Sean Carroll

Category: Science

  • What Questions Can Science Answer?

    One frustrating aspect of our discussion about the compatibility of science and religion was the amount of effort expended arguing about definitions, rather than substance. When I use words like “God” or “religion,” I try to use them in senses that are consistent with how they have been understood (at least in the Western world) through history, by the large majority of contemporary believers, and according to definitions as you would encounter them in a dictionary. It seems clear to me that, by those standards, religious belief typically involves various claims about things that happen in the world — for example, the virgin birth or ultimate resurrection of Jesus. Those claims can be judged by science, and are found wanting.

    Some people would prefer to define “religion” so that religious beliefs entail nothing whatsoever about what happens in the world. And that’s fine; definitions are not correct or incorrect, they are simply useful or useless, where usefulness is judged by the clarity of one’s attempts at communication. Personally, I think using “religion” in that way is not very clear. Most Christians would disagree with the claim that Jesus came about because Joseph and Mary had sex and his sperm fertilized her ovum and things proceeded conventionally from there, or that Jesus didn’t really rise from the dead, or that God did not create the universe. The Congregation for the Causes of Saints, whose job it is to judge whether a candidate for canonization has really performed the required number of miracles and so forth, would probably not agree that miracles don’t occur. Francis Collins, recently nominated to direct the NIH, argues that some sort of God hypothesis helps explain the values of the fundamental constants of nature, just like a good Grand Unified Theory would. These views are by no means outliers, even without delving into the more extreme varieties of Biblical literalism.

    Furthermore, if a religious person really did believe that nothing ever happened in the world that couldn’t be perfectly well explained by ordinary non-religious means, I would think they would expend their argument-energy engaging with the many millions of people who believe that the virgin birth and the resurrection and the promise of an eternal afterlife and the efficacy of intercessory prayer are all actually literally true, rather than with a handful of atheist bloggers with whom they agree about everything that happens in the world. But it’s a free country, and people are welcome to define words as they like, and argue with whom they wish.

    But there was also a more interesting and substantive issue lurking below the surface. I focused in that post on the meaning of “religion,” but did allude to the fact that defenders of Non-Overlapping Magisteria often misrepresent “science” as well. And this, I think, is not just a matter of definitions: we can more or less agree on what “science” means, and still disagree on what questions it has the power to answer. So that’s an issue worth examining more carefully: what does science actually have the power to do?

    I can think of one popular but very bad strategy for answering this question: first, attempt to distill the essence of “science” down to some punchy motto, and then ask what questions fall under the purview of that motto. At various points throughout history, popular mottos of choice might have been “the Baconian scientific method” or “logical positivism” or “Popperian falsificationism” or “methodological naturalism.” But this tactic always leads to trouble. Science is a messy human endeavor, notoriously hard to boil down to cut-and-dried procedures. A much better strategy, I think, is to consider specific examples, figure out what kinds of questions science can reasonably address, and compare those to the questions in which we’re interested.

    Here is my favorite example question. Alpha Centauri A is a G-type star a little over four light years away. Now pick some very particular moment one billion years ago, and zoom in to the precise center of the star. Protons and electrons are colliding with each other all the time. Consider the collision of two electrons nearest to that exact time and that precise point in space. Now let’s ask: was momentum conserved in that collision? Or, to make it slightly more empirical, was the magnitude of the total momentum after the collision within one percent of the magnitude of the total momentum before the collision?

    This isn’t supposed to be a trick question; I don’t have any special knowledge or theories about the interior of Alpha Centauri that you don’t have. The scientific answer to this question is: of course, the momentum was conserved. Conservation of momentum is a principle of science that has been tested to very high accuracy by all sorts of experiments, we have every reason to believe it held true in that particular collision, and absolutely no reason to doubt it; therefore, it’s perfectly reasonable to say that momentum was conserved.

    A stickler might argue, well, you shouldn’t be so sure. You didn’t observe that particular event, after all, and more importantly there’s no conceivable way that you could collect data at the present time that would answer the question one way or the other. Science is an empirical endeavor, and should remain silent about things for which no empirical adjudication is possible.

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  • Guest Post: Evalyn Gates on Cosmic Magnification (or — Invasion of the Giant Blue Space Amoebas)

    Evalyn Gates Scientists like to argue, contra Walt Whitman, that understanding something increases our appreciation of its beauty, rather than detracting from it. The image below, as Evalyn Gates explains, is a perfect example. Evalyn is an astronomer at the University of Chicago, and the author of a great new book on the science of gravitational lensing, Einstein’s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (Amazon, Barnes & Noble, Powell’s). This post is an introduction to how gravitational lensing gives us some of the most visually arresting and scientifically informative images in all of astronomy.

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    I had the pleasure of meeting up with Sean and some other old friends at the World Science Festival in NYC last month, and over champagne at the opening night reception (science has its benefits) Sean graciously invited me to write a guest post on gravitational lensing. It’s a broad topic, mainly because lensing is proving to be such an incredibly useful tool for many areas of cosmology and astronomy, but I have to admit that the visual beauty of the images produced by lensing is part of the appeal for me.
    I’m also enamored of the visceral connection between these images and lensing phenomena that all of us encounter in daily life – and the access into a complex theory that this connection affords. The giant arcs, Einstein Rings, and multiple copies of a single distant galaxy or quasar that have now been observed in hundreds of images are concrete visualizations of otherwise abstract concepts of general relativity – they effectively trace out the warps in spacetime created by massive objects, revealing the outline of the cosmos much as the technique of “rubbing” can reveal the writing on an ancient gravestone.

    This image, from a recent paper by Adi Zitrin and Tom Broadhurst is both scientifically and visually irresistible:

    zitrinbroadhurstfigure1.jpg

    First, the image itself is really cool. The bright white/yellow galaxies are members of a cluster known as MACS J1149.5+2223, while the blue amoeba-like objects that appear to be invading the cluster are actually five images of a single distant (z ~ 1) spiral galaxy.

    This galaxy has been lensed by the warp in spacetime created by the cluster. Light from the galaxy, which lies almost directly behind the center of the cluster but much farther away from us, travels along several curved paths through the cluster lens, producing multiple magnified images of the galaxy. The inset box shows a computer generated model of the unlensed source galaxy, enlarged by a factor of four so that the details, including the spiral arm structure, are visible. Without the lensing power of the cluster, we would see this galaxy as a single small blue smudge.

    In general, lensing will both magnify and distort (shear) images of a background source. This lens is fairly unique in that we see large but relatively intact images of the spiral galaxy, which implies that the mass distribution in the central region of the cluster must be nearly uniform. The images in the upper left (#1) and lower right (#2) are especially striking. #1 is magnified but very minimally distorted, while #2, the largest image with a magnification of over 80, seems to be curling its tentacles about one of the galaxies in the cluster.

    A close look also reveals the negative parity (mirror symmetry) of the remaining three images – the spiral arms appear to circle in the opposite direction – as expected from lensing. The total magnification of the distant galaxy (the sum of all five images) is about 200, the largest known to date – supporting the authors’s claim that this is “the more powerful lens yet discovered.”

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  • Looking for Dark Matter in the Moon’s Shadow

    moonshadow.jpg Here’s an extremely clever and fun idea (via arxiv blog). A while back the PAMELA experiment claimed to see an excess of high-energy positrons in cosmic rays — a signal that could come from imperfectly-understood astrophysical objects such as pulsars, or might be produced by something more exotic like dark matter annihilations. Some damper on enthusiasm for this idea was introduced by new results from the Fermi observatory, but it wasn’t completely conclusive, since Fermi’s detectors can’t actually distinguish between positrons and electrons.

    So now Pierre Colin and collaborators have hit upon a cute way to distinguish between electrons and positrons: treat the magnetosphere of the Earth like the interior of a giant particle detector. Ever since cloud chambers, physicists have put magnetic fields in their detectors to help distinguish between positively charged particles and negatively charged particles, which get pushed in opposite directions. Well, the Earth has a magnetic field, so maybe we can use that. The problem is that the positrons and electrons would still all hit a telescope such as MAGIC, so the fact that they were deflected by the magnetic field wouldn’t be very relevant.

    But Colin et al. suggest a trick: using the Moon’s shadow. Let’s imagine that the excess positrons really are coming from dark matter annihilating in the galactic center. When the moon is near the position of the galactic center in the sky, it will block out some of those particles, casting a shadow on ground-based telescopes. That’s already interesting, but the fun part is that positrons and electrons will be deflected by the Earth’s magnetic field, so the positron shadow will be in a slightly different position than the electron shadow! Using that effect, it may be possible to distinguish between the signals.

    I am completely unable to judge how feasible this actually is. But the idea is sufficiently imaginative, I’m sure rooting for it.

  • Does Philosophy Make You a Better Scientist?

    Steve Hsu pulls out a provocative quote from philosopher of science Paul Feyerabend:

    The withdrawal of philosophy into a “professional” shell of its own has had disastrous consequences. The younger generation of physicists, the Feynmans, the Schwingers, etc., may be very bright; they may be more intelligent than their predecessors, than Bohr, Einstein, Schrodinger, Boltzmann, Mach and so on. But they are uncivilized savages, they lack in philosophical depth — and this is the fault of the very same idea of professionalism which you are now defending.

    It’s probably true that the post-WWII generations of leading physicists were less broadly educated than their pre-war counterparts (although there are certainly counterexamples such as Murray Gell-Mann and Steven Weinberg). The simplest explanation for this phenomenon would be that the center of gravity of scientific research switched from Europe to America after the war, and the value of a broad-based education (and philosophy in particular) has always been less in America. Interestingly, Feyerabend seems to be blaming philosophers themselves — “the withdrawal of philosophy into a `professional’ shell” — rather than physicists or any wider geosocial trends.

    But aside from whether modern physicists (and maybe scientists in other fields, I don’t know) pay less attention to philosophy these days, and aside from why that might be the case, there is still the question: does it matter? Would knowing more philosophy have made any of the post-WWII giants better physicists? There are certainly historical counterexamples one could conjure up: the acceptance of atomic theory in the German-speaking world in the late nineteenth century was held back considerably by Ernst Mach‘s philosophical arguments. On the other hand, Einstein and Bohr and their contemporaries did manage to do some revolutionary things; relativity and quantum mechanics were more earth-shattering than anything that has come since in physics.

    The usual explanation is that the revolutionary breakthroughs simply haven’t been there to be made — that Feynman and Schwinger and friends missed the glory days when quantum mechanics was being invented, so it was left to them to move the existing paradigm forward, not to come up with something revolutionary and new. Maybe, had these folks been more conversant with their Hume and Kant and Wittgenstein, we would have quantum gravity figured out by now.

    Probably not. Philosophical presuppositions certainly play an important role in how scientists work, and it’s possible that a slightly more sophisticated set of presuppositions could give the working physicist a helping hand here and there. But based on thinking about the actual history, I don’t see how such sophistication could really have moved things forward. (And please don’t say, “If only scientists were more philosophically sophisticated, they would see that my point of view has been right all along!”) I tend to think that knowing something about philosophy — or for that matter literature or music or history — will make someone a more interesting person, but not necessarily a better physicist.

    This might not be right, though. Maybe, had they been more broad and less technical, some of the great physicists of the last few decades would have made dramatic breakthroughs in a field like quantum information or complexity theory, rather than pushing harder at the narrow concerns of particle physics or condensed matter. Easy to speculate, hard to provide much compelling evidence either way.

  • arxiv Find: The Local Density of Dark Matter

    One of the big hopes of particle- and astro-physicists over the next few years is to experimentally pin down the nature of dark matter. In a perfect world, we’ll make the dark matter particle at the LHC, observe gamma rays produced when dark matter annihilates in the galaxy, and detect it directly in experiments here on Earth. The world isn’t always perfect, but sometimes it’s even better, so everyone is sitting on the edges of their seats waiting to hear what the experiments tell us.

    For the direct-detection strategy here on Earth, we build giant detectors and wait for ambient dark-matter particles to interact with something in the detector. If the dark matter is a weakly interacting massive particle (WIMP), that’s not so hard; the difficult part is distinguishing a purported signal from various backgrounds. To know what the signal should be, of course, we need to know how many dark matter particles are zipping through the laboratory. It should be a good number: roughly speaking, there would be about one weak-scale-sized dark matter particle per coffee-cup-volume in the universe, and in our galaxy these particles will typically be trucking along at around 300 kilometers per second.

    Still, you’d like an accurate estimate of how much dark matter there is supposed to be in your detector. That’s what Riccardo Catena and Piero Ullio claim to have provided:

    A novel determination of the local dark matter density
    Authors: Riccardo Catena, Piero Ullio

    Abstract: We present a novel study on the problem of constructing mass models for the Milky Way, concentrating on features regarding the dark matter halo component. We have considered a variegated sample of dynamical observables for the Galaxy, including several results which have appeared recently, and studied a 7- or 8-dimensional parameter space – defining the Galaxy model – by implementing a Bayesian approach to the parameter estimation based on a Markov Chain Monte Carlo method. The main result of this analysis is a novel determination of the local dark matter halo density which, assuming spherical symmetry and either an Einasto or an NFW density profile is found to be around 0.39 GeV cm$^{-3}$ with a 1-$sigma$ error bar of about 7%; more precisely we find a $rho_{DM}(R_0) = 0.385 pm 0.027 rm GeV cm^{-3}$ for the Einasto profile and $rho_{DM}(R_0) = 0.389 pm 0.025 rm GeV cm^{-3}$ for the NFW. This is in contrast to the standard assumption that $rho_{DM}(R_0)$ is about 0.3 GeV cm$^{-3}$ with an uncertainty of a factor of 2 to 3. A very precise determination of the local halo density is very important for interpreting direct dark matter detection experiments. Indeed the results we produced, together with the recent accurate determination of the local circular velocity, should be very useful to considerably narrow astrophysical uncertainties on direct dark matter detection.

    So they’re claiming the density is about .39 GeV per cubic centimeter (where one GeV is about the mass of the proton), whereas the standard figure is something closer to .30 GeV per cubic centimeter. More importantly, they claim to trust their estimate to a precision of about 7%, while the usual number is supposed to be uncertain by a factor of 2 or 3.

    I’m not expert enough to judge whether they are right, but it would certainly be very impressive to pin down the density to such high precision. They do assume spherical symmetry, however, which I suspect is not a very good assumption. There are ongoing arguments about how lumpy the distribution of galactic dark matter really is, and I can easily imagine that lumpiness can distort the local density by much more than 7%. But work like this is going to be very important in interpreting the results, if (when?) we do directly detect the dark matter.

  • Timelessness

    After the FQXi Essay Contest, I was asked to comment on some of the essays besides my own, but I never did. Mostly because I didn’t take the time to read them all (there were an awful lot), but also because I just don’t know what to say about many of them. In her essay (which I liked), Fotini Markopoulou divides the world in two:

    There are two kinds of people in quantum gravity. Those who think that timelessness is the most beautiful and deepest insight in general relativity, if not modern science, and those who simply cannot comprehend what timelessness can mean and see evidence for time in everything in nature. What sets this split of opinions apart form any other disagreement in science is that almost no one ever changes their mind…

    That’s just about right (although perhaps there are also other splits with the same quality). Julian Barbour, whose essay finished first in the judging, has famously championed the view that time does not exist, even writing quite a successful book about it. In a recent Bloggingheads discussion with Craig Callender, Barbour talks a bit more about his view.

    To which all I can muster is: I don’t get it. There are a set of technical arguments, which for the most part I do get, that can be used to make it seem as if time does not exist. In ordinary classical mechanics, we can perform some formal tricks to remove the time variable from the conventional equations of physics. More dramatically, in general relativity or quantum gravity we can express Einstein’s equation (at least in certain circumstances) in a form where time does not appear. On the other hand, we can usually re-write any of these equations in a form where time does appear (at least, again, in certain circumstances).

    But none of these technical arguments are really the point. What I don’t understand — and this is a sincere lack of understanding on my part, not an indirect claim that this perspective is wrong — is what’s supposed to be so great about timelessness. What are we supposed to gain from thinking in this way? What problems is it supposed to solve?

    Put it this way: clearly time appears to exist, at first glance. Even the timelessness crowd somehow manages to submit their essay competition entries by the deadline, and finish their Bloggingheads dialogues within an hour. So the claim “time does not exist” certainly doesn’t mean the same kind of thing as “unicorns do not exist.” It must mean (I suppose) that, while we all find time very useful in our everyday lives, there is a deeper level of description in which time doesn’t appear at all; it only emerges in some sort of approximate description of reality. But that approximate description seems extremely valid and useful, including all of the phenomena in the observable universe. Surely it behooves us to take this purportedly-non-fundamental notion seriously, and attempt to understand some of its puzzling features? Moreover, even if “time” doesn’t turn out to be fundamental, why would that tempt you into saying that it doesn’t exist? Protons are made of quarks, but you don’t hear particle physicists going around claiming that protons don’t exist.

    The problem is not that I disagree with the timelessness crowd, it’s that I don’t see the point. I am not motivated to make the effort to carefully read what they are writing, because I am very unclear about what is to be gained by doing so. If anyone could spell out straightforwardly what I might be able to understand by thinking of the world in the language of timelessness, I’d be very happy to re-orient my attitude and take these works seriously.

  • Holes of Silence

    Black holes are black because you can’t go faster than the speed of light. So what about the speed of sound?

    Of course there is no problem in having something go faster than sound, but sound waves themselves are stuck with that speed limit. That fairly elementary fact inspired Bill Unruh years back to propose a clever idea: a black hole that you could make in the laboratory, but using sound rather than light. He called them dumb holes, although I’m not sure people get the right idea when they hear that name.

    I used to think that this was an amusing thought experiment, but was believed to be unrealistic to actually attempt. But now Lahav et al. have apparently done it! (Via Swans on Tea and arXiv blog.)

    A sonic black hole in a density-inverted Bose-Einstein condensate
    Authors: O. Lahav, A. Itah, A. Blumkin, C. Gordon, J. Steinhauer

    Abstract: We have created the analogue of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. The black hole is realized via a counterintuitive density inversion, in which an attractive potential repels the atoms. This allows for measured flow speeds which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow speed equals the speed of sound is the event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound.

    The idea is simply that you get a fluid flowing faster than its speed of sound in some region, so that the sound waves cannot escape the “horizon” bounding that region. (The flow speed has to change within the material; taking a balloon full of air and putting it on a supersonic jet doesn’t count.)

    But the reason this could some day be very exciting is when quantum mechanics gets into the game. Just like black holes, dumb holes should have “Hawking radiation” — but instead of particles, the holes should emit quantized sound waves (conventionally known as “phonons”). That would be very interesting to observe, although the experimental state of the art isn’t there yet.

    To be clear, we wouldn’t be learning much about quantum gravity if we observed Hawking phonons from dumb holes. The underlying physics is still that of atoms (and, in this case, a Bose-Einstein condensate), not that of general relativity. Indeed, one of Unruh’s original motivations was to show that the physics on small scales didn’t affect the prediction of Hawking radiation. So the prediction of Hawking phonons should be rock-solid, no matter how little we know about quantum gravity. Still, it would be very cool.

  • Susskind Lectures on General Relativity

    Via Dmitry Podolsky, a series of YouTube videos from Stanford encompassing an entire course by Lenny Susskind on general relativity. I didn’t look closely enough to figure out exactly what level the lectures are pitched at, but it looks like a fairly standard advanced-undergrad or beginning-grad introduction to the subject. (For which I could recommend an excellent textbook, if you’re interested.) This is the first lecture; there are more.

    Einstein's General Theory of Relativity | Lecture 1

    It’s fantastic that Stanford is giving this away. I don’t worry that it will replace the conventional university. The right distinction is not “people who would physically go to the lectures” vs. “people who will just watch the videos”; it’s between “people who can watch the videos” and “people who have no access to lectures like this.” And Susskind is a great lecturer.

  • 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.)