Sean Carroll

Author: Sean Carroll

  • 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.

  • Social Mediation

    People writing books, I have to imagine, are much like people with babies. This newborn thing has been the center of their life, and will continue to be, for some time; and one naturally presumes that the rest of the world shares one’s fascination with it. This presumption, alas, may not always be true.

    You may have heard that I have a book coming out — pushed back to January, unfortunately. I haven’t shown any hesitation in blogging about substantive questions related to the topic of the book, nor do I see any reason to. And once it comes out I do want to do some sort of book club so that people can ask questions and have a conversation about what’s in the various chapters. So there will be no shortage of book-related stuff here on the blog.

    But there is a whole ‘nother level of bookish miscellany — admiring the illustrations, having blurbs come in for the back cover, setting up public talks, and all that. Now we’re pretty much into baby-picture territory; it might not be completely safe to assume that everyone else is as fascinated by all this as I am. But you don’t want to deprive those who are, right? So I’m sending all that stuff here:

    That will shield you from the worst of my enthusiasms. A bit, anyway.

    Not that I’m at all sure that this is the right thing to do. Back in my day, we didn’t have all these fancy social networks to play around in; you had your blog, and that was it. Now there’s been a bit of proliferation, and there’s no question that it’s changing the landscape. It can obviously be annoying to try to follow too many things at once, but on the other hand it’s nice to have more appropriate tools for distinct tasks. In the old days, I wouldn’t think much of writing a blog post with an amusing link and little else. Now I will just put that on my Twitter feed. So there are fewer blog posts overall, but the average amount of substance per post is higher. Is this an improvement? Not really sure.

    A lot of bloggers have Twitter feeds where they link to every one of their blog posts, which seems backwards to me. (So I usually don’t subscribe to those folks — nothing personal.) I once asked (on Twitter) whether people thought that was a useful service, and I received strong opinions on either side — but then I noticed that everyone who was in favor of linking to every blog post on Twitter was a blogger who linked to every one of their blog posts on Twitter. So I resist. But then again, I synchronize my Twitter feed to my Facebook status updates, which is considered unforgivably gauche in some circles. So who am I to complain?

  • 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.

  • Newton, P.I.

    When I was studying for my Ph.D., a fellow grad student and I asked our advisor if he could think of one single characteristic that was common to all of the best scientists he knew. Without too much hesitation, he answered: “Hard work.” That certainly wasn’t the answer we wanted to hear — you mean there isn’t some secret recipe to being brilliant? And of course hard work is not nearly enough to elevate you to the ranks of the world’s great scientists. But now that I have marinated for some time in the juices of experience myself, I see the truth of what he was getting at; there are a lot of smart people out there, so it makes sense that what elevates a few of them above their peers is an extraordinary focus on their work and a great amount of simple effort.

    So it should come as no surprise that Isaac Newton, the greatest physicist of all time, was a relentless worker. In his days at Cambridge, when he focused on the workings of the natural world, he would spend as little time as possible on anything that drew him away from the researches in his rooms. Over the couple of years he was writing the Principia Mathematica, he took things to extremes, going for extended periods without food or sleep. (He also, apparently, died a virgin. Extremes come in many guises.)

    Most contemporary physicists have heard that Newton eventually left Cambridge and more or less turned his back on scientific research, to take up activities in later life that we associate with varying degrees of disreputability: alchemy, religious studies, taking a bureaucratic position at the Royal Mint, using the Royal Society to attack his scientific rivals. Lots of us shrug and agree that many older scientists do all sorts of crazy things, and don’t wonder too much about the details.

    levenson-newtoncounter-us-cover1.jpgHappily, Tom Levenson (of The Inverse Square, and one of our honored guest bloggers) has provided us with a fascinating peek into a telling episode in Newton’s later life — his career as a criminal investigator. Not really “P.I.”, as Newton was acting in his capacity as a government official, the Warden of the Mint. The story is closer to something from Law and Order or CSI — remarkably close, in fact. In Newton and the Counterfeiter, Levenson tells the tale of how Newton took up what should have been a cushy sinecure, and ended up devoting his extraordinary Newtonian powers to the pursuit and prosecution of one William Chaloner, the counterfeiter of the title. Poor Chaloner, suffice it to say, never knew what hit him.

    I should say right up front that this is not a book about physics. Some time back Tom asked me to read some pages from his draft, to make sure the physics was coming out right, but he assured me that physics played a very minor role in the book. That baffled me a bit, because — well, it is Isaac Newton, right? But this is a work of biography and intellectual history, and offers a fascinating “street-level view” of the dawn of the Age of Reason. I can recommend it without hesitation to anyone who likes good stories, which I presume is just about anyone.

    The book does begin with some stage-setting about Newton’s scientific work in Cambridge — it is Isaac Newton, right? But it picks up when our protagonist finally wrangles a position in London as Warden of the Mint. Not supposed to be a taxing job; one of the attractions for Newton was that he was going to have plenty of time available for his research. Mostly, at that time, on alchemy and religion — one of the enlightening chapters looks at how Newton actually went about his alchemical work, which is both engrossing and baffling to the modern reader.

    History did not cooperate. The 1690’s was a transformative time for the English currency system, including the introduction of paper money, trade imbalances with the Continent, massive debts run up by William III’s wars in France, and an epidemic of counterfeiting and “coin-clipping,” by which people would shave off the edges of silver coins and melt them down to make new ones. In response, the Mint eventually gave in and undertook a comprehensive re-coinage — a program that was on track to become a complete fiasco until Newton stepped in. Remember that he was not simply an abstract theorist (although he was that); Newton was an extraordinarily careful experimenter, and he turned his practical side to the problem of re-coinage, with spectacular results.

    But the real fun comes in when Newton takes on Chaloner, one of the most notorious counterfeiters of the day. I don’t want to give away too much, because you really should buy the book. Suffice it to say that where Newton was gifted with an extraordinary intellect and a relentless work ethic, Chaloner was gifted with what we would today call “balls.” No scheme was too audacious to be undertaken, no lie was too grandiose to be told, no collection of co-conspirators was too extensive to be betrayed or turned against each other. Chaloner was a colorful character, whose story would have made entertaining reading no matter what era he was born into. But he made one unforgivable mistake: he attracted the particular ire of Isaac Newton, who turned the full force of his powers to tracking this miscreant down and bringing him to justice. Chaloner’s own gifts notwithstanding, it was not a fair fight.

    We tend to look at successful people and imagine that they are defined by their sphere of success. It’s hard for us today to think of Isaac Newton as anything other than a scientist. But he was good at what he did, whether it was piecing together the mysteries of classical mechanics or paying informers to spy on suspected criminals. Gil Grissom would approve — maybe not of all his methods, but certainly of his results.

  • AAAS 2010

    The internets have spoken, and it’s a good thing I listened. A few months ago I had the idea to organize a session at the upcoming meeting of the American Association for the Advancement of Science, in San Diego next February. It’s a giant cross-scientific-disciplinary meeting, offering a great chance for journalists and scientists in diverse fields to catch up on what’s happening in other areas.

    But I couldn’t decide between two possible topics, both of which are close to my heart: “The Origin of the Universe” or “The Arrow of Time.” (My original book subtitle was “The Origin of the Universe and the Arrow of Time,” before that was squelched by the marketing department and replaced with “The Quest for the Ultimate Theory of Time.” Quests are big these days, apparently.) So I did the natural thing: I Tweeted the question. And the internet spoke with a fairly unambiguous voice: “Arrow of Time” sounded more interesting. So that’s what I proposed.

    And now we’ve just been accepted, so it’s on for San Diego 2010. We have a fantastic line-up of speakers (and also me), spanning quite a range of topics:

    That’s the fun part about this topic; it ranges naturally from the birth of the universe to the operation of your brain. Should be a good symposium.

    Update: Unfortunately, Daniel Schacter won’t be able to make the symposium. Instead, we are very fortunate to have Kathleen McDermott of Washington University in St. Louis. Her research involves how we remember the past and forecast the future.

  • Kindling

    So I broke down and bought a Kindle. As usual, I tend to be open to trying new technologies, but don’t like being at the bleeding edge (where people get hurt). There’s no doubt that electronic reading devices have a long way to go, but there’s also little doubt that they’re the wave of the future, or at least a sizable part of it. And the technology seems to have reached a point where Kindle editions of books are a non-trivial part of the market. My own decision to get one was definitely influenced by the number of queries I received about whether my own book would have a Kindle edition. (Answer: yes.)

    And now it’s arrived! So the question is: what’s the first book I should buy? An obvious choice would be Infinite Jest, as the Infinite Summer project is underway and (as I have learned) toting a thousand-page book around on a cross-country flight is less than perfectly convenient. But, of course, I already own that book. And, as Matthew Yglesias points out, you don’t want to buy Kindle versions of impressive books that you can prominently display to buff up your credentials as a person of culture. And the worst would be to display a giant, impressive book on your shelves, but one that was clearly unread and in pristine condition, even though you really did read it, only you read it on your Kindle. Worst of all possible worlds.

    The idea, then, is to find a good book that I haven’t yet read, but not one that is too good — not good enough that I’d rather have the dead-tree edition. Any suggestions?

  • Science and Religion are Not Compatible

    Jerry Coyne, an evolutionary biologist at the University of Chicago, has recently published a book called Why Evolution is True, and started up a blog of the same name. He’s come out swinging in the science/religion debates, taking a hard line against “accomodationism” — the rhetorical strategy on the part of some pro-science people and organizations to paper over conflicts between science and religion so that religious believers can be more comfortable accepting the truth of evolution and other scientific ideas. Chris Mooney and others have taken up the other side, while Russell Blackford and others have supported Coyne, and since electrons are free there have been an awful lot of blog posts.

    At some point I’d like to weigh in on the actual topic of accomodationism, and in particular on what to do about the Templeton Foundation. But there is a prior question, which some of the discussion has touched on: are science and religion actually compatible? Clearly one’s stance on that issue will affect one’s feelings about accomodationism. So I’d like to put my own feelings down in one place.

    Science and religion are not compatible. But, before explaining what that means, we should first say what it doesn’t mean.

    It doesn’t mean, first, that there is any necessary or logical or a priori incompatibility between science and religion. We shouldn’t declare them to be incompatible purely on the basis of what they are, which some people are tempted to do. Certainly, science works on the basis of reason and evidence, while religion often appeals to faith (although reason and evidence are by no means absent). But that just means they are different, not that they are incompatible. (Here I am deviating somewhat from Coyne’s take, as I understand it.) An airplane is different from a car, and indeed if you want to get from Los Angeles to San Francisco you would take either an airplane or a car, not both at once. But if you take a car and your friend takes a plane, as long as you both end up in San Francisco your journeys were perfectly compatible. Likewise, it’s not hard to imagine an alternative universe in which science and religion were compatible — one in which religious claims about the functioning of the world were regularly verified by scientific practice. We can easily conceive of a world in which the best scientific techniques of evidence-gathering and hypothesis-testing left us with an understanding of the workings of Nature which included the existence of God and/or other supernatural phenomena. (St. Thomas Aquinas, were he alive today, would undoubtedly agree, as would many religious people who actually are alive.) It’s just not the world we live in. (That’s where they would disagree.)

    The incompatibility between science and religion also doesn’t mean that a person can’t be religious and be a good scientist. That would be a silly claim to make, and if someone pretends that it must be what is meant by “science and religion are incompatible” you can be sure they are setting up straw men. There is no problem at all with individual scientists holding all sorts of incorrect beliefs, including about science. There are scientists who believe in the Steady State model of cosmology, or that HIV doesn’t cause AIDS, or that sunspots are the primary agent of climate change. The mere fact that such positions are held by some scientists doesn’t make them good scientific positions. We should be interested in what is correct and incorrect, and the arguments for either side, not the particular beliefs of certain individuals. (Likewise, if science and religion were compatible, the existence of thousands of irreligious scientists wouldn’t matter either.)

    The reason why science and religion are actually incompatible is that, in the real world, they reach incompatible conclusions. It’s worth noting that this incompatibility is perfectly evident to any fair-minded person who cares to look. Different religions make very different claims, but they typically end up saying things like “God made the universe in six days” or “Jesus died and was resurrected” or “Moses parted the red sea” or “dead souls are reincarnated in accordance with their karmic burden.” And science says: none of that is true. So there you go, incompatibility.

    But the superficial reasonableness of a claim isn’t enough to be confident that it is true. Science certainly teaches us that reality can be very surprising once we look at it more carefully, and it’s quite conceivable that a more nuanced understanding of the question could explain away what seems to be obviously laid out right in front of us. We should therefore be a little more careful about understanding how exactly a compatibilist would try to reconcile science and religion.

    The problem is, unlike the non-intuitive claims of relativity or quantum mechanics or evolution, which are forced on us by a careful confrontation with data, the purported compatibility of “science” and “religion” is simply a claim about the meaning of those two words. The favored method of those who would claim that science and religion are compatible — really, the only method available — is to twist the definition of either “science” or “religion” well out of the form in which most people would recognize it. Often both.

    (more…)

  • The Principle of Non-Overlapping Food Groups

    A friend of mine, who is severely allergic to pork products, recently asked whether it would be okay for him to order a Western Omelet (ingredients: eggs, cheese, ham, onions, peppers). Superficially, this might seem like a fairly easy question: the incompatibilities between Western omelets and pork allergies seem pretty obvious. But I was able to use a sophisticated philosophical argument to convince him that everything would be okay.

    My inspiration was Stephen Jay Gould’s concept of NOMA, or Non-Overlapping Magisteria. This principle establishes the fundamental compatibility of science with religion, arguing that the two simply don’t address similar questions, and therefore cannot come into conflict. Science deals with the workings of the world (“is” questions), while religion deals with ethical behavior (“ought” questions), so there is way they can be incompatible.

    In this spirit, I have developed what I like to call the principle of Non-Overlapping Food Groups, or NOFOG for short. The basic argument is as follows: throughout history, humans have divided our culinary products into a set of grand groupings. Among these are the Egg Group and the Pork Group. Clearly these are non-overlapping: eggs come from chickens, while pork comes from pigs. Q.E.D.

    Now, I don’t know about you, but a Western Omelet falls squarely within the Egg Group where I am from. Growing up in our small house in the Pennsylvania suburbs, I would look forward to eggs every Sunday morning, most often in the form of a yummy Western Omelet. While the identification is not perfect, we won’t go far wrong by recognizing the Western Omelet as a crucial component of the Egg Group on which we all depend.

    Clearly, since the Egg Group is non-overlapping with the Pork Group, and my friend’s allergies are only to pork, the NOFOG principle justified encouraging his interest in ordering the omelet. I’ll be visiting him in the hospital tomorrow, hopefully he’s feeling better.

  • Scientific Conferences: Tool of the Jewish/Mavericky/Nonviolent/CIA Conspiracy

    Another contender for Best Video of All Time. Via hilzoy, an Iranian-government propaganda video from a while back. It reveals the secret (naturally) collaboration between John McCain, George Soros (“he uses his wealth and slogans like liberty, democracy, and human rights to bring supporters of America to power”), Gene Sharp, and Bill Smith, aimed at undermining the true will of the Iranian people. (Transcript.) I especially like the part where Smith says “we have achieved a lot through international scientific conferences.”

    It’s pretty clear that Iranian security is using 1984 as a how-to guide. Spying on your family as a social good.

    The situation in Iran is no laughing matter; it remains to be seen whether Ayatollah Khamenei has painted himself into a corner where further large-scale violence is inevitable. Our thoughts are with the Iranian people demanding their rights of self-government.