Sean Carroll

Author: Sean Carroll

  • General Relativity: In Pretty Good Shape

    If we celebrate provocative new experimental findings, we should also celebrate the careful null results (experiments that agree with existing theories) on which much of science is based. Back in October we pointed to a new analysis that used observations of gravitational lensing by large-scale structure to test Einstein’s general relativity on cosmological scales, with the intriguing result that it didn’t seem to fit. And the caveat that it probably would end up fitting once we understood things better, but it’s always important to follow up on these kinds of clues.

    So now we understand things a bit better, and a number of people have been working to dig into this apparent anomaly. Here is a new paper from this week, that presents their own way of using these kinds of data to test GR against large-scale structure.

    Testing General Relativity with Current Cosmological Data
    Authors: Scott F. Daniel, Eric V. Linder, Tristan L. Smith, Robert R. Caldwell, Asantha Cooray, Alexie Leauthaud, Lucas Lombriser

    Abstract: Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.

    One issue, as we noted way back when, is that it’s very hard to “test GR” without committing yourself to a model of the mass and energy sources that are causing the curvature of spacetime. So the game is to make some plausible assumptions and see where you go from there. This group seems to have assembled a sensible framework for testing deviations from Einstein, and come back with the answer that everything is on the right track.

    We keep getting new and better data, of course, so we’ll keep testing. I suspect Einstein will continue to be right, but probably a lot of people thought Newton would continue to be right a century ago.

  • Colbert Nation

    Every scientist who writes a popular-level book harbors a secret (or maybe not-so-secret) ambition: to be invited on the Colbert Report. Not only because Stephen Colbert is a funny guy, and it’s a good way to sell books — although there is that. The truth is that Colbert (and the Daily Show) love talking to scientists. The sad part of that truth is that more people are exposed to real scientists doing cutting-edge research by watching Comedy Central than by watching, shall we say, certain channels you might have thought more appropriate venues for such conversations. But the happy part is that Colbert and Jon Stewart help bring some fun to science, and expose it to an audience it might not otherwise reach.

    So, mark your calendars: I’m going to be on Colbert on Wednesday, March 3. (Scheduled to be, anyway — updates as events warrant.) I have a book to sell, not that I would have turned down the opportunity otherwise.

    The precedents are pretty formidable — below the fold I’ve put some of Colbert’s recent interviews with some famous physics/astronomy types. Two things seem immediately obvious: (1) for scientists, these folks are very good at doing entertaining interviews, and (2) Stephen Colbert is an amazingly good interviewer, managing to mix topical jokes and his usual schtick with some really good questions, and more than a bit of real background knowledge. I think this is going to take some preparation.

    Anyone want to venture some guesses as to what questions he might ask? Every little bit of anticipation helps.

    (Note on above link to the Onion: “Punkin Chunkin,” “Manhunter,” and “Heavy Metal Taskforce” are all real Science Channel shows. “Extreme Gravity” is, as far as I can tell, not.)

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  • What Got You Interested in Science?

    Yesterday’s book club raised the question of what first inspires young people to get interested in science. Many Cosmic Variance readers aren’t scientists at all, but a lot of you are. So — what first set you down this road? For purposes of this highly non-scientific investigation, let’s define “scientist” fairly broadly, as someone who has either received a bachelor’s degree in some scientific field, or is currently on the road to doing so (e.g. someone currently in high school or college). Even if you’re not currently a full-time scientist, we’ll count you if you got the degree.

    Here’s a poll based on my quick guesses as to what might be the leading causes of nudging people into science.

    What first inspired you to study science?
    Parent, relative, or friend.
    Role model outside friends and family.
    Teacher or a particular class.
    Science fair, mathletics, or other scholastic activity.
    Personal hobby or tinkering.
    Science books (non-fiction).
    Science fiction or fantasy literature.
    Movies, TV, radio.
    The internet (for you youngsters).
    Other
      
    Free polls from Pollhost.com

    I’d be very interested to hear if I’m leaving out some hugely influential category. And you can vote for more than one thing, if you think you were influenced by multiple sources. Among the many flaws of this kind of poll is that you might not actually remember what first inspired you — maybe it was hearing something on the radio, which made you go check out a book, but you remember the book and not the radio show. So be it; just try your best to be honest.

  • From Eternity to Book Club: Chapters Four and Five

    Welcome to this week’s installment of the From Eternity to Here book club. This week we’re tackling two chapters at once: Chapter Four, “Time is Personal,” and Chapter Five, “Time is Flexible.” That’s just because these chapters are relatively short; next time we’ll return to one chapter per week.

    Excerpt:

    Starting from a single event in Newtonian spacetime, we were able to define a surface of constant time that spread uniquely throughout the universe, splitting the set of all events into the past and the future (plus “simultaneous” events precisely on the surface). In relativity we can’t do that. Instead, the light cone associated with an event divides spacetime into the past of that event (events inside the past light cone), the future of that event (inside the future light cone), the light cone itself, and a bunch of points outside the light cone that are neither in the past nor the future.

    It’s that last bit that really gets people. In our reflexively Newtonian way of thinking about the world, we insist that some far away event either happened in the past, the future, or at the same time as some event on our own world line. In relativity, for spacelike separated events (outside one another’s light cones), the answer is “none of the above.” We could choose to draw some surfaces that sliced through spacetime, and label them “surfaces of constant time,” if we really wanted to. That would be taking advantage of the definition of time as a coordinate on spacetime, as discussed in Chapter One. But the result reflects our personal choice, not a real feature of the universe. In relativity, the concept of “simultaneous faraway events” does not make sense.

    These two chapters take on a task that is part of the responsibility of any good book on modern cosmology or gravity: explaining Einstein’s theory of relativity. Both special relativity and general relativity, hence two chapters. In retrospect they are pretty short, so an argument could be made that I should have just combined them into a single chapter.

    The special challenge of these chapters is precisely that many readers — but not all — will already have read numerous other popular-level expositions of relativity. But you have to do it. Fortunately, my favorite way of talking about relativity is a little bit different from the standard one, and lines up well with the overarching goal of understanding the meaning of “time.” In particular, I try to make the point that the secret to relativity is to think locally — to compare things happening right next to each other in spacetime, not events that are widely separated. You’re allowed to compare separated events, of course, but the answers are necessarily dependent on arbitrary choices of coordinates, and that leads to endless confusion. So you won’t read a lot about “length contraction” or “time dilation,” but you will read a lot about the actual amount of time measured along a trajectory.

    Unfortunately, a search for vivid examples of the maxim “freely-falling paths through spacetime experience the longest amount of proper time” led me directly to the most embarrassing mistake in the book. (At least, “most embarrassing mistake so far uncovered.”) Sordid details below the fold!

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  • Quantum Photosynthesis

    This is an idea that has been bouncing around for a while, but is now apparently seen in experiments: real-world photosynthesis taking advantage of quantum mechanics. (Story in Wired, via @symmetrymag. Here’s the Nature paper on which it’s all based.)

    pc645-view4 The idea is both simple and awesome: you want to transport energy through an “antenna protein” in a plant cell to the “reaction-center proteins” where it is chemically converted into something useful for the rest of the plant. Obviously you’d like to transport that energy in the most efficient way possible, but you’re in a warm and wet environment where losses are to be expected. But the plants somehow manage the nearly impossible, of sending the energy with nearly perfect efficiency through the judicious use of quantum mechanics.

    We can think about this in terms of Feynman’s way of talking about quantum mechanics: rather than a particle taking a unique path between two points, as in classical mechanics, a quantum particle takes every possible path, with simple paths getting a bit more weight than complicated ones. In the case of the protein, different paths for the energy might be more or less efficient at any particular moment, but this bit of quantum trickery allows the energy to find the best possible route at any one time. Imagine at rush hour, if your car could take every possible route from your home to the office, and the time it officially took would be whatever turned out to be the shortest path. How awesome would that be?

    The reason you can’t do that is that your car is a giant macroscopic object that can’t really be in two places at once, even though the world is governed by quantum mechanics at a deep level. And the reason for that is decoherence — even if you tried to put your car into a superposition of “take the freeway” and “take the local roads,” it is constantly interacting with the outside world, which “collapses the wave function” and keeps your car looking extremely classical.

    Proteins in plants aren’t as big as cars, but they’re still made of a very large number of atoms, and they’re constantly bumping into other molecules around them. That’s why it’s amazing that they can actually maintain quantum coherence long enough to pull off this energy-transport trick. Previous studies had hinted at the possibility, but only by cooling the proteins down and shielding them from external jiggling. This new work happens at room temperature in the context of marine algae, so it seems to indicate that it can happen in real environments.

    One step closer to building my teleportation machine. Get to work, quantum engineers!

  • Sport Science: Human vs. Bow

    Super Bowl Sunday is, of course, the great American holiday. Past years have seen inspirational performances by Joe Namath, Joe Montana, and Janet Jackson. This year pits the New Orleans Saints against the Indianapolis Colts. New Orleans, of course, is known as a city of saintly behavior, while Indianapolis’s claim to fame involves horsepower in some tangential way.

    When faced with contests of ritualized violence, we like to look for the science. So check out this video of Saints quarterback Drew Brees participating in a rigorous laboratory experiment by throwing the ol’ pigskin at an archery target. Joking aside, that is some pretty sick accuracy there.

    Impressive that a human arm beats a bow and arrow for accuracy (although it’s not completely clear that the distances and conditions were perfectly analogous). All in the wobble, apparently. But if I were defending my castle from the barbarian hordes or something, I’d still prefer archers over some guys throwing footballs.

  • Time Travel Done Right: A Book Excerpt

    From Eternity to Here addresses the problem of the arrow of time — why is the past different from the future? But Chapter Six is all about time travel, and in particular the interesting version in which you travel backwards in time. Whether it’s possible, what rules it would have to obey, and so on. And now — even though I’m sure there aren’t more than two or three of you out there who haven’t purchased the book already — you can get a sneak peek of part of that chapter. It’s going to be the cover story in the March issue of Discover, and the story is already available online.

    clockmedia And here’s a bit of multimedia bonus: to get the cool exploding-clock image, the intrepid editors worked with Biwa Studios to film high-speed video of exploding clocks, and you can see the whole videos online. They run the events forwards and backwards, just in case your personal arrow of time needs to be calibrated.

    One may ask, why is there a chapter about time travel in a book about time’s arrow? Just couldn’t resist the temptation to talk about everything related to “time”? In fact there is a deeper reason. In the real world, the laws of physics may or may not allow for closed timelike curves — physicist-speak for time machines. (Probably not, but we’re not as sure as we could be.) But apart from the difficulty in constructing them, time machines boggle our minds by offering up logical paradoxes — what’s to prevent you from traveling into the past and killing your parents before they met? There is a consistent way to handle these paradoxes, simply by insisting that they never happen. (And we’re still hopeful that the folks at Lost adhere to this principle, regardless of the surface interpretation of last night’s Season Six premiere.)

    The reason why that’s hard to swallow is because we can’t imagine anything that stops us from killing our parents, once we grant the existence of time machines. We conceptualize the past and future very differently — the past is settled once and for all, while we can still make choices about what happens in the future. That, of course, is the arrow of time. At the heart of what bothers us about time-travel paradoxes is the difficulty of establishing a uniform arrow of time in a universe where time loops back on itself.

    Of course the easy, and probably correct, way out is to simply believe that time machines don’t and can’t exist. But disentangling the demands of logic from the demands of common sense is always a rewarding exercise in its own right.

  • From Eternity to Book Club: Chapter Three

    Welcome to this week’s installment of the From Eternity to Here book club. Next up is Chapter Three: “The Beginning and End of Time.” Remember that next week we’re doing two chapters at once, Four and Five.

    For those who missed them, here’s the Science Friday discussion, and here’s the Firedoglake book salon with Chad. I should also point to some substantive review/discussions: Wall Street Journal, New Scientist, USA Today, and Overcoming Bias.

    Excerpt:

    For the most part, people interested in statistical mechanics care about experimental situations in laboratories or kitchens here on Earth. In an experiment, we can control the conditions before us; in particular, we can arrange systems so that the entropy is much lower than it could be, and watch what happens. You don’t need to know anything about cosmology and the wider universe to understand how that works.

    But our aims are more grandiose. The arrow of time is much more than a feature of some particular laboratory experiments; it’s a feature of the entire world around us. Conventional statistical mechanics can account for why it’s easy to turn an egg into an omelet, but hard to turn an omelet into an egg. What it can’t account for is why, when we open our refrigerator, we are able to find an egg in the first place. Why are we surrounded by exquisitely ordered objects such as eggs and pianos and science books, rather than by featureless chaos?

    This chapter is a fairly straightforward review of the modern understanding of cosmology, with a particular eye on those issues that will become important later in the book. We zip through the expansion, structure formation, and dark energy. There I got to tell a fun personal story of my wager with Brian Schmidt. At least I think it’s fun — including personal stories is not my natural tendency, but at the right moments it can help to humanize all the forbidding science. Hopefully this was one such moment.

    A few topics go beyond the standard cosmology summary. I discussed the Steady State theory a bit, because it’s a relevant historical example when we will much later turn to the question of what the universe should look like. I also dwell a bit on vacuum fluctuations and dark energy, because those will pay a crucial role in my personal favorite explanation for the arrow of time. And we close the chapter with a very brief overview of the evolution of entropy. It has to be brief, because we haven’t laid nearly enough groundwork to do the job right. This is a conscious choice, which may or may not work: rather than simply progressing on an absolutely logical path from foundations to conclusions, I felt free to mention points that would be important later, on the theory that they would come as less of a shock if we had established some familiarity. Again, hope that worked.

    Tom Levenson, who is an actual writer, advised me to omit “smoking a pipe” from the caption to Figure 7, on the theory that what is shown should not also be told. I left it in anyway. It’s my book!

  • @JHabermas

    Update: Totally snookered. Via Kieran Healy, the disappointing news that the Habermas account is fake. Yet more evidence that the internet is less than an ideal speech situation.

    ————————-

    I’m not the only person to find it endlessly amusing that Jürgen Habermas, octogenarian theorist of communicative rationality, has taken to Twitter. (The account seems to be legit, but it’s hard to be sure.) This is so over-determined that just last year Lauren Fisher gave a presentation entitled “If Habermas could Twitter.” Well, now we know.

    He’s still trying to master the 140-character limit, though. Here’s his latest set of tweets:

    habermas

    Well, yeah. The internet is (in some sense) an egalitarian public sphere, but it raises the danger of fragmentation into self-reinforcing interest groups. Remains to be seen how it will all ultimately play out.

  • Cranking up the PR Machine

    Like any good author, one of my duties is taking to the airwaves to flog my book. A list of upcoming events can be found at Booktour.com, and of course you can always subscribe to the Facebook page or Twitter feed. But I wanted to highlight some stuff coming up over the next two weeks:

    • Friday January 29, 12:30 p.m. Pacific: I’ll be appearing on NPR’s Science Friday with Ira Flatow. (That’s today/tomorrow, depending on when you’re reading this.) Listen online, or via your favorite public radio station.
    • Saturday January 30, 2:00 p.m. Pacific: From Eternity to Here will be the subject of a Firedoglake Book Salon. I’ll be answering questions online, and our host will be none other than Chad Orzel, who has a book of his own you should check out.
    • Saturday February 6, 10:00 a.m. Pacific: Crank up your avatars, I’m giving a talk in Second Life. (And if you don’t already have an avatar, it’s easy to get one. And you can shop for clothes!) Sponsored by MICA, it will be held at the large amphitheater on StellaNova.
    • Saturday February 6, 5:00 p.m. Pacific: For everyone here in Los Angeles and environs, I’m doing a good old-fashioned book reading/signing at Skylight Books, an awesome independent bookstore in Los Feliz. 5:00 on Saturday night — what better way to kick off the evening’s festivities?

    Hope to see you there, virtually or in person!