Sean Carroll

Category: Science

  • Guest Post: Jim Kakalios on the Quantum Mechanics of Source Code

    Jim Kakalios of the University of Minnesota has achieved internet demi-fame — he has a YouTube video with over a million and a half views. It’s on the science of Watchmen, the movie based on Alan Moore’s graphic novel. Jim got that sweet gig because he wrote a great book called The Science of Superheroes — what better credentials could you ask for?

    More recently Jim has written another book, The Amazing Story of Quantum Mechanics. But even without superheroes in the title, everything Jim thinks about ends up being relevant to movies before too long. The new movie Source Code features a twist at the end that involves — you guessed it — quantum mechanics. Jim has applied his physicist super-powers to unraveling what it all means, and was kind enough to share his thoughts with us in this guest post.

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    There is an interesting discussion taking place on the internets concerning the ending of the newly released film SOURCE CODE, that suggests that the film concludes with a paradox. I believe that any such paradox can be resolved – with Physics!

    This entire post is one big honkin’ SPOILER, so if you want to read about the final twist ending of a film without having seen said film – by all means, read on, MacDuff!

    In SOURCE CODE, Jake Gyllenhaal plays US helicopter pilot Colter Stevens, whose consciousness is inserted into another man’s body (Sean Fentress, a school teacher in Chicago) through a procedure that requires a miracle exception from the laws of nature (involving quantum mechanics and “parabolic calculus” – by the way, there is no such thing as parabolic calculus). Thanks to some technobabble (or as Q-Bert on Futurama would describe it – weapons grade bolognium) Colter’s mind can only enter Sean’s body in the last eight minutes of Sean’s life. As Sean is sitting on a city bound Chicago commuter train, on which a bomb will explode at 7:58 AM, killing everyone aboard, the goal is for Colter to ascertain who planted the bomb. He cannot stop it from exploding, he is told, because that has already happened. He cannot affect the past, but he can bring information obtained in the past back to his present time. Learning the identity of the bomber would enable the authorities to prevent the detonation of a threatened second “dirty atomic” bomb is downtown Chicago.

    While the above can be discerned from the movie trailer, what I am going to discuss next involves the actual ending of the film, and if you do not want this ending spoiled, you should stop reading now. (more…)

  • Open Science

    [Note: this post was published prematurely, then deleted, and is now back.]

    Michael Nielsen gave a great talk at TEDxWaterloo about the idea of “open science”:

    Open science: Michael Nielsen at TEDxWaterloo

    There’s a great deal of buzz about “openness” in certain sectors of the science community; largely this has passed physics and astronomy by, because we’re already pretty darn open. It’s hard to image something more open than arxiv, where everyone puts their papers up for free even before they’re published in a journal.

    But Michael’s talking about something much more ambitious: opening the process of creating science, not just publishing it. For experimentalists this would be difficult, for obvious reasons. (You think people who sweat to build an experiment are going to invite the public in to take a whirl?) For theory it is also hard, but the reasons are more subtle.

    The point is that credit in science is given out on the basis of getting your name on published papers. In the arxiv era, the papers don’t necessarily have to appear in a traditional journal — but that’s a topic for another day. The model is set in stone: you have an idea, you work out its consequences to the point where it’s publishable, and you write a paper. Without that last step, you’re not going to get any credit. (Very occasionally you will see references to “unpublished work” or “private communication,” but it’s rare and not really for big-ticket ideas.)

    So if I had an idea, I would either work it out myself or start working with students or collaborators. I certainly would not go around publicizing an undeveloped idea; I wouldn’t get any credit for it, and someone else could take it and develop it themselves. I might give seminars in which I mention the idea, but that’s only recommended once it’s to the point where a paper is on the horizon.

    Michael and others want to overthrow that model. Their shining example is this blog post by Tim Gowers. Gowers is a mathematician who proposed attacking an open math problem right there on his blog, by asking for comments from the crowd. If they succeeded, they could publish a paper under a collective pseudonym. He next chose a problem — developing a combinatorial approach to the Hales-Jewett theorem — and, several hundred comments later, announced that they had succeeded. Here’s the paper. Buoyed by this success, people have set up a Polymath Wiki to expedite tackling other problems in this way.

    Could this work for theoretical physics? I don’t see why not. But note that Michael spends a lot of his time in the talk pointing out the obvious — crowdsourcing doesn’t always work. I could easily imagine ways in which such a project could fail; too much noise and not enough signal, everyone with good ideas deciding they would rather work on them by themselves rather than sharing openly, etc.

    Might be worth a shot, though. I’m thinking of suggesting some ideas here on this blog and seeing whether we get any useful input. Let me sleep on it.

  • Science is Hard

    The 3-sigma bump reported by Fermilab on Wednesday has garnered a lot of attention. Understandable, since it might be a precious sign of particle physics beyond the Standard Model — but it’s also just a 3-sigma bump, and usually those go away.

    Via Matt Strassler and Lisa Randall, here’s a set of plots that helps indicate exactly how hard this game really is. The plots can all be found on this web page at CERN. In comments Matt and Peter note that they were made by Tommaso Tabarelli de Fatis, as explained on Tommaso Dorigo’s blog. Here is the original plot from the CDF paper:

    We’re looking at the number of events that produce a W boson and two jets, as a function of the energy of the jets. The bottom plot is all the data, while at the top they’ve subtracted off most of the Standard Model background, leaving only the predicted red curve from WW/WZ events. You see the extra little bump around 150 GeV, that’s what’s getting everyone so excited. It’s unlikely that the data are a good fit to the prediction; the “KS (Kolmogorov-Smirnov) probability” is given as 5×10-5, which means that it’s not bloody likely.

    But, just for giggles, let’s imagine that the energy of the jets wasn’t measured very accurately. Obviously the experimenter worked hard to get it right, and I would trust their judgment over my own any day, but you never know. Jets are complicated things with many particles in them, you can imagine being off by a bit.

    So here is the same plot, just scaling the jet energy by two percent: (more…)

  • NASA Gives Up on LISA

    Sorry to bump Julianne’s fun post further down the page, but lots of news today. This particular piece of news is not fun: NASA is abandoning LISA, the planned Laser Interferometer Space Antenna, as well as IXO, an X-ray satellite observatory (formerly “Constellation X”). Steinn has some of the ugly details. Short story: money is tight, and the James Webb Space Telescope is taking all of it. (Not that JWST is completely immune from danger itself…)

    LISA is not completely dead: the European Space Agency will keep the planning alive. But this is a serious step, not just a feint in a budget negotiation; the LISA International Science Team is being disbanded, told to pack up and go home. Hopefully the ESA will continue to push forward, and individual researchers in the US can somehow find money to still think about gravitational-wave astrophysics from space. It’s possible that a smaller mission could be put forward, but it’s not as if NASA has extra money they’re looking to spend right now.

    Of all the concepts for big astrophysics missions in space, LISA is my favorite. Unlike LIGO, which strains as hard as possible and hopefully will detect something once its upgraded, LISA would be bombarded with gravitational waves, and the trick will be picking out the interesting signals from above the ambient noise. (That’s a problem we don’t mind having.) I was part of the original Beyond Einstein roadmap team (pdf) that packaged LISA and Constellation-X together with a dark energy mission to create an ambitious but realistic plan for NASA cosmology that Congress and the OMB could get behind. That was in 2002, before wars and tax cuts and financial catastrophes sapped the government of its ability to pay for anything. The best-laid plans of mice and men and NASA panels, as the saying goes.

    LISA’s science is not just achievable, it’s incredibly interesting. It would detect thousands of binary systems within our galaxy, as well as numerous inspirals of middleweight black holes into supermassive ones in other galaxies, giving us incredibly detailed access to the spacetime metric near a black hole. As a side benefit, the wavelength is just right for looking at gravitational waves that might be produced in the early universe if the electroweak phase transition is especially violent. I remember giving a talk to particle physicists planning the International Linear Collider (another possibly doomed endeavor) back in 2003. It was great to see their eyes light up when I told them about this connection between satellite observatories and particle accelerators — at a meeting dominated by budget worries, it was a tiny oasis of actual science.

    Hopefully things will somehow work out, but there’s not a lot of reason for optimism at the moment. We’ll see how things go.

  • Anomalies at Fermilab

    The Tevatron accelerator at Fermilab is shutting down soon, for some unavoidable reasons (the LHC is taking over) and some frustrating ones (we’re out of money). But there may be life in the old beast yet; a couple of intriguing anomalies have particle theorists raising their eyebrows in charmingly understated excitement.

    Two different anomalies are getting attention right now. One, which has been around for a while but doesn’t seem to be going away, is a forward-backward asymmetry in top quark production. Unlike the LHC, which just smashes protons together, the Tevatron has a proton beam and an antiproton beam. Intriguingly, when collisions produce a top-antitop pair, they seem to be preferentially produced in the direction of the protons rather than the antiprotons. If you want a popular-level account, here’s Ron Cowen at Science News, while Jester gives you the technical details at Résonaances. I’ll just show you a pretty picture; the horizontal axis is “forward” cross section, while the vertical axis is the “backward” cross section, both in units of the Standard Model expectation. The center of the plot is where we should be, and as you can see we are just a bit off.

    A completely different anomaly seems to have just cropped up, this time in collisions that produce a W boson as well as two “jets” (particle-physics speak for “bunches of particles typically associated with the production of strongly-interacting stuff). Once again we have explanations in the MSM and on the blogs: Dennis Overbye at the NYT, and Flip Tanedo at US LHC Blogs. What happens here is that you just measure how many events you see as a function of how much energy is in the jets. Then you look for a “bump,” which as John has taught us is often a signature of a new particle that has been produced and then quickly decayed. Do you see the bump?

    (more…)

  • Quantum Smell

    Over on the Facebooks, Matt Strassler points to a BBC story about the role of quantum mechanics in explaining our sense of smell. There aren’t any equations in the article, and I haven’t read the research papers, but the idea seems to be that electrons move from one part of a protein to another part via quantum tunneling. The potential that allows this to happen is only set up if you have the right chemical involved, which is how the protein purportedly “smells” the existence of this chemical. The resulting mechanism is just absurdly sensitive — apparently fruit flies can smell the difference between hydrogen and deuterium (chemically identical, but tiny differences in atomic energy levels from having an extra neutron in the nucleus).

    It’s still a controversial theory, but apparently not crackpotty. The question of how important quantum mechanics (as opposed to just its classical limit) is for biological processes was brought up in our earlier post on quantum photosynthesis. Which reminds me in turn of this worthwhile talk by Seth Lloyd, on the basic topic of “quantum life” and photosynthesis in particular. In between learning about how quantum phenomena might remain relevant in the hot, warm environment of a plant, you can enjoy Lloyd’s principled stance not to use PowerPoint under any circumstances.

    Seth Lloyd on Quantum Life

  • Crawling Into Consciousness

    We’re not very good at defining what “consciousness” is, although we think we know it when we see it. One promising avenue of attack on the problem is to consider how consciousness may have developed over the course of the evolution of life. There’s a great blog post about this by Malcolm MacIver over on our sibling blog Science Not Fiction. He is thinking about an obviously-important event in the history of life — the moment when aquatic organisms first flapped up onto land and starting breathing air, if we may greatly simplify a complicated process — and asking about its consequences for consciousness.

    The idea is one of those deceptively simple ones that makes you wonder why you didn’t think of it all along. The point is: attenuation lengths. In water, you just can’t see very far; your vision becomes blurry after a matter of meters. Consequently, you don’t have much time — maybe seconds — to react to the world around you, whether what you see is prey, a danger, or a potential love interest. So the evolutionary pressure is to “make up your mind” extremely quickly, essentially right away.

    Now imagine you crawl up into the air. Suddenly, you can see for kilometers! Now a different mode of action becomes useful: thinking about hypothetical alternatives. Under water, too much Hamlet-like equivocation would have made you someone’s dinner before long; now, you can ask yourself whether it would be better to duck under a rock, scurry up a tree, or finally take a stand against that big bully.

    The ability to contemplate competing alternatives before making a decision is a crucial part of what we call consciousness. It’s related to another idea I believe I first got from Steven Pinker’s The Language Instinct, although I don’t remember the precise passage: the claim that what really separates the conscious from the non-conscious is the ability to use grammar. In particular, the subjunctive mood, in which we talk about hypothetical futures. (“If I were to go and bring you back some tasty fish, would you let me live?”) Lots of animals can communicate using something like “language,” but the ability to make agreements based on contrary-to-fact scenarios is what separates the shouters from the negotiators. And of course, the ability to contemplate hypothetical scenarios is an important prior step to being able to communicate about them.

    Be sure to read the comments, where man good questions are asked (“What about octopuses?” “Aren’t there senses other than vision?”) and also answered. Malcolm also provocatively tries to imagine what it would mean if we vastly improved our sensory capabilities, which of course technology is doing for us all the time. What’s next after consciousness?

  • LIGO to Collaboration Members: There Is No Santa Claus

    Ah, the life of an experimental physicist. Long hours of mind-bending labor, all in service of those few precious moments in which you glimpse one of Nature’s true secrets for the very first time. Followed by the moment when your bosses tell you it was all just a trick.

    Not that you didn’t see it coming. As we know, the LIGO experiment and its friend the Virgo experiment are hot on the trail of gravitational waves. They haven’t found any yet, but given the current sensitivity, that’s not too much of a surprise. Advanced LIGO is moving forward, and when that is up and running the situation is expected to change.

    But who knows? We could be surprised. It’s certainly necessary to comb through the data looking for signals, even if they’re not expected at this level of sensitivity.

    Of course, there is something of a bias at work: scientists are human beings, and they want to find a signal, no matter how sincerely they may rhapsodize about the satisfaction of a solid null result. (Do the words “life on a meteorite” mean anything to you?) So, to keep themselves honest and make sure the data-analysis pipeline is working correctly, the LIGO collaboration does something sneaky: they inject false signals into the data. This is done by a select committee of higher-ups; the people actually analyzing the data don’t know whether a purported signal they identify is real, or fake. It’s their job to analyze things carefully and carry the whole process through, right up to the point where you have written a paper about your results. Only then is the truth revealed.

    Yesterday kicked off the LIGO-Virgo collaboration meeting here in sunny Southern California. I had been hearing rumors that LIGO had found something, although everyone knew perfectly well that it might be fake — that doesn’t prevent the excitement from building up. Papers were ready to be submitted, and the supposed event even had a colorful name — “Big Dog.” (The source was located in Canis Major, if you must know.)

    Steinn Sigurðsson broke the news, and there’s a great detailed post by Amber Stuver, a member of the collaboration. And the answer is: it was fake. Just a drill, folks, nothing to see here. That’s science for you.

    When the real thing comes along, they’ll be ready. Can’t wait.

  • Fractal Black Holes on Strings

    Here’s a fascinating new result about black holes in five dimensions — actually from last October, but I missed it when it came out. I just noticed it this week because of a write-up by Gary Horowitz in Matters of Gravity, the newsletter of the gravity group of the American Physical Society. (I obviously missed David Berenstein’s post as well.)

    You might be thinking that black holes in five dimensions can’t be that interesting, since they are probably pretty similar to black holes in four dimensions, and after all we don’t live in five dimensions. But of course, there could be a fifth dimension of space that is compactified on a tiny circle. (Of course.) So then you have to consider two different regimes: the size of the circle is much larger than the size of the black hole — in which the fact that it’s compact doesn’t really matter, and you just have a regular black hole in five dimensions — or the size of the circle is smaller than the black hole — in which case, what?

    The answer is that you get a black string — a cylindrical configuration that stretches across the extra dimension. This was figured out a long time ago by Ruth Gregory and Raymond LaFlamme. But they were also clever enough to ask — what if you had that kind of cylindrical black hole, but it stretched across a relatively large extra dimension? That sounds like a configuration you can make, but it might be unstable — wiggles in the string could grow, leading it to pinch off into a set of distinct black holes. One way of seeing that something like that is likely is to calculate the entropy of each configuration; for long enough black strings, the entropy is lower than a collection of black holes with the same mass, and entropy tends to grow. Indeed, Gregory and LaFlamme showed that long black strings are unstable. However, it wasn’t clear what exactly would ultimately happen to them. (more…)

  • Dark Matter: Just Fine, Thanks

    Astrophysical ambulance-chasers everywhere got a bit excited this week, and why wouldn’t they? Here are some of the headlines we read:

    Wow. More evidence against dark matter? I didn’t know about the original evidence.

    Sadly (and I mean that — see below) there is no evidence against dark matter here. These items were sparked by a paper and a press release from Maryland astronomer Stacy McGaugh, with the rather more modest titles “A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies” and “Gas rich galaxies confirm prediction of modified gravity theory,” respectively.

    I’m the first person to defend journalists against unfair attacks, and we all know that headlines are usually not written by the people who write the actual articles. But we can legitimately point fingers at a flawed system at work here: these articles are a tiny but very clear example of what is wrong wrong wrong about our current model for informing the public about science.

    McGaugh’s new paper doesn’t give any evidence at all against dark matter. What it does is to claim that an alternative theory — MOND, which replaces dark matter with a modification of Newtonian dynamics — provides a good fit to a certain class of gas-rich galaxies. That’s an interesting result! Just not the result the headlines would have you believe.

    It’s obvious what happens here. Nobody would read an article entitled “Gas rich galaxies confirm prediction of modified gravity theory” — or at least, most editors doubtless feel, fewer people would be interested in that than in evidence that went directly against dark matter. So let’s just spice up the story a bit by highlighting the most dramatic possible conclusion we can imagine drawing, and burying the caveats until the end. Net result: a few more people read the articles than otherwise would have, while many more people just read the headlines and are left with less understanding of modern cosmology than they started with. Scientists and journalists together have a responsibility to do a better job than this at making things clear, not just making things sound exciting.

    But let me take this opportunity to lay out the problems with MOND. (more…)