Sean Carroll

Author: Sean Carroll

  • Python

    I’ve decided I need to become a programmer again. As an undergrad, and to a lesser extent as a grad student, I wrote code all the time. But since I started doing research, it’s been pencil-and-paper almost all the way through, with occasional dips into Mathematica or plotting programs.

    That must end, so I’ve decided to learn Python. I just need something simple for number-crunching and graphics, and everyone in the know seems to have nice things to say about the language. (Secretly I would like to play around with genetic algorithms and cellular automata, but I’m not going to admit that.) I tried to get Fortran, my previous language of choice, up and running on my Mac … it didn’t go well.

    So… any tips? Pointers to well-written resources and tutorials (online or in print) would be especially helpful. Keep in mind that the target audience is an aging theoretical physicist who hasn’t programmed in 20 years, and for that matter has been pretty much command-prompt free (working on my Mac) for the last five.

    The things I admit in public on this blog, sheesh.

  • arXiv Find: New Sources of Gravitational Waves During Inflation

    Here’s an interesting paper from yesterday’s hep-th batch of abstracts. One of the exciting prospects in observational cosmology over the next several years is finding signals of gravitational waves in the cosmic microwave background. These can be produced by inflation, and indeed simple models (one scalar field, no funny tricks) predict a “consistency relation” between characteristics of ordinary density perturbations and the gravitational waves. If signatures of the waves were detected (typically, by finding “tensor modes” in CMB polarization) and shown to be consistent with the simple prediction, it would be a huge boost for inflation.

    The world isn’t always so simple, of course. It’s not too hard to think of models that violate the consistency relation. Now Senatore, Silverstein, and Zaldarriaga are pointing to a new mechanism: gravitational waves produced directly by particle or string sources during inflation. (Silverstein is of course well-known as a string theorist, but her recent work in early-universe cosmology has been as good as anybody’s in the field.) The usual mechanism is simply quantum fluctuations of fields in their vacuum state; these folks are imagining that particular objects — particles or strings — are produced during inflation, which then act as sources for gravitational waves. In a simple model you would expect that this effect might be important at a fixed scale corresponding to the energy of the sources, but they argue that it’s not hard to come up with models where gravitational waves are produced at all scales.

    New Sources of Gravitational Waves during Inflation

    Leonardo Senatore, Eva Silverstein, Matias Zaldarriaga

    We point out that detectable inflationary tensor modes can be generated by particle or string sources produced during inflation, consistently with the requirements for inflation and constraints from scalar fluctuations. We show via examples that this effect can dominate over the contribution from quantum fluctuations of the metric, occurring even when the inflationary potential energy is too low to produce a comparable signal. Thus a detection of tensor modes from inflation does not automatically constitute a determination of the inflationary Hubble scale.

    The bad news here is in the last line — in simple models (there’s that word again) the amplitude of gravitational waves is simply proportional to the expansion rate of the universe during inflation. Now that’s no longer so obvious. But research isn’t about finding good news or bad news, it’s about finding the right answer. Being unreasonably confident in the predictions we make from our models is just as dangerous as not having compelling models at all.

  • More Hints of Dark Matter

    Also known as, “scientific progress goes boink.”

    One of the benefits of being a Master of Time and Space is that I get to see the future. For example, cosmologically-inclined folks have been wondering for a few weeks about this press release from the TAUP conference in Munich, which includes the lines “Latest results from the CRESST Experiment provide an indication of dark matter. The press conference will be held on 6. September 2011 starting at 2:00 pm.” Suspense! But I know what they’re going to say.

    As Neal Weiner points out, we don’t have to wait for the press conference; the friendly folks at the CRESST experiment have ambitiously decided to write a paper as well as giving a press conference, and that paper appeared on the arxiv this evening. (You remember Neal as a distinguished guest blogger; re-read that post to get your bearings in this complicated game.) Very short version: they claim to see some signal that is statistically significantly greater than background, consistent with a WIMP dark-matter particle with a mass of around 20-40 GeV. Slightly longer version:

    Results from 730 kg days of the CRESST-II Dark Matter Search

    (more…)

  • How Probability Works

    From Barry Greenstein’s insightful poker book, Ace on the River:

    Someone shows you a coin with a head and a tail on it. You watch him flip it ten times and all ten times it comes up heads. What is the probability that it will come up heads on the eleventh flip?

    A novice gambler would tell you, “Tails is more likely than heads, since things have to even out and tails is due to come up.”

    A math student would tell you, “We can’t predict the future from the past. The odds are still even.”

    A professional gambler would say, “There must be something wrong with the coin or the way it is being flipped. I wouldn’t bet with the guy flipping it, but I’d bet someone else that heads will come up again.”

    Yes I know the math student would really say “individual trials are uncorrelated,” not “we can’t predict the future from the past.” The lesson still holds.

    Happy Labor Day, everyone.

  • Ten Things Everyone Should Know About Time

    “Time” is the most used noun in the English language, yet it remains a mystery. We’ve just completed an amazingly intense and rewarding multidisciplinary conference on the nature of time, and my brain is swimming with ideas and new questions. Rather than trying a summary (the talks will be online soon), here’s my stab at a top ten list partly inspired by our discussions: the things everyone should know about time. [Update: all of these are things I think are true, after quite a bit of deliberation. Not everyone agrees, although of course they should.]

    1. Time exists. Might as well get this common question out of the way. Of course time exists — otherwise how would we set our alarm clocks? Time organizes the universe into an ordered series of moments, and thank goodness; what a mess it would be if reality were complete different from moment to moment. The real question is whether or not time is fundamental, or perhaps emergent. We used to think that “temperature” was a basic category of nature, but now we know it emerges from the motion of atoms. When it comes to whether time is fundamental, the answer is: nobody knows. My bet is “yes,” but we’ll need to understand quantum gravity much better before we can say for sure.

    2. The past and future are equally real. This isn’t completely accepted, but it should be. Intuitively we think that the “now” is real, while the past is fixed and in the books, and the future hasn’t yet occurred. But physics teaches us something remarkable: every event in the past and future is implicit in the current moment. This is hard to see in our everyday lives, since we’re nowhere close to knowing everything about the universe at any moment, nor will we ever be — but the equations don’t lie. As Einstein put it, “It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.”

    3. Everyone experiences time differently. This is true at the level of both physics and biology. Within physics, we used to have Sir Isaac Newton’s view of time, which was universal and shared by everyone. But then Einstein came along and explained that how much time elapses for a person depends on how they travel through space (especially near the speed of light) as well as the gravitational field (especially if its near a black hole). From a biological or psychological perspective, the time measured by atomic clocks isn’t as important as the time measured by our internal rhythms and the accumulation of memories. That happens differently depending on who we are and what we are experiencing; there’s a real sense in which time moves more quickly when we’re older.

    4. You live in the past. About 80 milliseconds in the past, to be precise. Use one hand to touch your nose, and the other to touch one of your feet, at exactly the same time. You will experience them as simultaneous acts. But that’s mysterious — clearly it takes more time for the signal to travel up your nerves from your feet to your brain than from your nose. The reconciliation is simple: our conscious experience takes time to assemble, and your brain waits for all the relevant input before it experiences the “now.” Experiments have shown that the lag between things happening and us experiencing them is about 80 milliseconds. (Via conference participant David Eagleman.)

    5. Your memory isn’t as good as you think. (more…)

  • Time Is Out of Joint

    Greetings from Norway, where we’re about to embark on what is surely the most logistically elaborate conference I’ve ever attended. Setting Time Aright starts here in Norway, where we hop on a boat and cross the North Sea to Copenhagen. The get-together is sponsored by the Foundational Questions Institute, although it came together in an unusual way; I was part of a group that was organizing a conference, and we applied to FQXi for funding, at which point they mentioned they were planning almost exactly the same conference at the same time. So we joined forces, and here we are. Unity ’11!

    The topic, if you haven’t guessed, is time. That’s a big subject, one that can hardly be done justice by sprawling books with hundreds of (admittedly quite charming) footnotes. You can see why the conference has to spread over two countries. We’re trying an experiment in interdisciplinarity: while the conference is a serious event meant for researchers, we have a wide variety of specialties represented, including biologists, computer scientists, philosophers, and neuroscientists, as well as the inevitable physicists and cosmologists. (There is also a public event, for those of you who find yourselves in Copenhagen next week.) I can’t wait to hear some of these talks, it should be a blast.

    My job is to open the conference with an introductory talk that hits on some of the big questions. Here are the slides, at least as they are right now; last-minute editing is always a possibility. I think I put enough in there to provoke almost everyone at the conference one way or another.

  • Andy Lawrence on Books about Astronomy and People

    Andy Lawrence, Edinburgh astronomer by day and e-Astronomer by night, has participated in a Five Books interview at The Browser. You’ll remember that I did one where I picked five books about relativity and cosmology. Most of the other interviewees (and they have a great list) have been a bit more playful, mixing in different genres. Andy takes a judicious middle tack, including some straight-up astronomy but also some biography.

    I was glad he picked Dennis Overbye’s Lonely Hearts of the Cosmos, which is one of my favorite books about science and scientists. It manages to show the human side of science in all its quirky glory, without either creating fake scandals or putting anyone on a pedestal.

  • Astro Coffee Briefs from Ohio State

    Kris Stanek alerted me to this fun idea from the astronomers at Ohio State: when they submit a paper to arxiv, they accompany it by a simple YouTube video that explains the basic idea. Called “Coffee Briefs,” there is only one such video so far, for a paper by Jennifer van Saders and Mark Pinsonneault. But they hope to make it regular series.

    The Sensitivity of Convection Zone Depth to Stellar Abundances: An Absolute Stellar Abundance Scale from Asteroseismology
    Jennifer L. van Saders, Marc H. Pinsonneault

    The base of the convection zone is a source of acoustic glitches in the asteroseismic frequency spectra of solar-like oscillators, allowing one to precisely measure the acoustic depth to the feature. We examine the sensitivity of the depth of the convection zone to mass, stellar abundances, and input physics, and in particular, the use of a measurement of the acoustic depth to the CZ as an atmosphere-independent, absolute measure of stellar metallicities. We find that for low mass stars on the main sequence with $0.4 M_{odot} le M le 1.6 M_{odot}$, the acoustic depth to the base of the convection zone, normalized by the acoustic depth to the center of the star, $tau_{cz,n}$, is both a strong function of mass, and varies at the 0.5-1% per 0.1 dex level in [Z/X], and is therefore also a sensitive probe of the composition. We estimate the theoretical uncertainties in the stellar models, and show that combined with reasonable observational uncertainties, we can expect measure the the metallicity to within 0.15 – 0.3 dex for solar-like stars. We discuss the applications of this work to rotational mixing, particularly in the context of the observed mid F star Li dip, and to distguishing between different mixtures of heavy elements.

    van Saders & Pinsonneault (2011): OSU Astronomy Coffee Brief

    This example might not be immediately accessible to non-experts, but I think the idea is to pitch the video at the level of astronomy grad students. Certainly the participants deserve a lot of credit for trying out an innovative way to talk about their research.

    The key to the ambition of making this a regular even is keeping it simple and easy. If it takes a couple of hours to put it together, no problem; if it takes a couple of days, enthusiasm will flag. I’m not sure what software was used to make the video and the simple graphics — iMovie, maybe? For the DNA computer video we showed some time back, it was quite an elaborate job, and you would worry that it would be onerous to do something like that for every paper one writes.

  • Oedipus and the Riddle

    Thanks to Richard O’Connell for suggesting this Jorge Luis Borges poem as appropriate to the time-travel theme.

    Oedipus and the Riddle

    Quadruped in the dawn, erect at noon,
    and wandering on three legs across the blind
    spaces of afternoon; so the eternal
    Sphinx saw her inconstant brother, Man.
    And to her rocky silence came a man
    who unlocked the riddle in the mirror;
    terrified, he saw the shattering image
    of his destruction and his error.
    We are Oedipus, doomed as he, to be
    the triple beast: child, saviour, suppliant-
    all that we will be, all that we have been.
    It would annihilate us in an instant
    to glimpse our monstrous being; mercifully
    God grants us issue and oblivion.

    Sadly, God grants us nothing of the sort. But happily, we are not annihilated by glimpsing our monstrous being. We may be disappointed, disillusioned, or discombobulated; but those are temporary conditions that we can strive to overcome. Embrace your monstrous being! It’s the only true strategy in the face of Time’s relentless march.

  • The Flow of Time

    I Tweeted the following inscrutable remark. Probably best left unexplained, but upon reflection I can’t resist.

    My consciousness freely travels up and down my world line, but sadly it only carries the memories appropriate to the moment it inhabits.

    The point is that (some) people don’t think about the flow of time in the right way, and this leads to a couple of unfortunate consequences: a difficulty in understanding the psychology of time, and a scattering of entertaining but illogical science-fiction scenarios.

    Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line. This seemingly conflicts with our intuitive idea that we exist at a moment, and move through time. Of course there is no real conflict — just two different ways of looking at the same thing. There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time.

    But if you don’t play too much attention to the way these two views fit together, you are tempted to imagine that “you” might actually, in some set of laws of physics if not actually in our own, go visit different moments in your own life, carrying along the consciousness of your “present” self. Something like that happens in SF stories from Slaughterhouse-Five to Back to the Future. But it’s not consistent — it requires the implicit introduction of a kind of “meta-time” that keeps track of when we visit the ordinary time with which we are familiar. That’s not how nature works; my tweet was trying to point out the inconsistency of taking this idea seriously, subject to the strictures of 140 characters or less. (To be earnestly explicit: if you did manage to travel up and down your world line at will, you would indeed have whatever memories were appropriate to the moment you were inhabiting — which means it would be exactly like not traveling at all.)

    Sometimes, unfortunately, people go further than science fiction. I’ve run into folks who believe that our conscious perception of time passing is actually evidence against modern physics — arguing that we need to change the known laws of physics to account for the flow of time. It’s always conceivable, in principle, that what we think we understand at a basic level is completely wrong. But the evidence had better be pretty overwhelming. The brain is a complicated thing, and I don’t think that our present inability to provide a complete and comprehensive theory of conscious perceptions should be held as compelling evidence that the laws of physics are in need of overthrowing.