Sean Carroll

Author: Sean Carroll

  • From Eternity to Book Club: Chapter Eleven

    Welcome to this week’s installment of the From Eternity to Here book club. Part Three of the book concludes with Chapter Eleven, “Quantum Time.”

    Excerpt:

    This distinction between “incomplete knowledge” and “intrinsic quantum indeterminacy” is worth dwelling on. If the wave function tells us there is a 75 percent chance of observing the cat under the table and a 25 percent chance of observing her on the sofa, that does not mean there is a 75 percent chance that the cat is under the table and a 25 percent chance that she is on the sofa. There is no such thing as “where the cat is.” Her quantum state is described by a superposition of the two distinct possibilities we would have in classical mechanics. It’s not even that “they are both true at once”; it’s that there is no “true” place where the cat is. The wave function is the best description we have of the reality of the cat.

    It’s clear why this is hard to accept at first blush. To put it bluntly, the world doesn’t look anything like that. We see cats and planets and even electrons in particular positions when we look at them, not in superpositions of different possibilities described by wave functions. But that’s the true magic of quantum mechanics: What we see is not what there is. The wave function really exists, but we don’t see it when we look; we see things as if they were in particular ordinary classical configurations.

    Title notwithstanding, the point of the chapter is not that there’s some “quantum” version of time that we have to understand. Some people labor under the impression that the transition from classical mechanics to quantum mechanics ends up “quantizing” everything, and turning continuous parameters into discrete ones, perhaps even including time. It doesn’t work that way; the conventional formalism of quantum mechanics (such as the Schrödinger equation) implies that time should be a continuous parameter. Things could conceivably change when we eventually understand quantum gravity, but they just as conceivably might not. In fact, I’d argue that the smart money is on time remaining continuous once all is said and done. (As a small piece of evidence, the context in which we understand quantum gravity the best is probably the AdS/CFT correspondence, where the Schrödinger equation is completely conventional and time is perfectly continuous.)

    However, we still need to talk about quantum mechanics for the purposes of this book, for one very good reason: we’ve been making a big deal about how the fundamental laws of physics are reversible, but wave function collapse (under the textbook Copenhagen interpretation) is an apparent counterexample. Whether it’s a real counterexample, or simply an artifact of an inadequate interpretation of quantum mechanics, is a matter of much debate. I personally come down on the side that believes that there’s no fundamental irreversibility, only apparent irreversibility, in quantum mechanics. That’s basically the many-worlds interpretation, so I felt the book needed a chapter on what that was all about.

    Along the way, I get to give my own perspective on what quantum mechanics really means. Unlike certain parts of the book, I’m pretty happy with how this one came out — feel free to correct me if you don’t completely agree. Quantum mechanics can certainly be tricky to understand, for the basic reason that what we see isn’t the same as what there is. I’m firmly convinced that most expositions of the subject make it seem even more difficult than it should be, by speaking as if “what we see” really does reflect “what there is,” even if we should know better.

    Two-slit kitty

    So I present a number of colorful examples of two-state systems involving cats and dogs. Experts will recognize very standard treatments of the two-slit experiment and the EPR experiment, but in very different words. Things that seem very forbidding when phrased in terms of interference fringes and electron spins hopefully become a bit more accessible when we’re asking whether the cat is on the sofa or under the table. I did have to treat complicated macroscopic objects with many moving parts as if they could be described as very simple systems, but I judged that to be a worthwhile compromise in the interests of pedagogy. And no animals were harmed in the writing of this chapter! Let me know how you think the strategy worked.

  • Obamacare

    Good news and bad news last night, as the House passed health care reform.

    The good news is: the House passed health care reform. The work isn’t completely done yet, of course. The House had already passed a heath care bill, months ago, but this isn’t it; last night they passed the Senate’s version of the Bill, which had some glaring flaws. Under ordinary circumstances the House and Senate would get together and hammer out a compromise between their two bills. But in the meantime Republicans picked up an extra Senate seat in Massachusetts after Teddy Kennedy died, and they had promised to filibuster the compromise package. (Because, after all, what courageous moral stand could be worth invoking arcane parliamentary procedures more than the fight to prevent millions of people from getting health insurance, especially if that was the life’s goal of the Senator whose death allowed you to improve from having twenty fewer votes than the opposition to only having eighteen fewer votes?)

    So Obama will sign the Senate bill that the House just approved, and then the Senate will consider a reconciliation bill also passed by the House last night. Under even-more-arcane procedures, the reconciliation measure can be passed without threat of filibuster. It requires only “majority vote,” a quaint notion in this highly baroque age.

    It’s not an especially huge bill, whatever you may have heard, but it will have an impact. Here is a list of the major impacts, and an interactive graphic to figure out how you will be affected. The most important features seem to be:

    • Establish health insurance exchanges, and provide subsidies for people below four times the poverty line.
    • Guarantee insurance for people with pre-existing conditions, and eliminate “rescissions” that take away insurance from people who get sick.
    • Push business to provide insurance for their employees, and self-employed individuals to buy insurance for themselves.
    • Close the “donut hole” in the existing Medicare payout structure.
    • Implement cost controls (mostly through slowing the growth of Medicare spending), thereby lowering the budget deficit by $130 billion over the first ten years, and by another $1 trillion over the next ten years.

    Overall, it’s a relatively incremental bill, placing bandages over some of the more egregious wounds in the current system, while leaving in place the essential structure through which we funnel billions of dollars to middlemen while paying far more for medical care per person than any other country without getting better results. For 90% of Americans, coverage and insurance will continue as before. Basically, this brings us a little closer to where Western Europe was a century ago.

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  • Physics on TV

    You never know where you’ll find it.

  • It’s a Dusty Universe Out There

    The primary goal of the European Space Agency’s Planck satellite is to provide a map of the cosmic microwave background with unprecedented precision. But along the way, you have to take into account that there is stuff in between us and the farthest edges of the universe — in particular, there’s all sorts of dust here in our home galaxy. You can even become famous just studying dust; one of the most highly cited papers in all of astrophysics is a 1997 map of galactic dust.

    Dust isn’t only an annoyance — it’s also pretty. Planck hasn’t released any data about the CMB yet, but they just released a map of the cold dust in our local vicinity, looking for all the world like an abstract expressionist painting. (I want to suggest a particular artist, but my mind is blanking.) Click to embiggen.

    planckdustsmall

    It’s a false-color image, of course; the dust is very cold (tens of degrees above absolute zero), and the image is constructed from microwaves, not from visible light. You can see the plane of the galaxy, and the filamentary structures arising from all the churning of the interstellar medium from supernovae, star formation, magnetic fields, and so on.

    Okay, pretty time is over. Let’s see the CMB.

  • From Eternity to Book Club: Chapter Ten

    Welcome to this week’s installment of the From Eternity to Here book club. This is a fun but crucial part of the book: Chapter Ten, “Recurrent Nightmares.”

    Excerpt:

    Fortunately, we (and Boltzmann) only need a judicious medium-strength version of the anthropic principle. Namely, imagine that the real universe is much bigger (in space, or in time, or both) than the part we directly observe. And imagine further that different parts of this bigger universe exist in very different conditions. Perhaps the density of matter is different, or even something as dramatic as different local laws of physics. We can label each distinct region a “universe,” and the whole collection is the “multiverse.” The different universes within the multiverse may or may not be physically connected; for our present purposes it doesn’t matter. Finally, imagine that some of these different regions are hospitable to the existence of life, and some are not. (That part is inevitably a bit fuzzy, given how little we know about “life” in a wider context.) Then—and this part is pretty much unimpeachable—we will always find ourselves existing in one of the parts of the universe where life is allowed to exist, and not in the other parts. That sounds completely empty, but it’s not. It represents a selection effect that distorts our view of the universe as a whole—we don’t see the entire thing, we only see one of the parts, and that part might not be representative. Boltzmann appeals to exactly this logic.

    After the amusing diversions of the last chapter, here we resume again the main thread of argument. In Chapter Eight we talked a bit about the “reversibility objection” of Lohschmidt to Boltzmann’s attempts to derive the Second Law from kinetic theory in the 1870’s; now we pick up the historical thread in the 1890’s, when a similar controversy broke out over Zermelo’s “recurrence objection.” The underlying ideas are similar, but people have become a bit more sophisticated over the ensuing 20 years, and the arguments have become a bit more pointed. More importantly, they are still haunting us today.

    One of the fun things about this chapter is the extent to which it is driven by direct quotations from great thinkers — Boltzmann, of course, but also Poincare, Nietzsche, Lucretius, Eddington, Feynman. That’s because the arguments they were making seem perfectly relevant to our present concerns, which isn’t always the case. Boltzmann tried very hard to defend his derivation of the Second Law, but by now it had sunk in that some additional ingredient was going to be needed — here we’re calling it the Past Hypothesis, but certainly you need something. He was driven to float the idea that the universe we see around us (which, to him, would have been our galaxy) was not representative of the wider whole, but was simply a local fluctuation away from equilibrium. It’s very educational to learn that ideas like “the multiverse” and “the anthropic principle” aren’t recent inventions of a new generation of postmodern physicists, but in fact have been part of respectable scientific discourse for over a century.

    Boltzmann's multiverse

    It’s in this chapter that we get to bring up the haunting idea of Boltzmann Brains — observers that fluctuate randomly out of thermal equilibrium, rather than arising naturally in the course of a gradual increase of entropy over billions of years. I tried my best to explain how such monstrosities would be the correct prediction of a model of an eternal universe with thermal fluctuations, but certainly are not observers like ourselves, which lets us conclude that that’s not the kind of world we live in. Hopefully the arguments made sense. One question people often ask is “how do we know we’re not Boltzmann Brains?” The realistic answer is that we can never prove that we’re not; but there is no reliable chain of argument that could ever convince us that we are, so the only sensible way to act is as if we are not. That’s the kind of radical foundational uncertainty that has been with us since Descartes, but most of us manage to get through the day without being overwhelmed by existential anxiety.

  • Report from Colbert

    Reporting back from a hotel in midtown Manhattan, having made it through the Colbert Report basically unscathed. In fact the experience was great from beginning to end. Update: here is the clip.

    <td style='padding:2px 1px 0px 5px;' colspan='2'Sean Carroll
    The Colbert Report Mon – Thurs 11:30pm / 10:30c
    www.colbertnation.com
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    Monday morning I talked on the phone with Emily Lazar, a researcher for the show. I was really impressed right from the start: it was clear that she wanted to make it easy for me to get across some substantive message, within the relatively confining parameters of what is basically a comedy show. From start to finish everyone I dealt with was a consummate pro.

    We got picked up at our hotel in a car that brought us to the Colbert studio, and hustled inside under relatively high security — people whispering into lapel microphones that we had arrived and were headed to the green room. Very exciting. The green room was actually green, which is apparently unusual. I got pep talks from a couple of the staff people, who encouraged me to keep things as simple as possible. They made an interesting point about scientists: they make the perfect foils for Stephen’s character, since they actually rely on facts rather than opinions.

    colbert

    Stephen himself dropped by to say hi, and to explain the philosophy of his character — I suppose there still are people out there who could be guests on the show who haven’t ever actually watched it. Namely, he’s a complete idiot, and it’s my job to educate him. But it’s not my job to be funny — that’s his bailiwick. The guests are encouraged to be friendly and sincere, but not pretend to be comedians.

    We got to sit in the audience as the early segments were taped, which were hilarious. I feel bad that my own interview is going to be the low point of the show, laughs-wise. But I went out on cue, and fortunately I wasn’t at all jittery — too much going on to have time to get nervous, I suppose.

    I had some planned responses for what I thought were the most obvious questions. Of which, he asked zero. Right off the bat Colbert managed to catch me off guard by asking a much more subtle question than I had anticipated — isn’t the early universe actually very disorderly? That would be true if you ignored gravity, but a big part of my message is that you can’t ignore gravity! The problem was, I had promised myself that I wouldn’t use the word “entropy,” resisting the temptation to lapse into jargon. But he had immediately pinpointed an example where the association of “low entropy” with “orderly” wasn’t a perfect fit. So I had to go back on my pledge and bring up entropy, although I didn’t exactly give a careful definition.

    As everyone warned me, the whole interview went by in an absolute flash, although it really lasts about five minutes. There was a fun moment when we agreed that “Wrong Turn Into Yesterday” would make a great title for a progressive-rock album. Overall, I think I could have done a better job at explaining the underlying science, but at least I hope I successfully conveyed the spirit of the endeavor. We’ll have to see how it comes across on TV.

    I shouldn’t end without including some good words about the bag of swag. Not only does every guest get a goodie bag that includes a bottle of excellent tequila, it also includes a $100 gift certificate for Donors Choose. How awesome is that?

    And as we left the studio, there were some young audience members lurking around hoping for a glimpse of the great man himself. They had to settle for me, but they sheepishly asked if I would pose for a picture with them. Not yet having perfected my diva act, I happily complied. I hope they take away some great memories of the night.

  • Free Energy and the Meaning of Life

    When we think about the “meaning of life,” we tend to conjure ideas such as love, or self-actualization, or justice, or human progress. It’s an anthropocentric view; try to convince blue-green algae that self-actualization is some sort of virtue. Let’s ask instead why “life,” as a biological concept, actually exists. That is to say: we know that entropy increases as the universe evolves. But why, on the road from the simple and low-entropy early universe to the simple and high-entropy late universe, do we pass through our present era of marvelous complexity and organization, culminating in the intricate chemical reactions we know as life?

    Yesterday’s book club post referred to a somewhat-whimsical vision of Maxwell’s Demon as a paradigm for life. The Demon takes in free energy and uses it to maintain a separation between hot and cold sides of a box of gas — a sustained departure from thermal equilibrium. But what if we reversed the story? Instead of thinking that the Demon takes advantage free energy to help advance its nefarious anti-thermodynamic agenda, what if we imagine that the free energy is simply using the Demon — that is, the out-of-equilibrium configurations labeled “life” — for its own pro-thermodynamic purposes?

    From a slide by Eric Smith

    Energy is conserved, if we put aside some subtleties associated with general relativity. But there’s useful energy, and useless energy. When you burn gasoline in your car engine, the amount of energy doesn’t really change; some of it gets converted into the motion of your car, while some gets dissipated into useless forms such as noise, heat, and exhaust, increasing entropy along the way. That’s why it’s helpful to invent the concept of “free energy” to keep track of how much energy is actually available for doing useful work, like accelerating a car. Roughly speaking, the free energy is the total energy minus entropy times temperature, so free energy is used up as entropy increases.

    Because the Second Law of Thermodynamics tells us that entropy increases, the history of the universe is the story of dissipation of free energy. Energy wants to be converted from useful forms to useless forms. But it might not happen automatically; sometimes a configuration with excess free energy can last a long time before something comes along to nudge it into a higher-entropy form. Gasoline and oxygen are a combustible mixture, but you still need a spark to set the fire.

    This is where life comes in, at least according to one view. Apparently (I’m certainly not an expert in this stuff) there are two competing theories that attempt to explain the first steps taken toward life on Earth. One is a “replicator-first” picture, in which the key jump from chemistry to life was taken by a molecule such as RNA that was able to reproduce itself, passing information on to subsequent generations. The competitor is a “metabolism-first” picture, where the important step was a set of interactions that helped release free energy in the atmosphere of the young Earth. You can read some background about these two options in this profile of Mike Russell (pdf), one of the leading advocates of the metabolism-first view.

    (more…)

  • From Eternity to Book Club: Chapter Nine

    Welcome to this week’s installment of the From Eternity to Here book club. Now for something of a palate-cleanser, in the form of Chapter Nine, “Information and Life.”

    Excerpt:

    Schrödinger’s idea captures something important about what distinguishes life from non-life. In the back of his mind, he was certainly thinking of Clausius’s version of the Second Law: objects in thermal contact evolve toward a common temperature (thermal equilibrium). If we put an ice cube in a glass of warm water, the ice cube melts fairly quickly. Even if the two objects are made of very different substances—say, if we put a plastic “ice cube” in a glass of water—they will still come to the same temperature. More generally, nonliving physical objects tend to wind down and come to rest. A rock may roll down a hill during an avalanche, but before too long it will reach the bottom, dissipate energy through the creation of noise and heat, and come to a complete halt before very long.

    Schrödinger’s point is simply that, for living organisms, this process of coming to rest can take much longer, or even be put off indefinitely. Imagine that, instead of an ice cube, we put a goldfish into our glass of water. Unlike the ice cube (whether water or plastic), the goldfish will not simply equilibrate with the water—at least, not within a few minutes or even hours. It will stay alive, doing something, swimming, exchanging material with its environment. If it’s put into a lake or a fish tank where food is available, it will keep going for much longer.

    This chapter starts with something very important: the relationship between entropy and memory. Namely, the reason why we can “remember” the past and not the future is that the past features a low-entropy boundary condition, while the future does not. I don’t go into great detail about this, and we certainly don’t talk very specifically about how real memories are formed in the brain, or even in a computer. But when we get to the next chapter, about recurrences and Boltzmann brains, it will be crucial to understand how the assumption of a low-entropy boundary condition enables us to reconstruct the past. It’s hard for people to wrap their brains around the fact that, without such an assumption, our “memories” or records of the past will generally be unreliable — knowledge of the current macrostate wouldn’t allow us to reconstruct the past any better than it allows us to predict the future. (Which is only logical, since it’s only this hypothesis that breaks time-reversal symmetry.)

    The rest of the chapter, meanwhile, is more about having fun and mentioning some ideas that are not directly related to our story, but certainly play a part in understanding the arrow of time. Information theory, life, complexity. I’m not an expert in any of these fields, but it was a lot of fun reading about them to pick out some things that fit into the broader narrative. The Maxwell’s Demon story, in particular, is one that every physicist should know (up through it’s relatively modern resolution), but relatively few do. And I think Jason Torchinsky did a great job with the illustrations of the Demon.

    maxwellsdemon

    A lot of big ideas here, of course, and much of this stuff is still very much in the working-out stage, not the settled-understanding stage. We’re still arguing about basic things like the definition of “complexity” and “life.” It’s relatively easy to state the Second Law and explain how the arrow of time is related to the growth of entropy, but there’s a tremendous amount of work still to be done before we completely understand the way in which the universe actually evolves from low entropy to high.

  • Just a Frog on the Dissection Table

    We’ve been studied. Bora points to a new paper by Inna Kouper in the Journal of Science Communication. The title is “Science blogs and public engagement with science: Practices, challenges, and opportunities,” which pretty much explains what it’s about. The author picks out a collection of eleven blogs — Pure Pedantry, Synthesis, MicrobiologyBytes, Bioethics, Wired Science, DrugMonkey, Scientific Activist, Pharyngula, Panda’s Thumb, and our own humble offering — and analyzes posts and comments to judge how effective these sites are at promoting science communication.

    The list of blogs chosen is — okay, I guess. I have no idea how it was constructed, and the paper doesn’t seem to provide much guidance. Bora has a critique of the methodology that wonders about that, and about exactly how objective the study is. It’s very hard to assign numbers to things like “ratio of informative posts vs. rants,” or “degree to which the cause of collegial communication was harmed by use of intemperate language.” The paper reads like someone read a bunch of blogs and typed up their personal impressions.

    For the most part I don’t disagree too strongly with the impressions, with the obvious caveat that it’s almost completely useless to study “science blogs” as a group. People don’t read randomly chosen collections of blogs; they read very intentionally chosen subsets that appeal to their own interests, and different reading lists will lead to wildly divergent impressions about what blogs are really like.

    More significantly, though, I can’t really agree with the moral that the author draws from these experiences. Here is the telling quote from the paper:

    The blogs employ a variety of writing and authoring models, and no signs of emerging or stabilizing genre conventions could be observed. Even though all blogs mentioned science or a particular scientific discipline in their descriptions, they differed in their voice representations, points of view, and content orientation.

    It’s hard to disagree with that, but I think it’s a good thing, and the author clearly does not. Blogs differ in many ways, and happily avoid the encroachment of stabilizing genre conventions. That’s one of the biggest benefits of opening up communication channels to a tremendous variety of content providers, rather than restricting things to just a few mainstream outlets; writers can have their voices, and readers can choose who to read, and everyone is happy.

    It’s clear that a lot of people want blogs to be just like some pre-existing communication medium, just with comments and occasional expertise. And there are blogs like that, if that’s what you’re into. And there are blogs that aren’t, likewise. I hope it stays that way.

  • Delayed But Not Denied

    First a programming notice: turns out I will not be on the Colbert Report tonight. Never fear — I was just bumped back to next week, Wednesday March 10 (11:30 p.m., 10:30 Central). Business as usual in TV land, no big deal. I was hoping that I was nudged in favor of a newly medaled Olympic hero, or at least minor royalty, but it looks like tonight’s guest will be Garry Wills. He’s one of my favorite writers, but still. Obviously some Catholic favoritism going on here.

    Small scheduling glitches aside, the Colbert Report and the Daily Show remain two of the best places to hear interviews with interesting academics on TV, especially with scientists. In USA Today, Dan Vergano writes about this curious state of affairs. Neil deGrasse Tyson brings up a good point, that Johnny Carson’s version of the Tonight Show used to feature interviews with heavyweights such as Carl Sagan and Margaret Mead. These days, not many non-satirical network news shows bring on scientists (or anthropologists, or for that matter philosophers or English professors) as a regular event.

    When Conan O’Brian took over the Tonight Show, the Science and Entertainment Exchange received a request from the producers to suggest some entertaining (and hopefully enlightening) scientists they could consider bringing on as guests. I don’t know if they ever followed up on that idea, and now I guess we’ll never know. Hopefully the success of Stewart and Colbert will convince the networks that Americans don’t necessarily turn the channel when faced with people who think carefully about the universe.