Sean Carroll

Author: Sean Carroll

  • Arguments For Things I Don’t Believe, 1: Research on String Theory is Largely a Waste of Time

    First in a prospective series of my own versions of the best arguments for conclusions I don’t personally share. I’m supposed to stick to statements that I believe are true, even if I don’t think they warrant the conclusion. The idea is to probe presuppositions, put our ideas to the test, and of course to implicitly diss the less-good arguments for things we don’t believe. And who knows, maybe we’ll come up with arguments that are so great we’ll change our minds! (By slipping into the royal “we” I’m encouraging others to play along.) So here we go: the best argument I can think of for why research on string theory is a waste of time.

    Traditionally, the greatest progress in physics has come through an intense interaction between theory and experiment. We have learned new things when experiments were good enough to bring us data that didn’t fit into the models of the time, but our theoretical understanding was also sufficiently developed that we had the tools to formulate useful hypotheses. While we know that classical general relativity and quantum mechanics are fundamentally incompatible and must someday be reconciled, straightforward dimensional analysis suggests that detailed experimental information about the workings of such a reconciliation (as opposed to true-but-vague statements like “gravity exists” or “spacetime is four-dimensional on large scales”) won’t be available at energies below the Planck scale, which is hopelessly out of reach at the current time.

    A defensible response to this lack of detailed experimental input would be to place the problem of quantizing gravity on the back burner while we think about other things. And this was indeed the strategy pursued by the overwhelming majority of theoretical physicists, up until the 80’s. Two things caused a change: the drying-up of the river of experimental surprises that had previously kept particle theory vibrant and unpredictable, and the appearance of string theory as a miraculously promising theory of quantum gravity. Even though the Planck scale was still just as inaccessible, string theory was so good that it became reasonable to hope that we could figure it all out just by using brainpower, even without Planckian accelerators.

    But it hasn’t worked out that way. Gadflies point to the landscape of low-energy manifestations of string theory as the nail in the coffin for any hopes to uniquely predict new particle physics from string theory. But that is only a subset of the more significant challenge, and understanding particle physics beyond the Standard Model was never the primary motivation of most string theorists anyway — it was quantizing gravity.

    The real problem is that string theory isn’t a theory. It’s just part of a theory, and we don’t know what that theory is, although sometimes we call it M-theory. As Aaron explains in a very nice post, the thing we understand is “perturbative” string theory, which is a fancy way of saying “the part of M-theory where small perturbations around empty space act like weakly-interacting strings.” We’ve known all along that colorful stories about loops of string propagating through spacetime only captured part of the story, but we’re beginning to catch on to how difficult it will be to capture the whole thing. The Second String Revolution in the 90’s taught us a great deal about M-theory, but it’s hard to know whether we should be more impressed with what we’ve been able to learn even without experimental input, or more daunted by the task of finishing the job.

    Within our current understanding of string theory, there is not a single experiment we can even imagine doing (much less actually, realistically hope to do) that would falsify string theory. We can’t make a single unambiguous prediction, even in principle. I used to think that string theory predicted certain “stringy” behavior of scattering cross-sections at energies near the Planck scale; but that’s not right, only perturbative string theory predicts such a thing. “String theory” is part of a larger structure that we don’t understand nearly well enough to make contact with the real world as yet, and it’s completely possible that another century or two of hard thinking won’t get us to that goal. It made sense to be optimistic in the 80’s that there was enough rigidity and uniqueness in the theory that we would be led more or less directly to contact with observation; but that’s not what has happened.

    The best reason to think that research on string theory is largely a waste of time is because it’s just too hard.

    Pretty convincing, eh? But I don’t buy it, even though I think I’ve adhered to my self-imposed rule that I believe every individual sentence above. It might turn out to be the case that another century or two of hard thinking won’t get us any closer to connecting string theory with the real world, but I don’t see any reason to be that pessimistic. The thing that’s really hard to get across at a popular level is that the theory really is rigid and unique, deep down; it’s the connections between “deep down” and the world around us that are the hard part. Count me as one of those who is more impressed with what we have learned than daunted by what we haven’t; if I were to bet, I would say that more thinking will continue to lead to more breakthroughs, and ultimately a version of M-theory that can rightly be called “realistic.”

    In the meantime, the advent of sexy new data from the LHC and elsewhere will draw a certain fraction of brainpower away from string theory and into phenomenology, but there will be plenty left over. The field as a whole will fitfully establish a portfolio of different approaches, as it usually does. And there will undoubtedly be surprises around the corner.

  • Pesky Democratic Process

    The LA Times has a front-page article, apparently free of irony, that laments the glacial rate of progress on constructing a world-class subway system for the city, and imagines wistfully how much easier it would be if only we lived in a one-party communist state. In particular, they look at the progress that Shanghai has made in building its own subway, and pout about all of those nefarious restrictions that Americans have to put up with because we give actual citizens a say in the process.

    “If the government wants to do something, even if the conditions are not ready for it, it will be done,” said Zheng Shiling, an influential Chinese architect who teaches at Tongji University in Shanghai.

    At the risk of only slight oversimplification, the system works like this: Planners draw subway lines on a map. Party officials approve them. Construction begins. If anything is in the way, it is moved. If they need to, Chinese planners “just move 10,000 people out of the way,” said Lee Schipper, a transport planner who has worked with several Chinese cities in his role as director of research for EMBARQ, a Washington-based transportation think tank. “They don’t have hearings.”

    Schipper recalled consulting with one Chinese metropolis whose ancient city wall stood in the way of a transportation project.

    “One of the members of the People’s Committee said, ‘Oh, I know how we’ll solve the problem. We’ll move the historic wall.’ ” It was, he said, as if a planner in Washington proposed moving the Potomac River to make way for construction.

    One searches the article in vain for the part where they say “Of course we live in a democracy, and some people think that there are certain benefits to that kind of system, even if the government does have to ask permission before tearing down historic sites,” but the moment never comes. Instead, we are treated to stirring stories of the plucky citizens of Shanghai, who don’t raise a peep when construction displaces them from their homes — no, indeed, they are happy to be displaced, as it gives them a chance at a new life! (It might be that voices of complaint are not heard because they are actually silenced, but that smudges up the narrative.)

    As a dweller in downtown LA, where a better subway system would be a life-altering good and the lamentations of fragile newcomers who are shocked at the presence of construction noise in a booming high-density urban core form a constant background chorus, I deeply sympathize with frustration at the demands the democratic process force onto city planning. But I’ll tolerate the delays if it means that, if the Mayor wants to tear down our apartments, he at least has to hold a hearing first.

  • Charity Without Religious Belief: Mother Teresa

    People sometimes argue back and forth about whether religious belief is a good thing, because it induces believers to be moral or charitable. In a big-picture sense, I think arguments of this form completely miss the point; beliefs should be judged on whether they are correct or incorrect, not on whether they cause people to do good or bad things. (If the belief is not correct, but it makes people do something good, can we say they’ve been tricked into acting that way?) Certainly, nobody is going to convince me to believe something if they admit that it’s false, but it would be good for me to believe — recommendations of that sort are usually aimed at other people, not the one handing them out. Besides which, as a matter of historical record it’s pretty clear that religion has led people to do some really good things and also led people to do some really bad things, and trying to weigh the effects on some imaginary scales seems just hopeless. Or at least, an interesting and possibly never-ending source of discussion for sociologists and historians of religion, but fortunately orthogonal to questions of the truth or falsity of religious claims.

    Still, I confess to being a bit amused by the news that, in the last years of her life, Mother Teresa didn’t believe in God. (Via Cynical-C.) Letters that she wrote have now been released as part of a book project, and they are shot through with serious doubts.

    Shortly after beginning work in Calcutta’s slums, the spirit left Mother Teresa.

    “Where is my faith?” she wrote. “Even deep down… there is nothing but emptiness and darkness… If there be God — please forgive me.”

    Eight years later, she was still looking to reclaim her lost faith.

    “Such deep longing for God… Repulsed, empty, no faith, no love, no zeal,” she said.

    As her fame increased, her faith refused to return. Her smile, she said, was a mask.

    “What do I labor for?” she asked in one letter. “If there be no God, there can be no soul. If there be no soul then, Jesus, You also are not true.”

    I’m not someone who has strong feelings about Mother Teresa either way, and it seems sad that her doubts put her in such apparent torment. (To the extent that these letters paint a reliable picture at all, of course.) And, in the department of “things that are perfectly obvious but must nevertheless be said explicitly because it’s the internet,” this is only one individual case, from which no grand conclusions should be drawn. Except the obvious: motivations for altruistic and charitable behavior can be very complicated. We should keep them separate from our attempts to understand how the universe works.

  • The Best Arguments for Things I Don’t Believe

    Have you ever heard someone arguing in favor of a position with which you disagree, but their arguments are so bad that you can’t help but think “Man, I could do a better job arguing for their side than they are, and I don’t even agree with them!” I thought it might be interesting to do exactly that — consider some interesting issues, and come up with my own versions of what the people who I think are wrong should be saying.

    The rules would be: (1) The claims would be somewhat judgmental, rather than straightforwardly empirical. I’m not going to waste my time arguing that the universe is not expanding, or anything like that. (2) I have to stick to making individual statements that I really do believe, even if I don’t think they are sufficient to support the ultimate conclusion. I reserve the right to come up with more rules as I think of them.

    Here are some possible claims to be considered:

    1. God exists.
    2. The Iraq war was a good idea.
    3. Women scientists shouldn’t complain about discrimination.
    4. Research on string theory is a waste of time.
    5. Talking about the multiverse is intrinsically non-scientific.
    6. We shouldn’t worry about global climate change.

    Any other suggestions? I’m sure there are lots of things I don’t believe, but could come up with better arguments for than I usually hear. It’ll be like being on the debate team again.

  • Ask a String Theorist! Or an Atomic Physicist.

    Over at Uncertain Principles, Chad Orzel is on vacation and has handed the keys to the blog over to Aaron Bergman and Nathan (last name mysterious), specialists in string theory and atomic physics, respectively. Good luck to them as they experience what the blogosphere is like from the other side.

    Aaron has begun to talk a little about the multiverse — here, here. He has thereby earned grumpy mutterings, rolled eyes, and “help” from some sensible physicists, some crackpots, some curmudgeons, his guest co-blogger, and even himself. I don’t quite understand what all the angst is about. (Actually I do understand, of course; this is one of those times when you adopt a rhetorical stance of pretending not to understand some alternative position in order to emphasize how unimpeachably correct your own position is.)

    People are very welcome to disagree with the presuppositions or conclusions of anthropic reasoning; that’s just how science goes, and is perfectly healthy. But in addition to the substantive disagreements, there’s a widespread urge to express dismay that it’s even being talked about all the time. Now, that urge can’t be sensibly directed toward the actual research being done, because on that score multiverse-type stuff is a tiny percentage of all the work that goes on. Peek at any day’s worth of abstracts on hep-th, hep-ph, gr-qc, or astro-ph; you might find something anthropic here or there if you’re lucky, but it’s a tiny minority. This stuff is not dominating science, or physics, or theoretical physics, or high-energy theory, or even string theory.

    No, the complaint is that considerations of parts of the universe that we can’t possibly see tend to receive an inordinate amount of attention in public discussions — on blogs, in books, in magazines and newspapers. Which is completely true, as a factual statement. At the risk of revealing a trade secret: the public discussion of different avenues of scientific research does not faithfully reflect the amount of research effort being put into those questions. Eek! I’d be willing to bet that it has always been like that. And yet, science marches on.

    You may ask why something like the multiverse exerts such an outsized pull on the public imagination. So let me break it down for you here: it’s fun. People like talking about other universes, and whether we could be living in a simulation, and what happened before the Big Bang. For one thing, anyone can dive in; you don’t need to be an expert on twistor space, or two-loop counterterms, or BRST invariance in order to pontificate about the conditions under which life could exist if the laws of physics were very different. (Comments from people who are more informed and thoughtful about the subject will generally be more useful, but anyone can say something.) For another, it’s just cool to contemplate these way-out possibilities. The lure of crazy ideas is what draws a lot of people to science in the first place.

    And that’s … okay, as Stuart Smalley would remind us. It would be very bad indeed if unmoored philosophizing about other universes became the dominant paradigm in science, or any subset thereof, but there’s zero danger of that. Really. But there’s no reason why people can’t have fun contemplating some of the more provocative and accessible ideas out there. On this very blog we will occasionally write lengthy discourses on some piece of technical work related to observations — and not get anywhere near the number of comments that a two-minute toss-off about the anthropic principle gets. And yet, science has not ground to a halt. I think the enterprise is sufficiently healthy to survive a few more posts about the multiverse.

  • Best Class Ever

    Female Science Professor talks about the frustrations associated with making sure your class has a decent room and all that fun stuff, especially when it’s a small interdisciplinary freshman seminar. The irony, of course, is that an off-the-beaten path course on a topic that the professor is really passionate about is much more likely to end up being the Best Class Ever for the enrolled students than any of the inevitable required courses, but they will always get the short end of the stick when it comes to scheduling and logistics.

    But it got me thinking about the concept of the Best Class Ever. What is it that makes a college course especially memorable, years down the line? After at least fifteen seconds of quality rumination over my own experiences, two common features stand out. First, the professor was absolutely enthusiastic about the material; they weren’t just punching a clock, they were truly into it. Second, a very delicate balance was struck, in which the material was ultimately understandable (and interesting, it goes without saying), but also extremely challenging. The best classes were those in which you learned an incredible amount, but only after really sweating for it. Other than that, my favorite classes didn’t really have much in common; they were a remarkably heterogeneous group.

    My favorite undergrad class, and also my favorite non-science class (among many strong contenders), was probably “Contemporary Political Images,” taught by philosopher-turned-social-theorist Jack Doody. We covered a lot of political and social theory — Marx, Rawls, Habermas, Leo Strauss, Alasdair MacIntyre, that kind of thing. It was a small seminar, and an indispensable ingredient of the class’s awesomeness was the talent and enthusiasm of the other students. Every week we were wrestling with Big Ideas about Virtue and The Good, and some of the best conversations were over breakfast in the dining hall before class. And years later, when Clarence Thomas mumbled something about Natural Law at his confirmation hearings, we all knew exactly what was going through his mind.

    My favorite class in grad school, and also my favorite science class (without quite as many strong contenders) was probably Nick Warner’s general relativity course at MIT. I was a grad student at the liberal-arts college up the river, but Ted Pyne and I happily hopped on the Red Line twice a week to attend this course, given the sorry state of Harvard’s GR offerings at the time. This was a big lecture course, with detailed hand-written notes handed out beforehand, and there wasn’t too much in-class discussion — Nick talked awfully fast, and it’s not easy to stop that much momentum once it gets built up. (But there was a weekly recitation where we could ask whatever crazy questions popped into our heads.) Every week we were pushed to the limit, and loved it. We must have loved it, as Ted and I taught our own seminar to our fellow grad students the next year, and I went on to teach the course as a postdoc, and then as a professor, and write up my own notes, which eventually made it into a book. In the foreword of which, Nick gets a hearty acknowledgment.

    So what were your best college classes ever? Feel free to provide supporting evidence and anecdotes, and reason inductively from there to a comprehensive theory of class awesomeness.

    (I won’t reveal the best class ever from a teacher’s perspective — like children, they’re all my favorites.)

  • Hey, I Uploaded a Video

    Just got back from a great trip to Beijing, very enjoyable if a bit tiring, where much musing was done on the Primordial Existential Question, about which more anon. But I also mused a bit about what this blog needs, and I came to the conclusion that must have been obvious to everyone else long ago: more videos of me.

    So, here you are. Thanks to some heroic efforts on the part of folks who would just as soon lurk behind the scenes, we now have video captured from the C-SPAN broadcast of our science panel at YearlyKos. Here is my talk, conveniently divided into two pieces to appease the YouTube gods. They are a little fuzzy, but you get the idea. I used the mysterious beauty of dark matter and dark energy as an excuse to make some didactic points about science and rationality and politics. (If I weren’t an atheist, I would have made a good preacher.) You can also find videos of Chris’s talk and Ed’s talk at their respective sites; Tara, who felt sorry for me for being given the impossible task of making the universe sound interesting, has the Q&A up as well.

    But! Behind the fold, the true payoff!

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  • Exponentially Unlikely Fetishes

    T-Rex of Dinosaur Comics realizes that his personal kink involves reversing the arrow of time. And further contemplation just makes things worse.

    entropy fetish
    The construction of jokes involving Boltzmann’s Brains and onanism is a temptation better left resisted.

    Below the fold: hott entropy action! Not safe for work, perverts!

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  • China is Scared of Blogs

    Greetings from the International Congress on Logic, Methodology and Philosophy of Science in Beijing. I once read, in Ray Monk’s biography of Bertrand Russell, about a year that Russell spent lecturing on philosophy in Beijing. He was extremely taken with the city and the country, predicting that it would flower into a leading role in the world. This momentarily puzzled me, as my vision of China didn’t seem in sync with Russell’s democratic ideals. But then common sense clicked in, and I realized that we were talking about a period just after World War One, during the Sun Yat-sen era. The new Republic of China was struggling to emerge out from Imperial rule, and the Communist takeover was decades in the future. One could have easily imagined that this sprawling country, united by a common language and a rich heritage of culture and innovation, would rapidly take its place among the free and prosperous nations of the world. The fact that it didn’t is one of the great tragedies of twentieth-century history.

    These days China is increasingly prosperous, but not quite free. Upon landing at Beijing International Airport, one fills out the usual customs declaration form, full of admonishments against bringing in firearms or questionable agricultural products. But there is an extra item on the list of dangerous imports: writings, recordings, or other collections of information that could be judged as a threat to the political, moral, or social good of the nation. The didn’t actually ask to search my laptop, but the warning was there.

    It’s well known that the People’s Republic of China (PRC) censors blogs, so I’ve been poking around using the internet connection here in my hotel room, trying to judge the extent to which this is true. (The flipside, of course, is the perilous situation of bloggers located in China; apparently they’ve been required to register in order to blog, but I don’t have the latest on that. I should mention that there are all sorts of blogs about China, not that I have any expertise about them.) Access to most websites is fine, but certain addresses are certainly being blocked. Of course it’s impossible for me to distinguish between the actions of the local ISP, the city of Beijing, or the Chinese government itself, but you draw conclusions using the data you have, not the data you wish you had.

    Any blog on Blogspot is definitely off-limits (so I can’t visit Preposterous Universe for old time’s sake). You can type in the address or click a link, and the browser will think for a minute, and then return a “Problem loading page: The connection has timed out” error. My impression is that that’s been true for a long time, although apparently it’s been on and off for a while now. Typepad blogs are also off-limits, so no Cocktail Party Physics for me, although that might be a recent development. Livejournal seems to be unavailable, and likewise Xanga, but blogs hosted on WordPress.com seem to be available. You can search on Google Blog Search or Bloglines, but Technorati is blocked. I haven’t found any individually-hosted blogs that were off-limits, although certain news sites like philly.com are mysteriously banned. The Eagles are in the middle of training camp, how am I supposed to keep up? Also, the New York Times is readily accessible, so make of that what you will. I also couldn’t reach the BBC, although I can actually watch the BBC news channel on my hotel room TV.

    Google, of course, is available, in the wake of their somewhat-infamous deal struck with the Chinese government. But Wikipedia is a little confusing — blocked at times, available at others. Apparently this is an ongoing skirmish. I typed in “China” to Google, and the first link was the the Wikipedia page, so I clicked there, and saw it no problem. Then I typed “China internet” into the Wikipedia search box, and was given a list of pages, including Internet Censorship in the People’s Republic of China. But when I clicked there, it briefly began to load, before switching to a “The connection was reset” error. A little spooky, to be honest. Right now I seem to be able to see most Wikipedia pages, although apparently not those specifically about the PRC (although the main China page is still okay). You might think, no problem, I can just look at the Google cache pages for whatever Wikipedia article I’m interested in. But no, you can’t; nothing in Google’s cache seems to be available. So much for infamous deals.

    None of which has prevented me from reading any of my favorite blogs. I just do what I always do, and read the feeds via Bloglines. They’re all perfectly visible, even for the blocked sites. Google reader works just as well. A lack of internet savvy on the part of the censors, or an intentional oversight? The one thing that one can’t do is leave comments (or start up your own blog, obviously), and maybe that’s the point.

    (I also notice that when I visit google.com, I am not automatically redirected to the local version google.cn, which seems to happen in European countries. Is this because the hotel’s service provider is rigged for foreigners, and ordinary citizens have different rules? Not sure.)

    It could be much worse, of course. I mean, here I am, typing away on my own blog, with little fear that the secret police are going to burst into my hotel room in the middle of the night to haul me away. But the biggest single reason I don’t have that fear is that I know that word would get around, and that it wouldn’t look good — free information protects free people. Amnesty International has a campaign, irrepressible.info, to protest against internet censorship around the world. The more noise people make about this issue, the more pressure governments will feel to keep the web free.

    Update: In the United States, we prefer to have our censoring-for-political-content performed by corporations, rather than directly by the government. Different cultures, different systems.

  • Unusual Features of Our Place In the Universe That Have Obvious Anthropic Explanations

    The “sensible anthropic principle” says that certain apparently unusual features of our environment might be explained by selection effects governing the viability of life within a plethora of diverse possibilities, rather than being derived uniquely from simple dynamical principles. Here are some examples of that principle at work.

    • Most of the planetary mass in the Solar System is in the form of gas giants. And yet, we live on a rocky planet.
    • Most of the total mass in the Solar System is in the Sun. And yet, we live on a planet.
    • Most of the volume in the Solar System is in interplanetary space. And yet, we live in an atmosphere.
    • Most of the volume in the universe is in intergalactic space. And yet, we live in a galaxy.
    • Most of the ordinary matter in the universe (by mass) consists of hydrogen and helium. And yet, we are made mostly of heavier elements.
    • Most of the particles of ordinary matter in the universe are photons. And yet, we are made of baryons and electrons.
    • Most of the matter in the universe (by mass) is dark matter. And yet, we are made of ordinary matter.
    • Most of the energy in the universe is dark energy. And yet, we are made of matter.
    • The post-Big-Bang lifespan of the universe is very plausibly infinite. And yet, we find ourselves living within the first few tens of billions of years (a finite interval) after the Bang.

    That last one deserves more attention, I think.