Sean Carroll

Category: Science

  • What the World Is Made Of

    I know you’re all following the Minute Physics videos (that we talked about here), but just in case my knowledge is somehow fallible you really should start following them. After taking care of why stones are round, and why there is no pink light, Henry Reich is now explaining the fundamental nature of our everyday world: quantum field theory and the Standard Model. It’s a multi-part series, since some things deserve more than a minute, dammit.

    Two parts have been posted so far. The first is just an intro, pointing out something we’ve already heard: the Standard Model of Particle physics describes all the world we experience in our everyday lives.

    Theory of Everything (intro)

    The second one, just up, tackles quantum field theory and the Pauli exclusion principle, of which we’ve been recently speaking. (Admittedly it’s two minutes long, but these are big topics!)

    Theory of Everything: What is Matter?

    The world is made of fields, which appear to us as particles when we look at them. Something everyone should know.

  • Everything is Connected

    They do things differently over in Britain. For one thing, their idea of a fun and entertaining night out includes going to listen to a lecture/demonstration on quantum mechanics and the laws of physics. Of course, it helps when the lecture is given by someone as charismatic as Brian Cox, and the front row seats are filled with celebrities. (And yes I know, there are people here in the US who would find that entertaining as well — I’m one of them.) In particular, this snippet about harmonics and QM has gotten a lot of well-deserved play on the intertubes.

    More recently, though, another excerpt from this lecture has been passed around, this one about ramifications of the Pauli Exclusion Principle. (Headline at io9: “Brian Cox explains the interconnectedness of the universe, explodes your brain.”)

    Brian Cox – Everything is Connected to Everything

    The problem is that, in this video, the proffered mind-bending consequences of quantum mechanics aren’t actually correct. Some people pointed this out, including Tom Swanson in a somewhat intemperately-worded blog post, to which I pointed in a tweet. Which led to some tiresome sniping on Twitter, which you can dig up if you’re really fascinated. Much more interesting to me is getting the physics right.

    One thing should be clear: getting the physics right isn’t easy. For one thing, going from simple quantum problems of a single particle in a textbook to the messy real world is often a complicated and confusing process. For another, the measurement process in quantum mechanics is famously confusing and not completely settled, even among professional physicists.

    And finally, when one translates from the relative clarity of the equations to a natural-language description in order to reach a broad audience, it’s always possible to quibble about the best way to translate. It’s completely unfair in these situations to declare a certain popular exposition “wrong” just because it isn’t the way you would have done it, or even because it assumes certain technical details that the presenter did not fully footnote. It’s a popular lecture, not a scholarly tome. In this kind of format, there are two relevant questions: (1) is there an interpretation of what’s being said that matches the informal description onto a correct formal statement within the mathematical formulation of the theory?; and (2) has the formalism been translated in such a way that a non-expert listener will come away with an understanding that is reasonably close to reality? We should be charitable interpreters, in other words.

    In the video, Cox displays a piece of diamond, in order to illustrate the Pauli Exclusion Principle. The exclusion principle says that no two fermions — “matter” particles in quantum mechanics, as contrasted with the boson “force” particles — can exist in exactly the same quantum state. This principle is why chemistry is interesting, because electrons have to have increasingly baroque-looking orbitals in order to be bound to the same atom. It’s also why matter (like diamond) is solid, because atoms can’t all be squeezed into the same place. So far, so good.

    But then he tries to draw a more profound conclusion: that interacting with the diamond right here instantaneously affects every electron in the universe. Here’s the quote: (more…)

  • Neutrinos and Cables

    I’m a little torn about this: the Twitter machine and other social mediums have blown up about this story at Science Express, which claims that the faster-than-light neutrino result from the OPERA collaboration has been explained as a simple glitch:

    According to sources familiar with the experiment, the 60 nanoseconds discrepancy appears to come from a bad connection between a fiber optic cable that connects to the GPS receiver used to correct the timing of the neutrinos’ flight and an electronic card in a computer. After tightening the connection and then measuring the time it takes data to travel the length of the fiber, researchers found that the data arrive 60 nanoseconds earlier than assumed. Since this time is subtracted from the overall time of flight, it appears to explain the early arrival of the neutrinos. New data, however, will be needed to confirm this hypothesis.

    I suppose it’s possible. But man, that would make the experimenters look really bad. And the sourcing in the article is just about as weak as it could be: “according to sources familiar with the experiment” is as far as it goes. (What is this, politics?)

    So it’s my duty to pass it along, but I would tend to reserve judgment until a better-sourced account comes along. Not that there’s much chance that neutrinos are actually moving faster than light; that was always one of the less-likely explanations for the result. But this isn’t how we usually learn about experimental goofs.

    Update from Sid in the comments: here’s a slightly-better-sourced story.

    Update again: and here is the official CERN press release. Not exactly admitting that a loose cable is at the heart of everything, or even that the result was wrong, but saying that there were problems that could potentially invalidate the result.

  • Darwinism of the Inanimate

    Via Laura Hollis at the Twitter machine, here’s an interesting paper by chemist Addy Pross. The author tries to extend the idea of Darwinian natural selection to the realm of inanimate objects.

    Toward a general theory of evolution: Extending Darwinian theory to inanimate matter
    Addy Pross

    Though Darwinian theory dramatically revolutionized biological understanding, its strictly biological focus has resulted in a widening conceptual gulf between the biological and physical sciences. In this paper we strive to extend and reformulate Darwinian theory in physicochemical terms so it can accommodate both animate and inanimate systems, thereby helping to bridge this scientific divide. The extended formulation is based on the recently proposed concept of dynamic kinetic stability and data from the newly emerging area of systems chemistry. The analysis leads us to conclude that abiogenesis and evolution, rather than manifesting two discrete stages in the emergence of complex life, actually constitute one single physicochemical process. Based on that proposed unification, the extended theory offers some additional insights into life’s unique characteristics, as well as added means for addressing the three central questions of biology: what is life, how did it emerge, and how would one make it?

    It’s a paper by a chemist, published in the Journal of Systems Chemistry, but doesn’t seem to require much in the way of specialized knowledge in order to read it, have a look. The central idea seems to be something called “dynamic kinetic stability.” A stable system is one that doesn’t change over time; a dynamic-kinetically stable system is one that doesn’t change in some particular features, but only by taking advantage of some other kind of change. (more…)

  • How To Think About Quantum Field Theory

    I continue to believe that “quantum field theory” is a concept that we physicists don’t do nearly enough to explain to a wider audience. And I’m not going to do it here! But I will link to other people thinking about how to think about quantum field theory.

    Over on the Google+, I linked to an informal essay by John Norton, in which he recounts the activities of a workshop on QFT at the Center for the Philosophy of Science at the University of Pittsburgh last October. In Norton’s telling, the important conceptual divide was between those who want to study “axiomatic” QFT on the one hand, and those who want to study “heuristic” QFT on the other. Axiomatic QFT is an attempt to make everything absolutely perfectly mathematically rigorous. It is severely handicapped by the fact that it is nearly impossible to get results in QFT that are both interesting and rigorous. Heuristic QFT, on the other hand, is what the vast majority of working field theorists actually do — putting aside delicate questions of whether series converge and integrals are well defined, and instead leaping forward and attempting to match predictions to the data. Philosophers like things to be well-defined, so it’s not surprising that many of them are sympathetic to the axiomatic QFT program, tangible results be damned.

    The question of whether or not the interesting parts of QFT can be made rigorous is a good one, but not one that keeps many physicists awake at night. (more…)

  • A 3.8-Sigma Anomaly

    Every professional football game begins with the flip of a coin, to determine who gets the ball first. In the case of the Super Bowl, the teams represent the National Football Conference (NFC) or American Football Conference (AFC). Interestingly, the last 14 coin flips have been won by the NFC.

    Working out the numbers, the chances of 14 coin flips in a row being equal is 1 in 8,192. (The linked article says 1 in 16,000, which comes from 2^14; but that first coin flip has to be something, so the chances of 14 in a row are really 1 in 2^13. The anomaly would be just as strange if the AFC had won every time.) That’s a better than 3.8-sigma effect! Enough to call a press conference, if this were particle physics.

    The question is … is this really a signal, or did we just get lucky? Is it a fair coin and the NFC has just been the happy recipient of a statistical fluctuation, or is there something fishy about the coin? Remember Barry Greenstein’s parable about how different people compute probabilities.

    And let it be a lesson the next time you’re excited about 3-sigma anomalies.

  • Your Favorite Deep, Elegant, or Beautiful Explanation

    The annual Edge Question Center has now gone live. This year’s question: “What is your favorite deep, elegant, or beautiful explanation?” Find the answers here.

    I was invited to contribute, but wasn’t feeling very imaginative, so I moved quickly and picked one of the most obvious elegant explanations of all time: Einstein’s explanation for the universality of gravitation in terms of the curvature of spacetime. Steve Giddings and Roger Highfield had the same idea, although Steve rightly points out that Einstein won’t really end up having the final word on spacetime. Lenny Susskind picks Boltzmann’s explanation of why entropy increases as his favorite explanation, and mentions the puzzle of why entropy was lower in the past as his favorite unsolved problem — couldn’t have said it better myself. For those of you how prefer a little provocation, Martin Rees picks the anthropic principle.

    But as usual, the most interesting responses to me are those from far outside physics. What’s your favorite?

    Full text of my entry below the fold. (more…)

  • Do I Not Live?

    Can we define “life” in just three words? Carl Zimmer of Loom fame has written a piece for Txchnologist in which he reports on an interesting attempt: biologist Edward Trifonov looked at other people’s definitions, rather than thinking about life itself. Sifting through over a hundred suggested definitions, Trifonov looked for what they had in common, and boiled life down to “self-reproduction with variations.” Just three words, although one of them is compound so I would argue that morally it’s really four.

    We’ve discussed this question before, and the idea of reproduction looms large in many people’s definitions of life. But I don’t think it really belongs. If you built an organism from scratch, that was as complicated and organic and lifelike as any living thing currently walking this Earth, except that it had no reproductive capacity, it would be silly to exclude it from “life” just because it was non-reproducing. Even worse, I realized that I myself wouldn’t even qualify as alive under Trifonov’s definition, since I don’t have kids and don’t plan on having any. (And no, those lawsuits were frivolous and the court records were sealed.)

    It’s the yellow-taxi problem: in a city where all cars are blue except for taxis, which are yellow, it’s tempting to define “taxi” as “a yellow car.” But that doesn’t get anywhere near the essence of taxi-ness. Likewise, living species generally reproduce themselves; but that’s not really what makes them alive. Not that I have the one true definition (and maybe there shouldn’t be one). But any such definition better capture the idea of an ongoing complex material process far from equilibrium, or it’s barking up the wrong Tree.

  • Happy Birthday, Stephen Hawking

    Sorry for the light blogging of late. Actual work intervenes, and it might remain that way for a while. But I’ll try to pop in whenever I can.

    Stephen Hawking is celebrating his 70th birthday today. That in itself is an amazing fact, just as it was amazing when he celebrated his 40th, and 50th, and 60th birthdays, as well as every other day he’s lived and thrived with a debilitating neuron disease. The extra fact that he continues to make contributions to science pushes beyond amazing to practically unbelievable.

    Everyone likes to tell Hawking stories, and this blog is no exception. So here is mine, meagre as it is. I’ve gotten more than enough mileage out of this one in person, I might as well put it on the blog so I won’t be tempted to tell it any more.

    At the end of 1992 I was a finishing grad student, applying for postdocs. One of the places I applied was Cambridge, to Hawking’s group at DAMTP. There is a slight potential barrier for American students to travel to the UK for postdocs, so they like to get out ahead of things and offer jobs early. Unfortunately I was out of my office the day Hawking called to offer me a position. Fortunately, my future-Nobel-Laureate officemate was there, and he took the call. He explained that Stephen Hawking had called to offer me a job — I was thrilled about the offer, but understood “Hawking called” as metaphorical. But no, Brian later convinced me that it actually was Hawking on the other end of the line, which he described as a somewhat surreal experience. Of course after the initial introduction the phone gets handed over to someone else, but still. (more…)

  • You Are A Machine

    For any remaining mind/body dualists out there: neuroscientist Patrick Haggard builds magnetic coils that he can hold close to your head, and use them to control your body via signals to your brain. “Transcranial magnetic stimulation” would be the technical term. (He thinks it means you don’t have free will, because he’s a neuroscientist and not a philosopher.)

    The machinery can’t force Prof Haggard to do anything really complicated – “You can’t make me sign my name,” he says, almost ruefully – but at one point, Christina is able to waggle his index finger slightly, like a schoolmaster. It’s very fine control, a part of the brain specifically in command of a part of the body. “There’s quite a detailed map of the brain’s wiring to the body that you can build,” he tells me.

    We sometimes say “the Large Hadron Collider is the most complex machine ever built,” but I’m not sure how it would directly compare to a human being. All part of the great bootstrap up to greater complexity, which will continue for a while until it all inevitably deteriorates into empty space.