Science

I’m back from the Beyond Belief II conference at the Salk Institute in La Jolla, which packed an extraordinary amount of intellectual stimulation into a few short days. Any conference where you wander into the opening reception, get drawn into a conversation about reductionism and meaning with Stuart Kauffman, Rebecca Goldstein, and Sir Harold Kroto, and end up closing down the bar, is bound to be a good one, and this did not disappoint. (The title notwithstanding, much of the conference had little to do with atheism or religion — the subtitle “Enlightenment 2.0” gave a better flavor.) The talks provided fodder for at least ten to twenty blog posts, of which I’ll probably get around to writing one or two.

One of the talks was by local neuroscientist V.S. Ramachandran, or “Rama” to his friends. (Like any good neuro person, his web page includes a fun collection of optical illusions.) He talked about his experiments with synesthesia, the phenomenon in which people see graphemes (e.g. numbers or letters) as associated with colors. I do that a little bit — five is certainly yellow, seven is red, and eight is blue — but it’s closer to a vague association than a vivid experience. Some people report very strong synesthetic reactions, and for a long time researchers have wondered whether the experience was mostly metaphorical or something stronger.

To test synesthesia, Rama and collaborators designed an experiment where they could measure the vividness of the colors associated with the numbers 2 and 5. They chose those because you can make them look almost identical, although reversed, by choosing a boxy font. Then they made up a picture (on left) of mostly fives, with a few twos scattered within there. Then they asked people to pick out the twos. Most ordinary folks could do it within about twenty seconds or so.

But true synesthetes could do it immediately. That’s because to them, the twos popped out as a brightly colored triangle (right). This established beyond much doubt that synesthesia was “real,” and more particularly that was a measurable phenomenon with real consequences.

This, in turn, strengthened the hypothesis that the origin of synesthesia was to be found in the structure of the brain. Indeed, it turns out that the region of the brain responsible for processing graphemes lies adjacent to the region responsible for processing colors.

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Martian ColorsRead More »

I love telling the stories of Neptune and Vulcan. Not the Roman gods, the planets that were originally hypothesized to explain the mysterious motions of other planets. Neptune was propsed by Urbain Le Verrier in order to account for deviations from the predicted orbit of Uranus. After it was discovered, he tried to repeat the trick, suggesting a new inner planet, Vulcan, to account for the deviations of the orbit of Mercury. It didn’t work the second time; Einstein’s general relativity, not a new celestial body, was the ultimate explanation.

In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.

We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.

But it’s enormously more difficult to invent a theory of modified gravity in which the direction of the gravitational force points toward some place other than where the ordinary matter is. So the way to rule out the modified-gravity hypothesis is to find a system in which the dark matter and ordinary matter are located in separate places. If you see a gravitational force pointing at something other than the ordinary matter, dark matter remains the only reasonable explanation.

And that’s precisely what the Bullet Cluster gives you. Dark matter that has been dynamically separated from the ordinary matter, and indeed you measure the gravitational force (using weak lensing) and find that it points toward the dark matter, not toward the ordinary matter. So, we had an interesting question — dark matter or modified gravity? — and now we know the answer: dark matter. You might also have modified gravity, but one’s interest begins to wane, and we move on to trying to figure out what the dark matter actually is.

But some people don’t want to give up. A recent paper by Brownstein and Moffat claims to fit the Bullet Cluster using modified gravity rather than dark matter. If that were right, and the theory were in some sense reasonable, it would be an interesting and newsworthy result. So, you might think, the job of any self-respecting cosmologist should be to work carefully through this paper (it’s full of equations) and figure out what’s going on. Right?

I’m not going to bother. The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).

And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.

But the data say otherwise. So most of us suck it up and get on with our lives. Don’t get me wrong: I’m happy that some people are continuing to work on a long-shot possibility such as replacing dark matter with modified gravity. But it’s really a long shot at this point. There is a tremendous presumption against it, and you would have to have a correspondingly tremendous theory to get people interested in the possibility. I don’t think it’s worth writing news stories about, in particular: it gives people who don’t have the background to know any better the idea that more or less everything is still up for grabs. But we do learn things and make progress, and at this point it’s completely respectable to say that we’ve learned that dark matter exists. Not what all of us were rooting for, but the universe is notoriously uninterested in adapting its behavior to conform to our wishes.

John Wilkins at Evolving Thoughts has a great post about the development of the modern definition of “Life” (which, one strongly suspects, is by no means fully developed). Once we break free of the most parochial definitions involving carbon-based chemistry, we’re left with the general ideas that life is something complex, something that processes information, something that can evolve, something that takes advantage of local entropy gradients to make records and build structures. (Probably quantum computation does not play a crucial role, but who knows?) One of the first people to think in these physical terms was none other than Erwin Schrödinger, who was mostly famous for other things, but did write an influential little book called What Is Life? that explored the connections between life and thermodynamics.

Searching for a definition of “Life” is a great reminder of the crucial lesson that we do not find definitions lying out there in the world; we find stuff out there in the world, and it’s our job to choose definitions that help us make sense of it, carving up the world into useful categories. When it comes to life, it’s not so easy to find a definition that includes everything that we would like to think of as living, but excludes the things we don’t.

For example: is the Milky Way galaxy alive? Probably not, so find a good definition that unambiguously excludes it. Keep in mind that the Milky Way, like any good galaxy, metabolizes raw materials (turning hydrogen and helium into heavier elements) and creates complexity out of simplicity, and does so by taking advantage of a dramatic departure from thermal equilibrium (of which CV readers are well aware) to build organization via an entropy gradient.

Update: Unbeknownst to me, Carl Zimmer had just written about this exact topic in Seed. Hat tip to 3QD.

The National Academy of Sciences panel charged with evaluating the Beyond Einstein program has come out with its recommendations. Briefly: the first priority should be the Joint Dark Energy Mission (where “joint” means “with the Department of Energy”), but we should keep up some amount of work on LISA, the Laser Interferometer Space Antenna. Steinn has the lowdown, so you should go there for details.

I am happy to know that JDEM will go forward (if NASA listens to the panel, about which I’m less sure than Steinn seems to be); very happy that LISA gets at least some support, although if I were the European Space Agency I’d certainly be shopping around for more reliable partners; slightly bemused that little effort seemed to go into pushing a CMB probe; and very sad to see X-ray astronomy get the shaft, as Constellation-X and EXIST seem right out of the picture. We can only hope for happier times ahead.

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.

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.