Sean Carroll

Author: Sean Carroll

  • Things Going On

    Miscellaneous happenings, including a couple of talks I’ll be giving — one on another coast, one in another plane of existence.

    • 3 Quarks Daily has announced a series of four annual prizes, for blog posts in Science, Arts & Literature, Politics, and Philosophy. Science is the first one up, and they’re asking for nominations — the deadline is soon (June 1) so head over there and make suggestions. The final winner will be chosen by a well-known person in the appropriate field; this year’s Science judge will be Stephen Pinker. You are of course welcome to suggest your favorite CV post, because we like the attention. But this would also be a great opportunity to give a boost to that lesser-known blog that you really like and think should get more attention. (There are a lot of good blogs out there.) And if you are someone with a blog, don’t feel shy about nominating a post of your own — most readers don’t keep a mental file of your best posts over the last year.
    • The World Science Festival is happening in New York (the U.S.’s second most interesting city) from June 10 to 14. I’ll be there, speaking at two different events. On Friday June 12 there is the WSF Spotlight, which is an informal forum with short talks and a lot of discussion. Participants include Kristin Baldwin (cell biologist), Dominic Johnson (political scientist), Christopher McKay (solar system researcher), and Frank Wilczek (not sure what he does for a living). I believe alcoholic beverages will be available; it’s that kind of event. Then on Saturday June 13 I’ll be on a panel discussing Time Since Einstein, with David Albert, George Ellis, Michael Heller, John Hockenberry, Fotini Markopoulou-Kalamara, and Roger Penrose. (I predict already that insufficient time will be a popular complaint about the time panel.)
    • In Second Life, I’m giving a talk tomorrow morning at 10 am Pacific, sponsored by the Meta Institute for Computational Astrophysics. It will be a colloquium-level talk about “Dark Forces,” concentrating on building models of interacting dark matter and dark energy. Second Lifers can beam right there thanks to this elegant and finely-crafted link: http://slurl.com/secondlife/StellaNova/76/200/32.
    • Max Brockman (son of John, doyen of Edge) has edited a new collection of essays: What’s Next? Dispatches from the Future of Science. There’s an essay by me in there on “Our Place in an Unnatural Universe.” You should buy it, because it would be like reading a set of interesting blog posts, but on paper. And most of these folks don’t have blogs!

  • Epistemological Honesty on the Bench

    Barack Obama has nominated Sonia Sotomayor to fill David Souter’s seat on the Supreme Court. I don’t know much about her on the merits; I was idiosyncratically rooting for Kathleen Sullivan, who I had met while I was a grad student and impressed me as uncommonly brilliant. One thing that immediately strikes you about Sotomayor is her personal history — raised in housing projects in the Bronx by a single Mom, she fought her way up to graduate summa cum laude from Princeton, and then to law school at Yale where she edited the Law Review. Doesn’t mean she’ll be a great Justice, but it’s an impressive record.

    The opposition research has been out for a while, of course, because that’s how politics works. One of the things brought up by Sotomayor’s critics is this clip, where she talks about the difference in emphasis between a district court and an appellate court. (Appellate courts need to look beyond the facts of the case to consider implications of setting precedent for future decisions.)

    This clip drives people crazy, because she says that the courts of appeals are “where policy is made.” You’re not supposed to say that! (As Sotomayor immediately jokes.) The legislatures make the laws, and the courts are merely referees, interpreting the words of the statutes by lights of their objective and unchanging meanings.

    In reality, of course, Sotomayor is simply telling the truth — a cardinal sin in law as well as politics. In law and politics, and for that matter theology, we are presented with a sacred text of one form or another. And we are supposed to pretend that the text has a One True Meaning — we may, of course, argue at great length about the proper procedure for divining what that meaning actually is, but admitting that the text is inherently ambiguous (or even contradictory) is not allowed. We need to act as if the authors of Leviticus and the Framers of the Constitution were trying to say something very clear about contemporary debates, if only we had the interpretational acumen to figure out what it was.

    Which is why, as much as I enjoy the rest of the world of human endeavor, science will always be my true home. Our job is to interpret the natural world, which really is unambiguous and non-contradictory, if only we can make sense of its behavior. Other fields have a professional obligation to pretend that there are right and wrong answers, but we actually have them. Yet another way in which being a scientist is so much easier than other jobs.

  • Another Step Toward Skynet

    There should be some government program that forces scientists to watch dystopian science-fiction movies, so they can have some idea of the havoc their research is obviously going to cause. I just stumbled across an interview with Nobel Laureate Gerald Edelman, that has been on the site for a couple of months. (Apparently the Discover website is affiliated with some sort of magazine, to which you can subscribe.)

    Edelman won the Nobel for his work on antibodies, but for a long time his primary interest has been in consciousness. He believes (as all right-thinking people do) that consciousness is ultimately biological, and is interested in building computer models of the phenomenon. So we get things like this:

    Eugene Izhikevitch [a mathematician at the Neurosciences Institute] and I have made a model with a million simulated neurons and almost half a billion synapses, all connected through neuronal anatomy equivalent to that of a cat brain. What we find, to our delight, is that it has intrinsic activity. Up until now our BBDs had activity only when they confronted the world, when they saw input signals. In between signals, they went dark. But this damn thing now fires on its own continually. The second thing is, it has beta waves and gamma waves just like the regular cortex—what you would see if you did an electroencephalogram. Third of all, it has a rest state. That is, when you don’t stimulate it, the whole population of neurons stray back and forth, as has been described by scientists in human beings who aren’t thinking of anything.

    In other words, our device has some lovely properties that are necessary to the idea of a conscious artifact. It has that property of indwelling activity. So the brain is already speaking to itself. That’s a very important concept for consciousness.

    terminator_robot.jpg
    Oh, great. We build giant robots, equip them with lasers, and now we teach them how to gaze at their navels, and presumably how to dream. What can possibly go wrong?

  • Ten Things You Didn’t Know About Orgasm

    Mary Roach, author of Bonk, gives a TED talk about Ten Things You Didn’t Know About Orgasm. All based on stuff that appeared in highly reputable scientific journals, but still I find the story about the brushing-teeth woman hard to credit.

    (Aimed at adults, or at children who live in families who don’t think sex is dirty.)

    Via Rebecca Skloot’s Twitter feed, so there’s no reason to take it seriously. (Because it’s from Twitter, I mean, not because it’s from Rebecca.)

  • You Have Fifteen Minutes to Get It Right

    Thanks to the arcane magicks of Discover web producer Gemma Shusterman, this blog now offers the latest in commenting technology: the ability to edit your own comments! That’s right: after you type in the comment and hit “Submit,” for the next fifteen minutes you will be able to go back and change what you wrote. We expect that this will totally eliminate the appearance of typos or mistakes of judgment in any further comments. (Note that, in order to actually leave a comment, you have to click on the post title.)

    You’ll also notice that the name of the author now appears at the top of the post, which is very helpful. We have a list of upgrades we’re trying to gradually implement, in order to make your blog-reading experience as pleasant as possible.

  • Congrats to JoAnne and John!

    Of course we hand out congratulations for newly minted Ph.D.’s; but there are other milestones worth congratulating as well. Yesterday our very own JoAnne got married, at a beautiful winery nestled in the Santa Cruz Mountains. It comes at the end of some trying times, so all the more worthy of celebration. Congrats to the happy couple! (Now back to blogging, okay?)

    JoAnne & John’s Wedding

  • Rules for Time Travelers

    With the new Star Trek out, it’s long past time (as it were) that we laid out the rules for would-be fictional time-travelers. (Spoiler: Spock travels to the past and gets a sex change and becomes Kirk’s grandfather lover.*) Not that we expect these rules to be obeyed; the dramatic demands of a work of fiction will always trump the desire to get things scientifically accurate, and Star Trek all by itself has foisted half a dozen mutually-inconsistent theories of time travel on us. But time travel isn’t magic; it may or may not be allowed by the laws of physics — we don’t know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it’s more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.

    0. There are no paradoxes.

    This is the overarching rule, to which all other rules are subservient. It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won’t happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.

    1. Traveling into the future is easy.

    We travel into the future all the time, at a fixed rate: one second per second. Stick around, you’ll be in the future soon enough. You can even get there faster than usual, by decreasing the amount of time you experience elapsing with respect to the rest of the world — either by low-tech ways like freezing yourself, or by taking advantage of the laws of special relativity and zipping around near the speed of light. (Remember we’re talking about what is possible according to the laws of physics here, not what is plausible or technologically feasible.) It’s coming back that’s hard.

    2. Traveling into the past is hard — but maybe not impossible.

    If Isaac Newton’s absolute space and time had been the correct picture of nature, we could simply say that traveling backwards in time was impossible, and that would be the end of it. But in Einstein’s curved-spacetime universe, things are more flexible. From your own personal, subjective point of view, you always more forward in time — more technically, you move on a timelike curve through spacetime. But the large-scale curvature of spacetime caused by gravity could, conceivably, cause timelike curves to loop back on themselves — that is to say, become closed timelike curves — such that anyone traveling on such a path would meet themselves in the past. That’s what respectable, Einstein-approved time travel would really be like. Of course, there’s still the little difficulty of warping spacetime so severely that you actually create closed timelike curves; nobody knows a foolproof way of doing that, or even whether it’s possible, although ideas involving wormholes and cosmic strings and spinning universes have been bandied about.

    3. Traveling through time is like traveling through space.

    I’m only going to say this once: there would be no flashing lights. At least, there would only be flashing lights if you brought along some strobes, and decided to start them flashing as you traveled along your closed timelike curve. Likewise, there is no disappearance in a puff of smoke and re-appearing at some other time. Traveling through time is just like traveling through space: you move along a certain path, which (we are presuming) the universe has helpfully arranged so that your travels bring you to an earlier moment in time. But a time machine wouldn’t look like a booth with spinning wheels that dematerializes now and rematerializes some other time; it would look like a rocket ship. Or possibly a DeLorean, in the unlikely event that your closed timelike curve started right here on Earth and never left the road.

    Think of it this way: imagine there were a race of super-intelligent trees, who could communicate with each other using abstract concepts but didn’t have the ability to walk. They might fantasize about moving through space, and in their fantasies “space travel” would resemble teleportation, with the adventurous tree disappearing in a puff of smoke and reappearing across the forest. But we know better; real travel from one point to another through space is a continuous process. Time travel would be like that.

    4. Things that travel together, age together.

    If you travel through time, and you bring along with you some clocks or other objects, all those things experience time in exactly the same way that you do. In particular, both you and the clocks march resolutely forward in time, from your own perspective. You don’t see clocks spinning wildly backwards, nor do you yourself “age” backwards, and you certainly don’t end up wearing the clothes you favored back in high school. Your personal experience of time is governed by clocks in your brain and body — the predictable beating of rhythmic pulses of chemical and biological processes. Whatever flow of time is being experienced by those processes — and thus by your conscious perception — is also being experienced by whatever accompanies you on your journey.

    5. Black holes are not time machines.

    Sadly, if you fell into a black hole, it would not spit you out at some other time. It wouldn’t spit you out at all — it would gobble you up and grow slightly more corpulent in the process. If the black hole were big enough, you might not even notice when you crossed the point of no return defined by the event horizon. But once you got close to the center of the hole, tidal forces would tug at you — gently at first, but eventually tearing you apart. The technical term is spaghettification. Not a recommended strategy for would-be time adventurers.

    Wormholes — tunnels through spacetime, which in principle can connect widely-separated events — are a more promising alternative. Wormholes are to black holes as elevators are to deep wells filled with snakes and poisoned spikes. The problem is, unlike black holes, we don’t know whether wormholes exist, or even whether they can exist, or how to make them, or how to preserve them once they are made. Wormholes want to collapse and disappear, and keeping them open requires a form of negative energies. Nobody knows how to make negative energies, although they occasionally slap the name “exotic matter” on the concept and pretend it might exist.

    (more…)

  • arxiv Find: Atom interferometry tests of local Lorentz invariance

    What is the Secretary of Energy doing submitting papers to the arxiv when he’s supposed to be solving the world’s energy problems? I have enough trouble getting papers written when it’s my actual job.

    Atom interferometry tests of local Lorentz invariance in gravity and electrodynamics
    Authors: Keng-Yeow Chung, Sheng-wey Chiow, Sven Herrmann, Steven Chu, Holger Mueller

    Abstract: We present atom-interferometer tests of the local Lorentz invariance of post-Newtonian gravity. An experiment probing for anomalous vertical gravity on Earth, which has already been performed by us, uses the highest-resolution atomic gravimeter so far. The influence of Lorentz violation in electrodynamics is also taken into account, resulting in combined bounds on Lorentz violation in gravity and electrodynamics. Expressed within the standard model extension or Nordtvedt’s anisotropic universe model, we limit twelve linear combinations of seven coefficients for Lorentz violation at the part per billion level, from which we derive limits on six coefficients (and seven when taking into account additional data from lunar laser ranging). We also discuss the use of horizontal interferometers, including atom-chip or guided-atom devices, which potentially allow the use of longer coherence times in order to achieve higher sensitivity.

    We kid the Energy Secretary, but this is a very cool experiment. (I presume this is the interferometer?) Basically, you throw an atom up in the air, and catch it as it comes down. But you actually split the wave function of the atom into two different beams, depending on when it absorbs and emits a pulse of laser light. The beams leave the same place and are collected at the same place, but travel on slightly different paths; you can use interferometry to see whether these different paths have evolved differently.

    Which lets you test all kinds of things, from measuring the fine structure constant to looking for new forces to testing Lorentz invariance, as is happening here. But if it helps free us from dependence on foreign oil sources, I’d be surprised.

  • The Funniest Page on the Internet

    I was planning a mildly amusing joke about “Pastaeidolia,” hoping that Phil would forgive me, after seeing this map of the Paris subway system (via Bioephemera).

    paris4a.gif

    I mean, the resemblance is unmistakable, no?

    flying_spaghetti_monster_2-thumb-514x514.jpg

    There’s no way that can simply be a coincidence.

    But then I stumbled across the Flying Spaghetti Monster page at CreationWiki: The Encyclopedia of Creation Science. This is my new candidate for Funniest Page on the Internet. Marvel as they explain, with helpful charts and a compelling level of earnestness, why the FSM does not, in fact, deserve the same amount of respect as one should show to Creation Science.

    Flying Spaghetti Monster Evolution Intelligent Design Creationism
    Intended as parody Intended as science Intended as science Intended as a scientific model of intelligent design
    Creator (the Flying Spaghetti Monster itself) assumed to exist and identified Creator assumed not to exist Creator (designer) inferred from the evidence but not identified Creator assumed to exist and specifically identified
    Evidence for evolution claimed to be planted by the creator Evidence for evolution is not to be questioned Evidence for evolution challenged with academic arguments Evidence for evolution challenged with academic arguments
    Creator makes things appear older than they are as a test of faith Accepts uniformitarian ages Generally accepts uniformitarian ages Rejects uniformitarian ages as based on unprovable presuppositions
    Has no genuine support in the scientific community Has the support of the vast majority of scientists Has the support of (at least) thousands of scientists Has the support of (at least) thousands of scientists
    Has no supporting evidence Has supporting evidence that is highly contested Has supporting evidence Has supporting evidence

    Against professionals like that, we amateur humorists stand little chance.

  • Evolution and the Second Law

    Since no one is blogging around here, and I’m still working on my book, I will cheat and just post an excerpt from the manuscript. Not an especially original one, either; in this section I steal shamelessly from the nice paper that Ted Bunn wrote last year about evolution and entropy (inspired by an previous paper by Daniel Styer).

    ————————————

    Without even addressing the question of how “life” should be defined, we can ask what sounds like a subsequent question: does life make thermodynamic sense? The answer, before you get too excited, is “yes.” But the opposite has been claimed – not by any respectable scientists, but by creationists looking to discredit Darwinian natural selection as the correct explanation for the evolution of life on Earth. One of their arguments relies on a misunderstanding of the Second Law, which they read as “entropy always increases,” and then interpret as a universal tendency toward decay and disorder in all natural processes. Whatever life is, it’s pretty clear that life is complicated and orderly – how, then, can it be reconciled with the natural tendency toward disorder?

    There is, of course, no contradiction whatsoever. The creationist argument would equally well imply that refrigerators are impossible, so it’s clearly not correct. The Second Law doesn’t say that entropy always increases. It says that entropy always increases (or stays constant) in a closed system, one that doesn’t interact noticeably with the external world. But it’s pretty obvious that life is not like that; living organisms interact very strongly with the external world. They are the quintessential examples of open systems. And that is pretty much that; we can wash our hands of the issue and get on with our lives.

    But there’s a more sophisticated version of the argument, which you could imagine being true – although it still isn’t – and it’s illuminating (and fun) to see exactly how it fails. The more sophisticated argument is quantitative: sure, living beings are open systems, so in principle they can decrease entropy somewhere as long as it increases somewhere else. How do you know that the increase in entropy in the outside world is really enough to account for the low entropy of living beings?

    As we mentioned way back in Chapter Two, the Earth and its biosphere are systems that are very far away from thermal equilibrium. In equilibrium, the temperature is the same everywhere, whereas when we look up we see a very hot Sun in an otherwise very cold sky. There is plenty of room for entropy to increase, and that’s exactly what’s happening. But it’s instructive to run the numbers.

    The energy budget of the Earth, considered as a single system, is pretty simple. We get energy from the Sun, via radiation; we lose the same amount of energy to empty space, also via radiation. (Not exactly the same; processes such as nuclear decays also heat up the Earth and leak energy into space, and the rate at which energy is radiated is not strictly constant. Still, it’s an excellent approximation.) But while the amount is the same, there is a big difference in the quality of the energy we get and the energy we give back. Remember back in the pre-Boltzmann days, entropy was understood as a measurement of the uselessness of a certain amount of energy; low-entropy forms of energy could be put to useful work, such as powering an engine or grinding flour, while high-entropy forms of energy just sat there.

    Sun-Earth-entropy

    The energy we get from the Sun is of a low-entropy, useful form, while the energy we radiate back out into space has a much higher entropy. The temperature of the Sun is about twenty times the average temperature of the Earth. The temperature of radiation is just the average energy of the photons of which it is made, so the Earth needs to radiate twenty low-energy (long-wavelength, infrared) photons for every one high-energy (short-wavelength, visible) photon it receives. It turns out, after a bit of math, that twenty times as many photons directly translates into twenty times the entropy. The Earth emits the same amount of energy as it receives, but with twenty times higher entropy.

    The hard part is figuring out just what we mean when we say that the life forms here on Earth are “low-entropy.” How exactly do we do the coarse-graining? It is possible to come up with reasonable answers to that question, but it’s complicated. Fortunately, there is a dramatic shortcut we can take. Consider the entire biomass of the Earth – all of the molecules that are found in living organisms of any type. We can easily calculate the maximum entropy that collection of molecules could have, if it were in thermal equilibrium; plugging in the numbers (the biomass is 1015 kilograms, the temperature of the Earth is 255 Kelvin), we find that its maximum entropy is 1044. And we can compare that to the absolute minimum entropy it could have – if it were in an exactly unique state, the entropy would be precisely zero.

    So the largest conceivable change in entropy that would be required to take a completely disordered collection of molecules the size of our biomass and turn them into absolutely any configuration at all – including the actual ecosystem we currently have – is 1044. If the evolution of life is consistent with the Second Law, it must be the case that the Earth has generated more entropy over the course of life’s evolution by converting high-energy photons into low-energy ones than it has decreased entropy by creating life. The number 1044 is certainly an overly generous estimate – we don’t have to generate nearly that much entropy, but if we can generate that much, the Second Law is in good shape.

    How long does it take to generate that much entropy by converting useful solar energy into useless radiated heat? The answer, once again plugging in the temperature of the Sun and so forth, is: about one year. Every year, if we were really efficient, we could take an undifferentiated mass as large as the entire biosphere and arrange it in a configuration with as small an entropy as we can imagine. In reality, life has evolved over billions of years, and the total entropy of the “Sun + Earth (including life) + escaping radiation” system has increased by quite a bit. So the Second Law is perfectly consistent with life as we know it; not that you were ever in doubt.