems to mark out humans is some elevated form of consciousness, we are able to internalise and observe models of the world, but I’d say that’s not unique to us, what is perhaps unique is that process has iterated (and opened up the door to recursion) we are able to internalise and observe our observations: that’s consciousness.
What’s all this got to do with time. Well change is clearly a feature of the system we are part of. We are aware of change and we call it time.
One, unfortunately topical, primitive model of this is would be a version of Conway’s Game of Life, in which an individual structure has progressed to the state where it is aware that it is in some sort of Game, and that the Game state is changing and they call that process of change time.
So there you have it. We’re fluctuations in a changing field. Aware of our own awareness. We experience the change and we call it time. But it’s the same sort of change as the cycles of a sine curve*.
Time, consciousness, existence all solved in one internet comment.
I’m not sure about the Meaning of Life. I’ll have to get back to you on that one.
*Nod to Max Tegmark here, I don’t really know what his Theory about us all being mathematical objects is exactly but it’s in the same area isn’t it.
I’m just a simple guy that likes to read popular science books and magazines, but I’m very unsatisfied with this Type-B theory of time. I don’t think it’s enough to just say “come on, of course time is real.”
I don’t question that time is real in the sense that there is change, but on what basis do we conclude that the future and past exist? I’m lost on this one because unlike other difficult to accept propositions (many universes ), this one doesn’t seem to solve any problems, but instead just creates them ( e.g. time travel paradoxes ).
What is wrong with just saying that there is change and that change happens at different rates for different frames of reference?
Can I use the size of the universe as a clock?
Re: Q&A
It would seem difficult to slice the universe into distinct “ideas”
when all ideas in the universe get so interconnected.
What if particles aren’t fundamental but emergent from fields, which seem more fluidlike.
You drew moment slices as if they were planes, but shouldn’t they really be more like
spacelike regions exterior to the light cone?
Since you mentioned paradoxes associated with your colleague Kip Thorne, et al’s wormholes
you might also mention some of his resolutions of those paradoxes.
For a classical self-consistency principle (whatever happened happened) what’s to stop me:
https://authors.library.caltech.edu/6469/
For a quantum chronology protection (not possible) what’s to stop me:
https://en.wikipedia.org/wiki/Chronology_protection_conjecture#cite_note-8
After opening so many cans of worms, Newcomb’s paradox may be the one of the biggest.
I happen to be a one boxer, which may be part of why I resonate so well with your ideas,
but Everett seems like a consensus compared to this controversy,
even after nitpicking vagueness in the setup.
Prof. Carroll,
there has recently been a lot in the popular press about the writings of Prof Gisin, and that fact that intuitionist mathematics has a role to play in understanding the passage of time. any comments?
Good stuff… very much enjoyed the presentation on time. I do think there’s a strong reason to go for a growing block universe instead of just “going all the way” to the block universe… particularly quantum uncertainty… but that’s just my opinion. Thanks for a great description/explanation.
This may be the result of my lack of education in philosophy, but my answer to Newcomb’s paradox is: the whole question is quite silly.
If a predictor like that exists, and what it tells me is true, then to me this seems to say that the reality where all that happens is not the reality I live in right now. How can I possibly claim to make a decision if you change the rules of reality? Could I exist, with my currently held beliefs that influence my decisions, in that reality? If not, is it me at all?
It’s not even about time travel, not really. A time-traveling predictor in a single, coherent universe cannot say a prediction like that and have it be the absolute truth. You could argue that it’s an Everettian universe, where my choice (which doesn’t even seem to be a quantum choice in this paradox) splits the universe in different branches depending on which boxes I choose. Hmm, okay. Let’s go with that.
For all I know, based on the facts you’ve told me, any amount of money could appear or disappear on a whim (like maybe someone else’s choice makes my money, be it 0 or $1,001,000 or $1,000, disappear after 1 minute. So it doesn’t matter much what I choose since I can’t know if I’ll have that money once I reach a place where I can spend it. The power of choice is bending the rules of reality beyond recognition.
If you simply ask, what seems to be the best choice for someone in that situation, in that universe? Well, if you trust the predictor, of course you choose the opaque box with one million dollars. But now I don’t see the paradox or any meaningful thing to talk about any longer. It’s just a setup where one choice is the objectively best one.
If you ask me, the actual person in this reality, to make a choice based on those claims (but don’t require me to believe that they are absolutely true and the predictor is always correct), then… I think I’d just take the transparent box where I can see the money, and walk away. Who knows what crazy things you’ve set up to mess with my mind.
What do you think of Theo Quinn’s “Breakout Earth”? Could all change of motion come from internal information usage alone? It provides a neat singular framework for relativistic and quantum effects without needing any of the postulates. Very speculative, but very cool too.