Sean Carroll

Author: Sean Carroll

  • A History of Night

    April is going to be Poetry Month all month long! The New York Review of Books is celebrating. Here’s one from Jorge Luis Borges, translated by Alastair Reid.

    A History of Night

    Through the course of generations
    men brought the night into being.
    In the beginning were blindness and dream
    and thorns which gash the bare foot
    and fear of wolves.
    We shall never know who fashioned the word
    for the interval of darkness
    which divides the two half-lights.
    We shall never know in what century it stood
    for the starry spaces.
    Others began the myth.
    They made night mother of the tranquil Fates
    who weave all destiny
    and sacrificed black sheep to her
    and the rooster which announced her end.
    The Chaldeans gave her twelve houses;
    infinite worlds, the Stoic Portico.
    Latin hexameters molded her,
    and Pascal’s dread.
    Luis de León saw in her the homeland
    of his shivering soul.
    Now we feel her inexhaustible
    as an old wine
    and no one can think of her without vertigo,
    and time has charged her with eternity.

    And to think that night would not exist
    without those tenuous instruments, the eyes.

  • String Wars: The Aftermath

    An interesting short interview with Ed Witten in this week’s New Scientist. Mostly straightforward stuff, but it’s always good to hear what smart people are thinking. Witten is spending the year on sabbatical at CERN; like many people, he was sort of hoping to be there when the first physics results from the LHC appeared, but reality intervened an that’s looking increasingly unlikely. Happily, CERN has developed electronic means of communication whereby interesting findings may be promulgated to researchers who are not within close physical proximity to the lab.

    Longtime CV readers may be interested in Witten’s take on the String Wars:

    The 1980s and 90s were dotted with euphoric claims from string theorists. Then in 2006 Peter Woit of Columbia University in New York and Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, published popular books taking string theory to task for its lack of testability and its dominance of the job market for physicists. Witten hasn’t read either book, and compares the “string wars” surrounding their publication – which played out largely in the media and on blogs – to the fuss caused by the 1995 book The End of Science, which argued that the era of revolutionary scientific discoveries was over. “Neither the publicity surrounding that book nor the fact that people lost interest in talking about it after a while reflected any change in the intellectual underlying climate.”

    That sounds about right. For the most part, actual string theorists simply went about their business, trying to figure out what this fascinating but difficult theory really is. The irony is that a major point of the anti-string books was that the public hype concerning string theory didn’t paint an accurate picture of its more problematic features — which was true. But the backlash books gave the public a misleading impression in the other direction, leading to the somewhat amusing appearance of my own piece in New Scientist explaining that the theory was for the most part chugging along as before. Hype cuts in every direction, and it feeds on drama, not on accuracy.

    There is certainly some feeling that the near-term growth area in high-energy theory is not string theory, but phenomenology (or arguably particle astrophysics). Certainly those are the people who seem to be getting the jobs these days. The explanation there is pretty straightforward: data! Or at least the promise thereof. It’s hard to do physics with little to go on other than thought experiments, but one gets by when relatively few real experiments are available. Increasingly, that’s no longer the case.

    But it’s been a long time since we’ve had a good string-wars thread, so here you go. For old time’s sake.

  • Death by Physics

    I’m not supposed to give away too much here. But recall that Hollywood loves science, and occasionally we can help them out with an interesting idea or two. So it’s possible that if you were to watch tonight’s episode of Bones (8 p.m., 7 Central, on Fox), our plucky heroes Booth and Brennan could be investigating a murder that makes clever use of expertise in physics. It’s even possible that the murder technique was dreamed up in part by one of our previous guest-bloggers, which might very well be reflected in the name of the research institute where the dastardly deed takes place.

    deathbyphysics.jpg

    I’ve probably said more than I should already.

  • The Earth’s Elder

    The largest organism on Earth, and probably the oldest multicellular organism, is named Pando. Kind of a cutesy name for such an impressive specimen, don’t you think?

    800px-aspenoverview0172.JPG

    If you were to meet Pando — which you could easily do, if you paid a visit to Fishlake National Forest in Utah — it would look like a forest of Quaking Aspen trees. But if you happened to be equipped to do DNA testing on plant specimens, you would realize that all of the trees were genetically identical. That’s because they’re all part of the same tree, sharing a common root system. One tree springs from a seed, long ago, and spreads out roots; but then more trees erupt from those roots, and the process simply continues. Individual “trees” might die, but that’s like you or me losing a toenail; Pando lives on. It weighs in at over six million kilograms, and is likely more than 80,000 years old (although it might be much older).

    I have nothing especially profound to say about Pando, I just think it’s cool. But when you have arrow-of-time on the brain, everything resonates. Unlike most other multicellular organisms, there’s no reason why Pando should ever die, absent dramatic external factors. As long as its environment remains hospitable, Pando could live forever. Monocellular organisms, of course, do this all the time; they split into “children” which are genetically identical (up to mutations), so it’s legitimate to say that any given bacterium has lived for many millions of years. The birth/growth/death cycle is not absolutely necessary to the existence of life — it’s just useful, if life wants to avoid the very real possibility that the environment does dramatically change for the worse. Giving birth to children with slightly different genetic makeups — and then getting out of their way, by dying — gives the species a fighting chance to adapt and survive in the face of dramatic changes around it. (Update: some termites have a different strategy.)

    Meanwhile, Pando abides. Good for it.

  • Perceiving Randomness

    The kind way to say it is: “Humans are really good at detecting patterns.” The less kind way is: “Humans are really good at detecting patterns, even when they don’t exist.”

    I’m going to blatantly swipe these two pictures from Peter Coles, but you should read his post for more information. The question is: which of these images represents a collection of points selected randomly from a distribution with uniform probability, and which has correlations between the points? (The relevance of this exercise to cosmologists studying distributions of galaxies should be obvious.)

    randompoints.gif

    The points on the right, as you’ve probably guessed from the set up, are distributed completely randomly. On the left, there are important correlations between them.

    Humans are not very good at generating random sequences; when asked to come up with a “random” sequence of coin flips from their heads, they inevitably include too few long strings of the same outcome. In other words, they think that randomness looks a lot more uniform and structureless than it really does. The flip side is that, when things really are random, they see patterns that aren’t really there. It might be in coin flips or distributions of points, or it might involve the Virgin Mary on a grilled cheese sandwich, or the insistence on assigning blame for random unfortunate events.

    Bonus link uncovered while doing our characteristic in-depth research for this post: flip ancient coins online!

  • Post-Christian America

    We’re a long way from the day when the United States could reasonably be described as a non-religious nation. But we’re getting there. It’s sometimes hard to see the forest for the trees, but the longer-term trends are pretty unambiguous. (Which is not to say it’s impossible they will someday reverse course.) I suspect that, hand-wringing about arrogance and “fundamentalist atheists” notwithstanding, the exhortations of Richard Dawkins and his ilk have had something to do with it. If nothing else, they provide clear examples of people who think it’s perfectly okay to not believe in God, and be proud of it. That’s not an insignificant factor. It’s most likely a small perturbation on top of more significant long-term cultural trends, but it’s there.

    Newsweek reports the facts: the number of self-identified Christians in the U.S. has fallen by 10 points over the last twenty years, from 86 to 76 percent. The number of people who are unaffiliated with any religion has jumped forward, from 5 percent in 1988 to 12 percent today. And the number who are willing to label themselves “atheists” has, it’s reasonable to say, skyrocketed — from 1 million in 1990 to 3.6 million today. That’s still less than two percent of the population, so let’s not get carried away. But it’s double the number of Episcopalians! (I was raised as an Episcopalian. Always been a shameless front-runner.)

    Here’s how Jon Meacham sums it up in Newsweek:

    There it was, an old term with new urgency: post-Christian. This is not to say that the Christian God is dead, but that he is less of a force in American politics and culture than at any other time in recent memory. To the surprise of liberals who fear the advent of an evangelical theocracy and to the dismay of religious conservatives who long to see their faith more fully expressed in public life, Christians are now making up a declining percentage of the American population.

    I’ve said it before, but it’s time for us atheists to diversify our portfolio, as far as popular culture is concerned — skepticism and mocking of creationists are all well and good, but we need to put forward a positive agenda for living our lives without the comforting untruths handed down by religion. I’m doing my part by joining the Epicurus fan page on Facebook.

  • Will the Internet Replace Universities?

    Via Brad DeLong, an article by Kevin Carey in the Chronicle of Higher Education starts with the obvious — the internet is killing newspapers as we knew them — and asks whether the same will happen to universities.

    Much of what’s happening was predicted in the mid-1990s, when the World Wide Web burst onto the public consciousness. But people were also saying a lot of retrospectively ludicrous Internet-related things — e.g., that the business cycle had been abolished, and that vast profits could be made selling pet food online. Newspapers emerged from the dot-com bubble relatively unscathed and probably felt pretty good about their future. Now it turns out that the Internet bomb was real — it just had a 15-year fuse.

    Universities were also subject to a lot of fevered speculation back then. In 1997 the legendary management consultant Peter Drucker said, “Thirty years from now, the big university campuses will be relics…. Such totally uncontrollable expenditures, without any visible improvement in either the content or the quality of education, means that the system is rapidly becoming untenable.” Twelve years later, universities are bursting with customers, bigger, and (until recently) richer than ever before.

    But universities have their own weak point, their own vulnerable cash cow: lower-division undergraduate education. The math is pretty simple: Multiply an institution’s average net tuition (plus any state subsidies) by the number of students (say, 200) in a freshman lecture course. Subtract whatever the beleaguered adjunct lecturer teaching the course is being paid. I don’t care what kind of confiscatory indirect-cost multiplier you care to add to that equation, the institution is making a lot of money — which is then used to pay for faculty scholarship, graduate education, administrative salaries, the football coach, and other expensive things that cost more than they bring in.

    I’m not sure I buy it. Let’s think about what good purposes a college or university might serve. Off the top of my head, I can think of several:

    1. Classroom-based education. Certainly important.
    2. Extracurricular learning. This includes everything from “participating in actual academic research” to “serving on the school newspaper.”
    3. Meeting different kinds of people. Not only do students get exposed to professors, and an academic way of thinking about problems, but they also meet other students, hopefully from a wide variety of backgrounds.
    4. Establishing independence. For many people, going to college is the first time one lives away from home, and begins to establish an identity separate from one’s family.
    5. Belonging to a community. From the university itself to numerous smaller subcultures within, college provides an opportunity to belong. As great as the Teaching Company is, it doesn’t have a basketball team in the Final Four.

    Feel free to add your own. We can argue whether online learning can be effective in replacing the first of these — after all, hearing a recorded lecture is not the same as hearing it live. But it would appear very difficult to replace the others. The four years one spends at college are often the most formative (and perhaps the most enjoyable) years of one’s life. It’s not clear, of course, how much people are willing to pay for those other purposes, as important as they may be.

    On the other hand, there is a long-established bargain at big research universities that could conceivably come unraveled at the hands of the internet. Namely: it is research and scholarship that attracts the faculty and establishes the academic reputation of a school, but it is teaching that brings in students and tuition dollars. This is not an arrangement based entirely on avarice; the top research schools bring in a lot more money from grants and gifts than they do from student tuitions. But it reflects a deep philosophical split, that might signal an underlying instability: from within academia, the purpose of the university is seen as the production of new scholarship; from outside academia, the purpose of universities is seen as the teaching of students.

    In the case of newspapers, the internet made it harder to tightly bundle straightforward news with advertising and sections of the paper any one reader might not be interested in. In the case of universities, will the internet make it harder to bundle teaching and research? Quick, name the largest private university in the U.S. The answer is the University of Phoenix, founded in 1976, where 95% of faculty are part-time and the large majority of teaching happens completely online.

    It could happen that more education-providing corporations (one hesitates to call them “universities”) could develop better ways to provide online classroom educations to a large number of students who are interested in the first purpose listed above but are unwilling to pay for the second. If that model catches on, it will cause dramatic upheaval in the economy of traditional universities. And, much as I love the internet, that would be too bad.

  • The Inverse-What Law?

    An arxiv find, via David Hogg (via Facebook, via the internet).

    The gravitational force law in the Solar System
    Authors: Jo Bovy (NYU), Iain Murray (Toronto), David W. Hogg (NYU, MPIA)

    Abstract: If the Solar System is long-lived and non-resonant (that is, if the planets are bound and have evolved independently through many orbital times), and if the system is observed at any non-special time, it is possible to infer the dynamical properties of the Solar System (such as the gravitational force or acceleration law) from a snapshot of the planet positions and velocities at a single moment in time. We consider purely radial acceleration laws of the form ar= –A [r/r0]-α, where r is the distance from the Sun. Using only an instantaneous kinematic snapshot (valid at 2009 April 1.0) for the eight major planets and a Bayesian probabilistic inference technique, we infer 1.989<α<2.052 (95-percent confidence). Our results confirm those of Newton (1687) and contemporaries, who inferred α=2 (with no stated uncertainty) via the comparison of computed and observationally inferred orbit shapes (closed ellipses with the Sun at one focus; Kepler 1609). Generalizations of the methods used here will permit, among other things, inference of Milky-Way dynamics from Gaia-like observations.

    So: instead of noting that an inverse-square behavior for the force of gravity fits the data, assume that gravity obeys an inverse power law and fit for the power. (It’s two, to within the errors.) Of course there have been many higher-precision tests of gravity in the Solar System than this one; the new thing here is that the data are simply the positions and velocities of all the planets at one particular moment in time, no direct dynamical measurements. A little bit of Bayesian voodoo magic, and there you go.

    What I want to know is, what makes the authors so convinced that their instantaneous kinematic snapshot is valid tomorrow?

  • Why Can’t We Visualize More Than Three Dimensions?

    Physicists and mathematicians who think about higher-dimensional spaces are, if they allow their interest to somehow become public knowledge, inevitably asked: “How can you visualize more than three dimensions of space?” There are at least three correct answers: (1) You can’t. (2) You don’t have to; manipulating abstract symbols is enough to help you figure things out. (3) There are tricks to help you pseudo-visualize higher-dimensional objects by cleverly projecting them into three dimensions; see here and here.

    But really, why can’t we visualize things in more than three dimensions of space? Could a Flatlander, living in a world with only two spatial dimensions, learn to visualize our three-dimensional world? Could we somehow, through practice or direct intervention in the brain, train ourselves to truly visualize more dimensions?

    I can think of a couple of explanations why it’s so hard, with different ramifications. One would be simply that our imaginations aren’t good enough to project our consciousness into a constructed world so very different from our own. Could you, for example, really imagine what it’s like to live in two dimensions? Sure, you can visualize Flatland from the outside, but what about asking what it’s like to really be a Flatlander? The best I can do is to imagine a line, flickering with colors, surrounded by darkness on either side. But the darkness is still there, in my imagination.

    The other possible explanation is that the process of visualization takes up a three-dimensional space in our actual brain, preventing us from “tuning a dimensionality knob” on our imaginations. The truth is certainly more complicated than that (and I’m not experts, so anyone who is should chime in); the visual cortex itself is effectively two-dimensional, but somehow our brain reconstructs a three-dimensional image of the space around us.

    Maybe this could be a new tantric discipline: visualization in higher dimensions. Or maybe the Maharishi already offers a course?

  • Ada Lovelace Day: Chien-Shiung Wu

    240x240_wu.jpg March 24 was designated Ada Lovelace Day. To honor the world’s first computer programmer, bloggers posted something about a woman who made a significant contribution to science or technology. Serious bloggers wrote detailed and engaging pieces, but we overdue authors don’t have time for that. So instead, only one day late, here’s a short excerpt from my book draft, about Chien-Shiung Wu and the discovery of parity violation.

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    It came as quite a surprise in the 1950’s when parity was shown not to be a symmetry of nature, largely through the efforts of three Chinese-born American physicists: Tsung-Dao Lee, Chen-Ning Yang, and Chien-Shiung Wu. The idea of parity violation had been floating around for a while, suggested by various people but never really taken seriously. In physics, credit traditionally accrues not just to someone who makes an offhand suggestion, but to someone who takes that suggestion seriously enough to put in the work and turn it into a respectable theory or a decisive experiment. In the case of parity violation, it was Lee and Yang who sat down and performed a careful analysis of the problem. They discovered that there was ample experimental evidence that electromagnetism and the strong nuclear force both were invariant under P, but that the question was open as far as the weak nuclear force was concerned.

    Lee and Yang also suggested a number of ways that one could search for parity violation in the weak interactions. They finally convinced Wu, who was an experimentalist specializing in the weak interactions and Lee’s colleague at Columbia, that this was a project worth tackling. She recruited physicists at the National Bureau of Standards to join her in performing an experiment on Cobalt-60 atoms in magnetic fields at very low temperatures.

    As they designed the experiment, Wu became convinced of the project’s fundamental importance. In a later recollection, she explained vividly what it is like to be caught up in the excitement of a crucial moment in science:

    Following Professor Lee’s visit, I began to think things through. This was a golden opportunity for a beta-decay physicist to perform a crucial test, and how could I let it pass? — That Spring, my husband, Chia-Liu Yuan, and I had planned to attend a conference in Geneva and then proceed to the Far East. Both of us had left China in 1936, exactly twenty years earlier. Our passages were booked on the Queen Elizabeth before I suddenly realized that I had to do the experiment immediately, before the rest of the Physics Community recognized the importance of this experiment and did it first. So I asked Chia-Liu to let me stay and go without me.

    As soon as the Spring semester ended in the last part of May, I started work in earnest in preparing for the experiment. In the middle of September, I finally went to Washington, D. C. for my first meeting with Dr. Ambler. … Between experimental runs in Washington, I had to dash back to Columbia for teaching and other research activities. On Christmas eve, I returned to New York on the last train; the airport was closed because of heavy snow. There I told Professor Lee that the observed asymmetry was reproducible and huge. The asymmetry parameter was nearly -1. Professor Lee said that this was very good. This result is just what one should expect for a two- component theory of the neutrino.

    Your spouse and a return to your childhood home will have to learn to wait – Science is calling! Lee and Yang were awarded the Nobel Prize in Physics in 1957; Wu should have been included among the winners, but she wasn’t.