Sean Carroll

Author: Sean Carroll

  • Are You a Cognitive Miser?

    Jack is looking at Anne, but Anne is looking at George. Jack is married, but George is not. Is a married person looking at an unmarried person?

    A) Yes.

    B) No.

    C) Cannot be determined.

    (more…)

  • Fake Style

    The latest Twitter phenomenon is FakeAPStylebook, an amusing take on guidelines to proper journalistic writing. Some tips include:

    • STAR WARS Episodes IV-VI are to be referred to as “The Original Trilogy.” Episodes I-III are not to be referred to at all.
    • Always capitalize Satan. You don’t want to get dead goats from those people.
    • The correct spelling is “Rocktober,” not “Roctober,” which is the month of giant birds.
    • Replace “situation deteriorated/worsened” with “shit [just] got real.” Ex: On day three of the hostage crisis, shit got real.

    Amusing enough, but I have to admit that I originally read “Fake AP Stylebook” as “Fake APS Stylebook,” as if it were the (fake) American Physical Society rather than the (fake) Associated Press that was handing out advice. After all, the real APS is quite a bit quirkier than the AP; they insist that no article title begin with “The,” and for a while there they were insisting that “Lagrangian” be spelled “Lagrangean.” (Everyone has their quirks; Nature has banned the words “paradigm” and “scenario” from its pages entirely.)

    So I’m sure we can do better. Any good suggestions for improved physics style? I promise to tweet anything sufficiently amusing.

  • Has Fermi Seen New Evidence for Dark Matter?

    Speaking of successful NASA/DOE collaborations, there’s an interesting new paper on astro-ph claiming that the Fermi gamma-ray satellite has found evidence for a gamma-ray excess in the vicinity of the galactic center — similar to what you might expect from high-energy electrons produced by annihilations or decays of dark matter.

    The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze
    Authors: Gregory Dobler, Douglas P. Finkbeiner, Ilias Cholis, Tracy R. Slatyer, Neal Weiner

    Abstract: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal in the inner Galaxy with the same spatial morphology as the microwave haze observed by WMAP, confirming the synchrotron origin of the microwaves. Using spatial templates, we regress out pi0 gammas, as well as ICS and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with ICS from a hard population of electrons. The morphology and spectrum are consistent with it being the ICS counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition to confirming that the microwave haze is indeed synchrotron, the distinct spatial morphology and very hard spectrum of the ICS are evidence that the electrons responsible for the microwave and gamma-ray haze originate from a harder source than supernova shocks. We describe the full sky Fermi maps used in this analysis and make them available for download.

    In English: if the dark matter is a weakly-interacting massive particle (WIMP), individual WIMPs should occsasionally annihilate with other WIMPs, giving off a bunch of particles, including electron/positron pairs as well as high-energy photons (gamma rays). Indeed, searching for such gamma rays was one of the primary motivations behind the Fermi mission (formerly GLAST). And it makes sense to look where the dark matter is most dense, in the center of the galaxy. But it’s a very hard problem, for a simple reason — there’s lots of radiation coming from the center of the galaxy, most of which has nothing to do with dark matter. Subtracting off these “backgrounds” (which would be very interesting in their own right to galactic astronomers) is the name of the game in this business.

    But Doug Finkbeiner at Harvard has for a while now been suggesting that there was already evidence for something interesting going on near the galactic center — not in the form of high-energy photons, but in the form of low-energy photons. The so-called WMAP haze is alleged to be radiation emitted when high-energy electrons are being accelerated by magnetic fields, leading to low-energy photons (synchrotron radiation). And Finkbeiner and collaborators claim that a careful analysis of data from WMAP (whose primary mission was to observe the cosmic microwave background) reveals exactly the kind of radiation you would expect from annihilations near the galactic center.

    If that model is right, it gives us some guidance about what to look for in the gamma rays themselves, which Fermi is now observing. And according to this new paper, this is what we see.

    Excess gamma rays from the galactic center, from Dobler et al.

    That’s one of many images, and has been extensively processed; see paper for details. The new paper claims that there is an excess of gamma rays, and that it has just the right properties to be arising from the same population of electrons that gave rise to the WMAP haze. These much higher-energy photons arise from inverse Compton scattering — electrons bumping into photons and pushing them to higher energies — rather than synchrotron emission. So we’re not talking about gammas that are produced by dark-matter annihilations, but ones that might arise from electrons and positrons that are produced by such annihilations. The authors pointedly do not claim that what we see must arise from dark matter, or even delve very deeply into that possibility.

    There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars. The search for new physics – or an improved understanding of conventional astrophysics – will be the topic of future work.

    That’s as it should be; whether or not the gamma-ray haze is real is a separate question from whether dark matter is the culprit. But on a blog we can speculate just a bit. Therefore I’m going to go out on a limb and say: maybe it is! Or maybe not. But a wide variety of promising experimental techniques are attacking the problem of detecting the dark matter, and we’ll be hearing a lot more in the days to come.

  • How to Go After Dark Energy?

    It’s well known that dark energy is a mystery — both for scientists, and apparently for funding agencies who are trying to figure out how best to learn more about this stuff that makes up about 73% of the energy of the universe. I haven’t been paying close attention to the ins-and-outs of this saga (there are more rewarding ways to give yourself an ulcer), but last I had heard the National Academy of Sciences had given very high priority to a satellite observatory meant to pin down the properties of dark energy. This was the JDEM idea — Joint Dark Energy Mission, where “joint” indicates a partnership between NASA and the Department of Energy. (They don’t always play well together, but the Fermi satellite is a notable recent success.)

    Now, via Dan Vergano’s Twitter feed, I see a story in Nature News to the effect that things have become murky once again. The proposals got too expensive, so NASA turned to the European Space Agency for help, but ended up giving away things the DOE thought were in their domain, so they threatened to take their toys and go home, giving up on the idea of a satellite altogether.

    The story is complicated by disagreement over how important it is to measure the dark energy equation-of-state parameter, the number characterizing how quickly the energy density changes (if at all). It’s frequently said that “we know nothing” about dark energy, but that’s not true; we know that it’s smoothly distributed and nearly-constant in density through time. We even have a very natural candidate for what it is: the vacuum energy. There is of course the problem that the vacuum energy is much smaller than it should be, but that problem is there whether it’s strictly zero or just really small. Other models still have that problem, and tend to add other fine-tunings on top. It would be great, and we would certainly learn a lot, if the dark energy were not simply vacuum energy; but right now we have no compelling reason to think it’s not, so it’s a bit of a long shot.

  • Defending Science Isn’t Always Pretty

    This month’s issue of WIRED features a great story by Amy Wallace: “An Epidemic of Fear: How Panicked Parents Skipping Shots Endangers Us All.” It’s an overview of the anti-vaccination movement in the United States, a topic that should be very familiar to anyone who reads Discover‘s baddest astronomer. At ScienceBlogs, Orac and Abel Pharmboy gives big thumbs-up to the article.

    The anti-vaccination movement is a little weird — they claim that vaccines, which are universally credited with wiping out smallpox and polio and other bad things, are responsible for causing autism and diabetes and other also-bad things, all just to make a buck for pharmaceutical companies. The underlying motivation seems to be a combination of the conviction that things must happen for a reason — if a child develops autism, there must be an enemy to blame — and a general distrust of science and technology. Certainly the pro-science point of view is fairly unequivocal; like any medicine, vaccines should be used properly, but they have done great good for the world and there are very real dangers of increased risk for epidemics if enough children stop receiving them. Good for WIRED for taking on the issue and publishing an uncompromisingly pro-science piece on it.

    But the anti-vax movement is more than just committed; they’re pretty darn virulent. And since the article came out, author Amy Wallace has been subject to all sorts of attacks. She’s been documenting them on her Twitter feed, which I encourage you to check out. Some lowlights:

    • I’ve been called stupid, greedy, a whore, a prostitute, and a “fking lib.” I’ve been called the author of “heinous tripe.”
    • J.B. Handley, the founder of Generation Rescue, the anti-vaccine group that actress Jenny McCarthy helps promote, sent an essay titled “Paul Offit Rapes (intellectually) Amy Wallace and Wired Magazine.” In it, he implied that Offit had slipped me a date rape drug.
    • Just now, I got an email so sexually explicit that I can’t paraphrase it here. Except to say it contained the c-word. And a reference to dead fish.
    • In his book, Autism’s False Prophets, Dr. Offit writes about scientists who have been intimidated into staying silent about autism/vaccines. If scientists – who are armed with facts and trained to interpret them – are afraid, can it be any surprise that a lot of parents are, too?


    It’s pretty horrifying stuff. But there is good news: Wallace also reports that the large majority of emails she has received were actually in favor of the piece, and expressed gratitude that she had written it. There are strong forces arrayed against science, but the truth is on our side, and a lot of people recognize it. It gives one a bit of hope.

  • The Big Blog Theory

    Recent years have seen a notable increase in the number of successful TV shows with some sort of scientific component — Numbers, CSI, House, Bones, Lie to Me, Fringe, and so on. But there’s no doubt which network show has the most accurate science on TV; that would be the CBS comedy The Big Bang Theory.

    And it’s not because the writers are all physics Ph.D.’s who have traded in equations and laboratories for a glamorous life in Hollywood. It’s because the Big Bang Theory is one of the very few shows to have a full-time science advisor: David Saltzberg, a particle physicist at UCLA. David confers with the writers, reads every script, provides complicated-looking equations for the white boards in Sheldon and Leonard’s apartment, and suggests the occasional physics joke.

    And now David, encouraged by some of his well-meaning friends, is going to be explaining the science behind the show in his new blog:

    The show is a comedy, but the science here is completely serious — read about dark matter, quantum mechanics, monopoles, and all sorts of good stuff. I’m sure much of this was explained carefully in the original scripts, but landed on the cutting-room floor in interests of time.

    The Big Bang Theory, of course, raises strong feelings among scientists. Right here at Discover, you can read both pro and anti feelings about the show. The complaints are mostly about the cheerful reliance on various stereotypes that we would just as soon see stamped out. All four of the main scientist characters are socially maladjusted guys; the one main non-scientist is a blonde woman with severe science-phobia.

    I think the critique of sexism is mostly fair. In the real world, plenty of brilliant socially-maladjusted scientists are female! (To be fair, Penny represents the everyperson character to which the audience is supposed to relate; in almost every activity not related to science or technology, she is much more competent than the boys.) The critique that all these nerdy scientist characters somehow damage the image of science I find much less compelling — even though, in the real world, plenty of brilliant scientists aren’t socially maladjusted at all. It is, after all, a sitcom, not a public-service announcement; sitcoms get a lot of their mileage out of stereotypes. And as socially awkward as the scientist characters are, they are also portrayed as lovable and warm people at heart. Shows like this humanize science, and who knows what ten-year-old kid will see an episode and start thinking that physics is a career to which real people can actually aspire.

    Now if we could just get across the idea that even young girls can aspire to these careers, we’d be getting someplace.

  • Political Life’s Mysteries

    My personal blog-reading strategy is to cycle around, subscribing to any individual blog for a while in my newsreader and then dropping it after a while. You can’t read everything. So I used to read Matthew Yglesias, but haven’t been recently. I clearly need to start again, because this (via Brad DeLong) is extremely smart and powerful.

    I’ve come to be increasingly baffled by the high degree of cynicism and immorality displayed in big-time politics. For example, Senators who genuinely do believe that carbon dioxide emissions are contributing to a global climate crisis seem to think nothing of nevertheless taking actions that endanger the welfare of billions of people on the grounds that acting otherwise would be politically problematic in their state. In other words, they don’t want to do the right thing because their self-interest points them toward doing something bad. But it’s impossible to imagine these same Senators stabbing a homeless person in a dark DC alley to steal his shoes. And what’s more, the entire political class would be (rightly!) shocked and appalled by the specter of a Senator murdering someone for personal gain. Yet it’s actually taken for granted that “my selfish desires dictate that I do x” constitutes a legitimate reason to do the wrong thing on important legislation.

    It is kind of a mystery. Why is it a heinous crime for one individual to act directly against another, but business as usual for a powerful politician to act knowingly in ways that will bring harm to the nation or the world? Is it just that one death is a tragedy, a million is a statistic?

  • Talking About Time

    I’m in the middle of jetting hither and yon, talking to people about the arrow of time. (Wouldn’t it be great if I had a book to sell them?) Right now, as prophesyed, I’m at the Quantum To Cosmos Festival at the Perimeter Institute. They’re extremely on the ball over here, so every event is being recorded by the ultra-professional folks at TVO, and instantly available on the web. So here is the talk I gave on Saturday night — a public-level discussion of entropy and how it connects to the history of our universe.

    Yes, that’s a pretty suave picture of me on the image capture. What can I say? I’m just one of those lucky folks with an effortless magic in front of the camera.

    _stwVar[“player”]= “generic_singlev2”;_stwVar[“width”]= “600”;_stwVar[“height”]= “425”;_stwVar[“autostart”]= “0”;_stwVar[“skintemplate”]= “stw_dark”;_stwVar[“clientid”]= “2121”;_stwVar[“clientcheck”]= “9Huudq3”;_stwVar[“mediaid”]= “570901”;_stwVar[“lang”]= “en”;_stwVar[“activesprinkler”]= “0”;_stwVar[“clientname”]= “perimeterinstitute”;_stwVar[“mediafileid”]= “893391”;embed();

    If you prefer to get your talks about entropy unadulterated by voice and motion, and don’t mind a more technical presentation, I’ve put the slides from my recent Caltech colloquium online. These are aimed basically at grad students in physics, so there is an equation or two, and the caveats are spelled out more clearly. But the punchline is the same.

    ouaot

  • The Sun is Going to Die?

    From Spike Jonze’s new film, Where the Wild Things Are, based on the classic Maurice Sendak book. Neither the book nor (apparently) the movie are pablum. Via io9 (spoilers at that link).

  • Spooky Signals from the Future Telling Us to Cancel the LHC!

    A recent essay in the New York Times by Dennis Overbye has managed to attract quite a bit of attention around the internets — most of it not very positive. It concerns a recent paper by Holger Nielsen and Masao Ninomiya (and some earlier work) discussing a seemingly crazy-sounding proposal — that we should randomly choose a card from a million-card deck and, on the basis of which card we get, decide whether to go forward with the Large Hadron Collider. Responses have ranged from eye-rolling and heavy sighs to cries of outrage, clutching at pearls, and grim warnings that the postmodernists have finally infiltrated the scientific/journalistic establishment, this could be the straw that breaks the back of the Enlightenment camel, and worse.

    Since I am quoted (in a rather non-committal way) in the essay, it’s my responsibility to dig into the papers and report back. And my message is: relax! Western civilization will survive. The theory is undeniably crazy — but not crackpot, which is a distinction worth drawing. And an occasional fun essay about speculative science in the Times is not going to send us back to the Dark Ages, or even rank among the top ten thousand dangers along those lines.

    The standard Newtonian way of thinking about the laws of physics is in terms of an initial-value problem. You specify the state of the system (positions and velocities) at one moment, then the laws of physics tell you how it will evolve into the future. But there is a completely equivalent alternative, which casts the laws of physics in terms of an action principle. In this formulation, we assign a number — the action — to every possible history of the system throughout time. (The choice of what action to assign is simply the choice of what laws of physics are operative.) Then the allowed histories, the ones that “obey the laws of physics,” are those for which the action is the smallest. That’s the “principle of least action,” and it’s a standard undergraduate exercise to show that it’s utterly equivalent to the initial-value formulation of dynamics.

    In quantum mechanics, as you may have heard, things change a tiny bit. Instead of only allowing histories that minimize the action, quantum mechanics (as reformulated by Feynman) tells us to add up the contributions from every possible history, but give larger weight to those with smaller actions. In effect, we blur out the allowed trajectories around the one with absolutely smallest action.

    Nielsen and Ninomiya (NN) pull an absolutely speculative idea out of their hats: they ask us to consider what would happen if the action were a complex number, rather than just a real number. Then there would be an imaginary part of the action, in addition to the real part. (This is the square-root-of-minus-one sense of “imaginary,” not the LSD-hallucination sense of “imaginary.”) No real justification — or if there is, it’s sufficiently lost in the mists that I can’t discern it from the recent papers. That’s okay; it’s just the traditional hypothesis-testing that has served science well for a few centuries now. Propose an idea, see where it leads, toss it out if it conflicts with the data, build on it if it seems promising. We don’t know all the laws of physics, so there’s no reason to stand pat.

    NN argue that the effect of the imaginary action is to highly suppress the probabilities associated with certain trajectories, even if those trajectories minimize the real action. But it does so in a way that appears nonlocal in spacetime — it’s really the entire trajectory through time that seems to matter, not just what is happening in our local neighborhood. That’s a crucial difference between their version of quantum mechanics and the conventional formulation. But it’s not completely bizarre or unprecedented. Plenty of hints we have about quantum gravity indicate that it really is nonlocal. More prosaically, in everyday statistical mechanics we don’t assign equal weight to every possible trajectory consistent with our current knowledge of the universe; by hypothesis, we only allow those trajectories that have a low entropy in the past. (As readers of this blog should well know by now; and if you don’t, I have a book you should definitely read.)

    To make progress with this idea, you have to make a choice for what the imaginary part of the action is supposed to be. Here, in the eyes of this not-quite-expert, NN seem to cheat a little bit. They basically want the imaginary action to look very similar to the real action, but it turns out that this choice is naively ruled out. So they jump through some hoops until they get a more palatable choice of model, with the property that it is basically impotent except where the Higgs boson is concerned. (The Higgs, as a fundamental scalar, interacts differently than other particles, so this isn’t completely ad hoc — just a little bit.) Because they are not actually crackpots, they even admit what they’re doing — in their own words, “Our model with an imaginary part of the action begins with a series of not completely convincing, but still suggestive, assumptions.”

    Having invoked the tooth fairy twice — contemplating an imaginary part of the action, then choosing its form so as to only be relevant where the Higgs is concerned — they consider consequences. Remember that the effect of the imaginary action is non-local in time — it depends on what happens throughout the history of the universe, not just here and now. In particular, given their assumptions, it provides a large suppression to any history in which large numbers of Higgs bosons are produced, even if they won’t be produced until some time in the future.

    (more…)