Sean Carroll

Author: Sean Carroll

  • Preaching to the Unconverted

    And now for something somewhat different. After I posted my article on “Does the Universe Need God?“, there were a few responses at the Intelligent Design blog Uncommon Descent, including a list of questions by Vincent Torley. Vincent then went the extra mile by inviting me to write a guest post for UD. Not my usual stomping grounds, but I ultimately agreed, precisely for that reason.

    Here’s the post, which I’m cross-posting below. This might be controversial, as a lot of people on my side of things will say that there’s little point in engaging with people on the other side. And admittedly, this is a subject where feelings can be pretty entrenched. But you never know — not everyone has their mind made up on every issue, and it’s good to try to explain yourself to unsympathetic audiences on occasion. That’s all I tried to do here — to explain how I think about these things, not necessarily to pick a fight or even persuade any skeptics. I tried pretty hard to be as clear and unpretentious as I can be. (Success is for you to decide.) In a world of shouting and diatribe, I remain optimistic that real communication can occasionally occur! We’ll see how it goes.

    ——

    I wanted to thank Vincent Torley and Denyse O’Leary for the opportunity to write a guest blog post, and apologize for how long it’s taken me to do so. I’ve written an article for the forthcoming Blackwell Companion to Science and Christianity, entitled Does the Universe Need God?, in which I argued that the answer is “no.” Vincent posed a list of questions in response. After thinking about it, I decided that my answers would be more clear if I simply wrote a coherent argument, rather than addressing the questions individually.

    My goal is to try to explain my own thinking to an audience that is not predisposed to agree. We can roughly break people up into two groups: naturalists such as myself, who think that the best explanation we have for the universe involves physical quantities obeying laws of Nature and nothing else; and those who believe that a better explanation can be found by invoking a powerful being/designer/creator/God. (For the sake of simplicity I’m going to use “God” to refer to this notion, but feel free to substitute the more accurate description of your choice.) Obviously there are many nuances that are being passed over by this simple distinction, but hopefully it will suffice for this moment.

    The dispute between these two camps isn’t one where people often change their minds at the drop of an argument. Minds do change, in either direction — but typically after extended periods of reflection, not suddenly in response to a single killer blog post. So persuasion is not my goal here; only explanation. I’ve succeeded if an open-minded person who disagrees with me reads the post and still disagrees, but at least understands why I hold my positions. (After giving an earlier talk, one of the theologians in the audience told me that I had persuaded him — not that God didn’t exist, but that the argument from design wasn’t the way to get to Him. That sort of real-time response is more than one can generally hope for.)

    What I want to do is to elaborate on some crucial aspects of how science is done that bear directly on the issues raised by my article and some of the responses to it that I’ve seen. In particular, I want to talk about simplicity, laws, openness, explanation, and clarity. This isn’t supposed to be a comprehensive treatise on the philosophy of science, nor is it especially rigorous, or anything really new — just some thoughts on issues relevant to this conversation.

    I will be taking one thing for granted: that what we’re interested in doing here is science. There are many kinds of consideration that may lead people to theism or atheism that have nothing whatsoever to do with science; likewise, one may believe that there are ways of understanding the natural world that go beyond the methods of science. I have nothing to say about that right now; that’s a higher-level discussion. I’m just going to presume that we all agree that we’re trying to be the best scientists we can possibly be, and ask what that means.

    With all that throat-clearing out of the way, here’s what I have to say about these five issues.

    (more…)

  • DNA Takes Square Roots

    Around these parts we’ve been known to discuss whether it makes any sense to say that the universe is a computer. There’s little doubt, of course, that parts of the universe are computer-like. And in case you are wondering, you can now officially remove DNA from your personal list of “things I suspect are not computers.”

    Caltech researchers Lulu Qian and Erik Winfree have managed to coax 130 strands of DNA into performing what is unquestionably a calculation: taking the square root of a number. (Ars Technica post; Science paper behind paywall; open-access background paper.) Not a big number: we’re talking about four-digit binary numbers, so 15 at the biggest. And not very efficiently: with prodding, the calculation took eight hours. Moore’s Law isn’t really in danger here.

    Still, pretty cool stuff. Mostly it’s interesting because it seems scalable: the authors claim that this kind of circuit architecture could be made much larger. It’s not the first biochemical circuit; RNA and bacterial colonies have been made into logic gates. But it’s the first to do something as elaborate as taking a square root.

    Best of all, the authors decided to illustrate their method for a wide audience by means of a … whimsical YouTube video! Let’s hope this idea catches on.

    The seesaw magic book: the computational power of DNA molecules

  • Unsolicited Advice: Non-Academic Careers

    Since I know nothing very useful about the job market outside academia, I solicited suggestions for specific pointers and helpful websites. A bushel of useful advice and thought-provoking comments resulted.

    My original idea was to summarize what I thought was the best advice, and turn it into a single post. This idea has been undermined by (1) me not knowing which advice is best, and (2) a wide variety of occasionally-contradictory advice, presumably all applicable in different circumstances.

    So instead here I’m just going to link to some of the most promising-looking resources that were mentioned. I encourage you to read the comments on the original post to get more ideas, and chime in here to keep the conversation going.

    Collections of Online Resources

  • Anomaly at the Tevatron Might Be Something Real?

    The Tevatron, Fermilab’s mighty but ancient (as these things go) particle accelerator, is scheduled to be shut down at the end of this year. But the old beast might have a trick or two left yet.

    Way back in April we talked about a couple of lingering anomalies in the Tevatron data that had risen to the level where theorists were intrigued enough to start building models. One of these — a forward/backward asymmetry in top-quark interactions — had been around for a while, and was taken seriously by a number of people. The other — a tiny bump near 150 GeV in the total number of events that produce a W boson and two jets — was relatively new, and was greeted by a bit of scoffing. The bump credibility took another hit when it was pointed out that it could be explained away by a simple (although completely hypothetical) systematic error — a miscalibration of the jet energies. Bump-hunting is hard, and experiments near the end of their lifetimes are more willing to share their anomalies than they would be if they knew they were going to keep going, since there’s little hope that new data will solve the problem.

    But there’s some hope. The real reason to be patient rather than excited by the bump at 150 GeV was that it was a 3-sigma effect, in a game where most 3-sigma effects go away. In particle physics, we generally take a solid 3-sigma result as “evidence for” something, and require 5 sigma — a much greater deviation from the expected numbers — to declare something a “discovery.”

    More data are now in! This is from the CDF experiment at Fermilab, as reported in a conference talk by Giovanni Punzi (pdf), and shared worldwide by Jester at Résonaances. There’s a reason why I mentioned Résonaances among the physics blogs above — it’s unquestionably the go-to place for new results in particle physics.

    And the anomaly is now — almost five sigma! It didn’t go away with more data, it became more prominent. It would be very hard at this point to simply attribute it to an energy miscalibration or something like that; if it is a systematic error, it’s a subtle one. But it doesn’t look like an error; it looks like a signal.

    Of course, it’s still very possible that it will go away. These things usually do. But when an interesting result is pushing five sigma, it’s perfectly okay to get a bit excited and start wondering what’s going on. One of the nice things about this bump is that it’s not very hard to come up with models that can explain it — all you need is a neutral boson, similar to the well-known Z boson of the weak interactions, with a mass near 150 GeV. This kind of idea is so well-known in the trade that it already has a name — the Z’ boson, imaginatively enough.

    Except it’s not that simple, of course — where would be the fun? When you start mindlessly adding new particles to the Standard Model, you have to check consistency with all sorts of experimental constraints. In particular, a naive Z’ boson would sometimes decay into leptons as well as quarks (the jets mentioned above). In that case, it would have been seen long ago in LEP, the electron-positron collider at CERN that previously lived in what is now the LHC’s tunnel. So what you really need is a “leptophobic” Z’, one that decays into quarks but not into leptons.

    Or something along those lines, or something completely different. See Résonaances once again for the lay of the theoretical land. Yes, there are possible explanations within supersymmetry; and yes, there are explanations that have nothing to do with supersymmetry.

    If this is real — still a very, very, big if — it’s the beginning of the “beyond the Standard Model era” in collider particle physics. Things aren’t going to snap into place overnight; there will be false starts, mysteries, and sudden epiphanies. That’s where the real fun is in science.

    Update: Note that the very preliminary word from the LHC is that they don’t yet see the same bump that CDF does. But from a glance at the figure it doesn’t look like they have nearly as much data yet, so that’s probably not surprising. The LHC has seen incredible jumps in luminosity recently, however, so they should be able to do a proper check before too long.

  • Best Science Blogging of the Year

    Okay we’re a little late with this, so be quick if you want to participate: it’s time for the Quark, the 3quarksdaily annual prize for science blogging. The deadline for nominations is tomorrow (Tuesday) night, so hurry up and nominate if you are so moved! This year’s judge is Lisa Randall — great to see a top-notch physicist in there.

    Part of the process involves a vote by readers, which I think is something that just doesn’t work on the internet. Bloggers with large followings and sufficient shamelessness to prod them into voting will always dominate over the negligible number of readers who actually read every post and try to make a fair decision. But so be it — it’s not stopping me from nominating one of my own posts! (I can’t imagine that anyone else keeps track of all the science blogging I’ve done over the last year.) But it would be great if the winner came from one of the other awesome bloggers out there. Just to pick a few semi-randomly, let me steer potential nominators to have a look at some of my favorite blogs:

    Sorry to all the great blogs I’m not including, this isn’t meant to be an exhaustive list. If you think I’m missing something, go nominate it! And then upbraid me in the comments here for my lack of fairness and good taste.

    And while we’re on the subject, Open Lab 2011 is also open for nominations. This is an ongoing process through 2011, so there’s no hurry — keep your eyes peeled for good blogging out there. Many submissions will be chosen to be collected into a published anthology, and this year they have a serious publisher — Scientific American Books, an imprint of Farrar, Straus and Giroux. The editor will be the lovely and talented Jennifer Ouellette, so being included carries an extra cachet this year.

  • Gil Scott-Heron

    Gil Scott-Heron has died at 62. I could mention how his spoken-word recordings were a noted precursor of hip-hop, but then the Onion would make fun of me.

    http://www.youtube.com/watch?v=_b2F-XX0Ol0

    Gil Scott-Heron – 'Where Did The Night Go'

  • Are Many Worlds and the Multiverse the Same Idea?

    When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea. There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea. (I think the branes are still a truly distinct notion.)

    At first blush it seems crazy — or at least that was my own initial reaction. When cosmologists talk about “the multiverse,” it’s a slightly poetic term. We really just mean different regions of spacetime, far away so that we can’t observe them, but nevertheless still part of what one might reasonably want to call “the universe.” In inflationary cosmology, however, these different regions can be relatively self-contained — “pocket universes,” as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different; they can have different particles, different forces, even different numbers of dimensions. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime.

    The situation in quantum mechanics is superficially entirely different. Think of Schrödinger’s Cat. Quantum mechanics describes reality in terms of wave functions, which assign numbers (amplitudes) to all the various possibilities of what we can see when we make an observation. The cat is neither alive nor dead; it is in a superposition of alive + dead. At least, until we observe it. In the simplistic Copenhagen interpretation, at the moment of observation the wave function “collapses” onto one actual possibility. We see either an alive cat or a dead cat; the other possibility has simply ceased to exist. In the Many Worlds or Everett interpretation, both possibilities continue to exist, but “we” (the macroscopic observers) are split into two, one that observes a live cat and one that observes a dead one. There are now two of us, both equally real, never to come back into contact.

    These two ideas sound utterly different. In the cosmological multiverse, the other universes are simply far away; in quantum mechanics, they’re right here, but in different possibility spaces (i.e. different parts of Hilbert space, if you want to get technical). But some physicists have been musing for a while that they might actually be the same, and now there are a couple of new papers by brave thinkers from the Bay Area that make this idea explicit.

    Physical Theories, Eternal Inflation, and Quantum Universe, Yasunori Nomura

    The Multiverse Interpretation of Quantum Mechanics, Raphael Bousso and Leonard Susskind

    Related ideas have been discussed recently under the rubric of “how to do quantum mechanics in an infinitely big universe”; see papers by Don Page and another by Anthony Aguirre, David Layzer, and Max Tegmark. But these two new ones go explicitly for the “multiverse = many-worlds” theme.

    After reading these papers I’ve gone from a confused skeptic to a tentative believer. This happened for a very common reason: I realized that these ideas fit very well with other ideas I’ve been thinking about myself! So I’m going to try to explain a bit about what is going on. However, for better or for worse, my interpretation of these papers is strongly colored by my own ideas. So I’m going to explain what I think has a chance of being true; I believe it’s pretty close to what is being proposed in these papers, but don’t hold the authors responsible for anything silly that I end up saying. (more…)

  • Happy Birthday Bob Dylan

    Seventy years old. Wow.

    Bob Dylan – Knockin' On Heaven's Door (MTV Unplugged)

    I recognize the first questioner at this press conference. I’m pretty sure he’s been at some of my own talks.

    http://youtu.be/6efuxHTiNwE

  • Physics and the Immortality of the Soul

    [Cross-posted at Scientific American Blogs. Thanks to Bora Z. for the invitation.]

    The topic of “Life after death” raises disreputable connotations of past-life regression and haunted houses, but there are a large number of people in the world who believe in some form of persistence of the individual soul after life ends. Clearly this is an important question, one of the most important ones we can possibly think of in terms of relevance to human life. If science has something to say about, we should all be interested in hearing.

    Adam Frank thinks that science has nothing to say about it. He advocates being “firmly agnostic” on the question. (His coblogger Alva Noë resolutely disagrees.) I have an enormous respect for Adam; he’s a smart guy and a careful thinker. When we disagree it’s with the kind of respectful dialogue that should be a model for disagreeing with non-crazy people. But here he couldn’t be more wrong.

    Adam claims that “simply is no controlled, experimental[ly] verifiable information” regarding life after death. By these standards, there is no controlled, experimentally verifiable information regarding whether the Moon is made of green cheese. Sure, we can take spectra of light reflecting from the Moon, and even send astronauts up there and bring samples back for analysis. But that’s only scratching the surface, as it were. What if the Moon is almost all green cheese, but is covered with a layer of dust a few meters thick? Can you really say that you know this isn’t true? Until you have actually examined every single cubic centimeter of the Moon’s interior, you don’t really have experimentally verifiable information, do you? So maybe agnosticism on the green-cheese issue is warranted. (Come up with all the information we actually do have about the Moon; I promise you I can fit it into the green-cheese hypothesis.)

    Obviously this is completely crazy. Our conviction that green cheese makes up a negligible fraction of the Moon’s interior comes not from direct observation, but from the gross incompatibility of that idea with other things we think we know. Given what we do understand about rocks and planets and dairy products and the Solar System, it’s absurd to imagine that the Moon is made of green cheese. We know better.

    We also know better for life after death, although people are much more reluctant to admit it. (more…)

  • arxiv Find: Breakdown of Classical Gravity?

    The single most interesting feature of attempts to replace dark matter with a modification of gravity is Milgrom’s discovery that in a wide variety of galaxies, there’s a unique place where ordinary gravity plus ordinary matter stops working: when the acceleration due to gravity (as Newton would have calculated it) drops below a fixed value a0 ≈ 10−10 m/s2. This is the basis of MOND, but the pattern itself is arguably more interesting than any current attempt to account for it. Very possibly it can be explained by the complicated dynamics of baryons and dark matter in galaxies — but in any event it should be explained somehow.

    The existence of this feature gives a strong motivation for testing gravity in the regime of very tiny accelerations. Note that this isn’t even a statement that makes sense in general relativity; particles move on geodesics, and the “acceleration due to gravity” is always exactly zero. So implicitly we’re imagining some global inertial frame with respect to which such acceleration can be measured. That’s a job for a future theory to make sense of; for the moment we’re forgetting that we know GR and thinking like Newton would have.

    So now Hernandez, Jimenez, and Allen have tried to test gravity in this weak-acceleration regime — and they claim it fails!

    The Breakdown of Classical Gravity?
    X. Hernandez, M. A. Jimenez, C. Allen

    Assuming Newton’s gravity and GR to be valid at all scales, leads to the dark matter hypothesis as a forced requirement demanded by the observed dynamics and measured baryonic content at galactic and extra galactic scales. Alternatively, one can propose a contrasting scenario where gravity exhibits a change of regime at acceleration scales less than $a_{0}$, and obtain just as good a fit to observations across astrophysical scales. A critical experiment in this debate is offered by wide orbit binary stars. Since for $1 M_{odot}$ systems the acceleration drops below $a_{0}$ at scales of around 7000 AU, an statistical survey of relative velocities and binary separations reaching beyond $10^{4}$ AU should yield a conclusive answer to the above debate. By performing such a study we show Kepler’s third law to fail precisely beyond $a approx a_{0}$ scales, precisely as predicted by modified gravity theories designed not to require any dark matter at galactic scales and beyond.

    Color me dubious, but interested in seeing further studies. It’s very hard to collect this kind of data, and note that it’s just a statistical survey of velocities, not a precise measurement of individual systems. In principle a statistical survey is fine; in practice, it opens up the possibility of hidden subtle systematic effects.

    Still, intriguing and worth checking out. Any time you have the chance to overthrow Sir Isaac Newton, you go for it.