Sean Carroll

Author: Sean Carroll

  • Grumpy Kvetching of the Day

    If I ever give up blogging for good, it will be because of comments like this:

    I just don’t get it. What a lame blog topic that should have been left on the cutting room floor. There is no science here. Evidently cited just to provide an opportunity to express a personal belief. Why not blog on the news of the day..the successfully trapping the first “anti-atom” and its potential implications? This is real news, real science and in keeping with your expertise. You could teach me something. Instead you give me this?

    Obviously the sensible reaction is to laugh and move on, but few of us achieve that level of Zen detachment in dealing with the world. Many of the comments at CV are great, and I’ve certainly learned a lot from the interactions here, but quite a high percentage are of this form. When you put a lot of work into the blog and care about how it turns out, this kind of stuff wears you down. Why are people like this? I understand that not every post will interest every person; is it really more satisfying to take time to lash out in the comment section (when you have never left a constructive comment yet), rather than just skipping to something else on the vast and endlessly amusing internet?

    [/grumpy]

  • Autonomously Moving Colloidal Objects that Resemble Living Matter

    That’s the name of a new paper by Akihisa Shioi, Takahiko Ban, and Youichi Morimune. Abstract:

    The design of autonomously moving objects that resemble living matter is an excellent research topic that may develop into various applications of functional motion. Autonomous motion can demonstrate numerous significant characteristics such as transduction of chemical potential into work without heat, chemosensitive motion, chemotactic and phototactic motions, and pulse-like motion with periodicities responding to the chemical environment. Sustainable motion can be realized with an open system that exchanges heat and matter across its interface. Hence the autonomously moving object has a colloidal scale with a large specific area. This article reviews several examples of systems with such characteristics that have been studied, focusing on chemical systems containing amphiphilic molecules.

    The journal is called Entropy, which I love. The paper discusses a variety of different systems that can travel, wiggle around, and respond to stimuli in ways that resemble living organisms. Not exactly building life in a test tube, but the boundary grows increasingly blurry.

  • Here’s What Needs to be Explained

    The results from this weekend’s question are in: “What is the one concept in science that you really think should be explained better to a wide audience?” I tried to collate the answers from Twitter and Facebook as well as here, at least up to the point where my patience evaporated. Answers below the fold, grouped into three categories: big concepts, specific ideas, and meta issues.

    Scott Aaronson wrote, “The skill of sharpening a question to the point where it could actually have an answer.” Which is a skill I should probably try to develop myself, as the question I asked was amenable to different interpretations. Many people answered “evolution,” but as Ed Yong pointed out on Twitter, evolution is actually explained quite well in many places. So when we ask what needs to be explained better, there are at least two issues at work: what we actually do a bad job at explaining, and what doesn’t succeed at penetrating out into the public consciousness. In contrast with evolution, for example, I would say that quantum mechanics is explained in many places, but very rarely is it explained well.

    The winner by a wide margin was the meta issue of “the scientific method.” Which raises another question: do we agree on what the scientific method is? I suspect not. But I am completely on board with the idea that “how science works” is not explained very well, and possibly a higher priority than any particular scientific concept.

    Others that did well: evolution, statistics, certainty/uncertainty, entropy, quantum mechanics, time, and gravity. I cannot refrain from pointing out that these last four were all addressed at some length in From Eternity to Here. Which makes me think that what people are really saying is, “more folks should read Sean’s book.” Only 40 more shopping days ’till Xmas…

    Also of note is that there wasn’t actually a great deal of consensus; the list of concepts that came up is quite long. Clearly we need to do a better job of explaining.

    Here are the answers:

    (more…)

  • What Should Be Explained Better?

    I tweeted this on an impulse:

    What is the one concept in science that you really think should be explained better to a wide audience?

    At least 140 characters restricts people to really only suggesting one thing. But I don’t want to leave the blog readers out, so have a go. See if you can stick to just one!

  • Mapping the Dark Matter

    Have any friends or colleagues who don’t believe in dark matter? Showing them this should help.

    Dark Matter in Abell 1689

    That ghostly haze is dark matter — or at least, an impression of the gravitational field created by the dark matter. This is galaxy cluster Abell 1689, in the constellation Virgo. (We feel compelled to add that information, in case you’re going to go looking for it in the night sky tonight or something.) It’s easy to see that the images of many of the galaxies have been noticeably warped by passing through the gravitational field of the cluster, a phenomenon known as strong gravitational lensing. This cluster has been studied for a while using strong lensing. The idea is that the detailed distribution of dark matter affects the specific ways in which different background images are distorted (similar to what was used to analyze the Bullet Cluster). Astronomers use up massive amounts of computer time constructing different models and determining where the dark matter has to be to distort the galaxies in just the right way. Now Dan Coe and collaborators have made an unprecedentedly high-precision map of where the dark matter is (paper here).

    This isn’t all about the pretty pictures. We have theoretical predictions about how dark matter should act, and it’s good to compare them to data. Interestingly, the fit to our favorite models is not perfect; this cluster, and a few others like it, are more dense in a central core region than simple theories predict. This is an opportunity to learn something — perhaps clusters started to form earlier in the history of the universe than we thought, or perhaps there’s something new in the physics of dark matter that we have to start taking into account.

    But the pretty pictures are certainly a reward in their own right.

  • The Pi-on

    I am in love with this comment and want to have its babies:

    pi appears as a constant in many formula of physics. General relativity says that it isn’t constant. Is it the origin of the pi particle, aka pion?

    A curmudgeonly literalist might, when faced with a question such as this, harrumph a simple “No.” A more loquacious sort might explain that general relativity does not say that π is not a contstant. Pi is not a parameter of physics like the fine-structure constant, which could conceivably be different or even variable from place to place. It’s a universal answer to a fixed question, to wit: what is the ratio of the circumference of a circle to its diameter, as measured in Euclidean geometry? The answer is of course 3.141592653589793…, or any number of representations in terms of infinite series.

    But the point of the question is that GR says we don’t live in Euclidean space; we move through a curved spacetime manifold. That’s okay. In a curved space, we could imagine defining the “diameter” of a circle as the maximum geodesic distance connecting two of its points, and taking the ratio of the circumference with that diameter, and indeed it would typically not give us 3.14159… But that doesn’t mean π is changing from place to place; it just means that the ratio of circumference to diameter (defined this way) in a curved space doesn’t equal π. If the circumference/diameter ratio is less than π, you are in a positively curved space, such as a sphere; if it is greater than π, you are in a negatively curved space, such as a saddle. Geometry can also be much more complicated than that, with different ratios depending on how the circle is oriented in space, which is why curvature is properly measured by tensors rather than by a simple number.

    Taken from Mathematics Illuminated, which says that pi really does depend on the geometry of space, which is crazy.
    Taken from Mathematics Illuminated, which says that pi really does depend on the geometry of space, which is crazy.

    (Parenthetically, one of the dumbest mathematical arguments ever given was put forward by the world’s smartest person, Marilyn Vos Savant. The columnist wrote an entire book criticizing Andrew Wiles’s proof of Fermat’s Last Theorem. Her argument: Wyles made use of non-Euclidean geometry, but what if geometry is really Euclidean? Touche!)

    However … despite the fact that π doesn’t really change from place to place in general relativity, the geometry does change from place to place, and there is a particle associated with those dynamics — the graviton. Although the formulation of the original question isn’t accurate, the spirit is very much in the right place. And I, for one, will henceforth be perpetually sad that the physics community missed a chance by attaching the word pion to the lightest quark-antiquark bound state, rather than to the particle associated with deviations from Euclidean geometry. That would have been awesome.

  • Against Space

    The Philosophy of Science Association meeting in Montreal was great fun. For one thing it was in Montreal; for another I got to hang out with Doctor Free-Ride; and as a bonus there were some interesting and provocative talks about the nature of time. I chatted with Tim Maudlin, Huw Price, Craig Callender, Nick Huggett, Chris Wuttrich, David Wallace, John Norton, and other people I always learn from when I talk to. Philosophers always force you to think hard about things.

    Here are the slides from my own talk, which was supposed to be about time but ended up being more about space. Not much in the way of original research, just some ruminations on what is and is not “fundamental” about spacetime (with the caveat that this might not be a sensible question to ask). I made two basic points, which happily blended into each other: first, that the distinction between “position” (space) and “momentum” is not a fundamental aspect of classical mechanics or quantum mechanics, but instead reflects the particular Hamiltonian of our world; and second that holography implies that space is emergent, but in a very subtle and non-local way. This latter point is one reason why many of us are skeptical of approaches like loop quantum gravity, causal set theory, or dynamical triangulations; these all start by assuming that there are independent degrees of freedom at each spacetime point, and quantum gravity doesn’t seem to work that way.

    Sadly the slides aren’t likely to be very comprehensible. There’s a lot of math, and the equations don’t come out completely clearly — my first time using Slideshare, so perhaps they would look better if I uploaded a pdf file rather than PowerPoint. (Hint: the slides are much more clear if you switch to full-screen mode by clicking on the bottom right.) Also I didn’t make any attempt to have the slides stand by themselves without the accompanying words. But at least this will serve as documentation that I really did give a talk at the conference, no just hang out in restaurants in Montreal.

  • Food In Space

    Editor’s note: Crap. I wrote this post within milliseconds after the first of these awesome images came out, but somehow didn’t publish it. Now they are all over the place, and the message is ancient news in internet-time. But the science is timeless!

    This tweet by Alicia Chang says it all: “Comet Hartley 2 looks like a peanut.”

    Comet Hartley 2

    This is the first close-up image from a fly-by of the comet by NASA’s Deep Impact mission. Expect more coming in. Despite the delicious appearance, however, it wouldn’t be prudent to take a bite; the comet is spewing out cyanide.

  • Wicked Company

    wickedcompany
    Via 3 Quarks Daily, an Economist review of what looks like a fun book: Philipp Blom’s A Wicked Company: The Forgotten Radicalism of the European Enlightenment.

    It is the story of the scandalous Paris salon run by Baron Paul Thierry d’Holbach, a philosophical playground for many of the greatest thinkers of the age. Its members included Denis Diderot (most famous as the editor of the original encyclopedia, but, Mr Blom argues, an important thinker in his own right), Jean-Jacques Rousseau, the father of romanticism, and the baron himself; even David Hume, a famous Scottish empiricist, paid the occasional visit.

    I have a special fondness for these guys, having taught a course about them. As much as I am a forward-thinking person, the modern mode of expression by freethinkers (pounding out passionate diatribes on our keyboards) isn’t quite as much fun as gathering in a salon among good food and drink to denounce hypocrisy and spread the Enlightenment message.

    Apparently Blom’s historical account has a contemporary message:

    Even today, and even in secular western Europe, the bald and confident atheism and materialism of Diderot and Holbach seems mildly shocking. We still cling stubbornly to the idea of an animating soul, a spiritual ghost in the biological machine. For Mr Blom, the modern, supposedly secular world has merely dressed up the “perverse” morality of Christianity in new and better camouflaged ways. We still hate our bodies, he says, still venerate suffering and distrust pleasure.

    This is the message of Mr Blom’s book, hinted at but left unstated until the closing chapters. He believes the Enlightenment is incomplete, betrayed by its self-appointed guardians. Despite all the scientific advances of the past two centuries, magical thinking and the cultural inheritance of Christianity remain endemic.

    Sounds pretty darn accurate. Let’s order some bottles of wine and get this job finished!

  • Physicalist Anti-Reductionism

    In a philosophical mood at the moment, because I’m about to head to Montreal for the Philosophy of Science Association biennial meeting. Say hi if you’re in the neighborhood! I’m on a panel Thursday morning with Nick Huggett, Chris Wüthrich, and Tim Maudlin, talking about the emergence of spacetime in quantum gravity. My angle: space is obviously not fundamental, though time might be.

    Here’s a Philosophy TV dialogue between John Dupré (left) and Alex Rosenberg (right). They are both physicalists — the believe that the world is described by material things (or fermions and bosons, if you want to be more specific) and nothing else. But Dupré is an anti-reductionist, which is apparently the majority view among philosophers these days. Rosenberg holds out for reductionism, and seems to me to do a pretty good job at it.

    John and Alex from Philosophy TV on Vimeo.

    To be honest, even though this was an interesting conversation and I can’t help but be drawn into very similar discussions, I always come away thinking this is the most boring argument in all of philosophy of science. Try as I may, I can’t come up with a non-straw-man version of what it is the anti-reductionists are actually objecting to. You could object to the claim that “the best way to understand complex systems is to analyze their component parts, ignoring higher-level structures” but only if you can find someone who actually makes that claim. You can learn something about a biological organism by studying its genome, but nobody sensible thinks that’s the only way to study it, and nobody thinks that the right approach is to break a giraffe down to quarks and leptons and start cranking out the Feynman diagrams. (If such people can be identified, I’d happily join in the condemnations.)

    A sensible reductionist perspective would be something like “objects are completely defined by the states of their components.” The dialogue uses elephants as examples of complex objects, so Rosenberg imagines that we know the state (position and momentum etc.) of every single particle in an elephant. Now we consider another collection of particles, far away, in exactly the same state as the ones in the elephant. Is there any sense in which that new collection is not precisely the same kind of elephant as the original?

    Dupré doesn’t give a very convincing answer, except to suggest that you would also need to know the conditions of the environment in which the elephant found itself, to know how it would react. That’s fine, just give the states of all the particles making up the environment. I’m not sure why this is really an objection.

    This is purely a philosophical stance, of course; it means next to nothing for practical questions. Nor does the word “fundamental” act in this context as a synonym for
    “important” or “interesting.” If I want to describe an elephant, the last thing I would imagine doing is listing the positions and momenta of all its atoms. But it’s worth getting the philosophy right. I could imagine hypothetical worlds in which reductionism failed — worlds where different substances were simply different, rather than being different combinations of the same underlying particles. It’s just not our world.