While we’re getting the multiverse out of our system, let me point to this interview with Leonard Susskind by Amanda Gefter over at New Scientist (also noted at Not Even Wrong). I’ve talked with Amanda before, about testing general relativity among other things, and she was nice enough to forward the introduction to the interview, which appears in the print edition but was omitted online.

Ever since Albert Einstein wondered whether the world might have been different, physicists have been searching for a “theory of everything” to explain why the universe is exactly the way it is. But one of today’s leading candidates, string theory, is in trouble. A growing number of physicists claim it is ill-defined, based on crude assumptions and hasn’t got us any closer to a theory of everything. Something fundamental is missing, they say (see New Scientist, 10 December, p 5).

The main complaint is that rather than describing one universe, the theory describes some 10⁵⁰⁰, each with different kinds of particles, different constants of nature, even different laws of physics. But physicist Leonard Susskind, who invented string theory, sees this huge “landscape” of universes not as a problem, but as a solution.

If all these universes actually exist, forming a huge “multiverse,” then maybe physicists can explain the way things are after all. According to Susskind, the existence of a multiverse could answer the most perplexing question in physics: why the value of the cosmological constant, which describes how rapidly the expansion of the universe is accelerating, appears improbably fine-tuned to allow life to exist. A little bigger and the universe would have expanded too fast for galaxies to form; a little smaller and it would have collapsed into a black hole. With an infinite number of universes, says Susskind, there is bound to be one with a cosmological constant like ours.

The idea is controversial, because it changes how physics is done, and it means that the basic features of our universe are just a random luck of the draw. He explains to Amanda Gefter why he’s defending it, and why it’s a possibility we simply can’t ignore.

Just because you can’t see the dark matter doesn’t mean you can’t take a picture of it. Via Universe Today, here’s a press release from Johns Hopkins announcing a beautiful new image of the reconstructed dark matter density in cluster CL 0152-1357 by Jee et al. (I couldn’t find the paper online, but you can get a higher-resolution version of the picture at Myungkook Jee’s home page.) The dark matter is in purple, the galaxies are in yellow.

How do you do that? It’s not because we’ve detected some form of light coming from the dark matter. Rather, we’ve detected (once again) its gravitational field — this time, via the tiny distortions in the shapes and positions of background galaxies (weak lensing). This is a form of gravitational lensing that is so subtle you could never detect it happening to a single galaxy — it would be impossible to distinguish between lensing and the intrinsic shape of the galaxy. But if you have a large number of background galaxies (which the universe is kind enough to provide us), you can use statistics to reconstruct the gravitational field through which the light travels, and hence figure out where the dark matter must be.

Of course we’re still trying to detect the dark matter, both directly (in ground-based experiments) and indirectly (looking for high-energy radiation produced by annihilating dark matter particles), not to mention using particle accelerators to actually produce candidate dark matter particles. Over the next ten or twenty years, probing the properties of dark matter is going to be one of the top priorities at the particle/astrophysics interface.

When the fall quarter started, there were six papers that I absolutely had to finish by the end of the term. Three have been completed, two are very close, and the last one — sadly, I think the deadline has irrevocably passed, and it’s not going to make it. So here’s the upshot.

About a year ago I gave a talk at the Philosophy of Science Association annual meeting in Austin. The topic of the session was “The Dimensions of Space,” and my talk was on “Why Three Spatial Dimensions Just Aren’t Enough” (pdf slides). I gave an overview of the idea of extra dimensions, how they arose historically and the role they currently play in string theory.

But in retrospect, I didn’t do a very good job with one of the most basic questions: how many dimensions does spacetime really have, according to string theory? The answer used to be easy: ten, with six of them curled up into a tiny manifold that we couldn’t see. But in the 1990’s we saw the “Second Superstring Revolution,” featuring ideas about D-branes, duality, and the unification of what used to be thought of as five distinct versions of string theory.

One of the most important ideas in the second revolution came from Ed Witten. Ordinarily, we like to examine field theories and string theories at weak coupling, where perturbation theory works well (QED, for example, is well-described by perturbation theory because the fine-structure constant α = 1/137 is a small number). Witten figured out that when you take the strong-coupling limit of certain ten-dimensional string theories, new degrees of freedom begin to show up (or more accurately, begin to become light, in the sense of having a low mass). Some of these degrees of freedom form a series of states with increasing masses. This is precisely what happens when you have an extra dimension: modes of ordinary fields that wrap around the extra dimension will have a tower of increasing masses, known as Kaluza-Klein modes.

In other words: the strong-coupling limit of certain ten-dimensional string theories is an eleven-dimensional theory! In fact, at low energies, it’s eleven-dimensional supergravity, which had been studied for years, but whose connection to string theory had been kind of murky. Now we know that 11-d supergravity and the five ten-dimensional string theories are just six different low-energy weakly-coupled limits of some single big theory, which we call M-theory even though we don’t know what it really is. (Even though the 11-d theory can arise as the strong-coupling limit of a 10-d string theory, it is itself weakly coupled in its own right; this is an example of strong-weak coupling duality.)

So … how many dimensions are there really? If one limit of the theory is 11-dimensional, and others are 10-dimensional, which is right?

I’ve heard respected string theorists come down on different sides of the question: it’s really ten-dimensional, it’s really eleven. (Some have plumped for twelve, but that’s obviously crazy.) But it’s more accurate just to say that there is no unique answer to this question. “The dimensionality of spacetime” is not something that has a well-defined value in string theory; it’s an approximate notion that is more or less useful in different circumstances. If you look at spacetime a certain way, it can look ten-dimensional, and another way it can look like eleven. In yet other configurations, thank goodness, it looks like four!

And it only gets worse. According to Juan Maldacena’s famous gravity-gauge theory correspondence (AdS/CFT), we can have a theory that is equally well described as a ten-dimensional theory of gravity, or a four-dimensional gauge theory without any gravity at all. It might sound like the degrees of freedom don’t match up, but ultimately infinity=infinity, so a lot of surprising things can happen.

This story is one of the reasons for both optimism and pessimism about the prospects for connecting string theory to the real world. On the one hand, string theory keeps leading us to discover amazing new things: it wasn’t as if anyone guessed ahead of time that there should be dualities between theories in different dimensions, it was forced on us by pushing the equations as far as they would go. On the other, it’s hard to tell how many more counterintuitive breakthroughs will be required before we can figure out how our four-dimensional observed universe fits into the picture (if ever). But it’s nice to know that the best answer to a seemingly-profound question is sometimes to unask it.

The laws of physics are safe for now.

It occasionally comes to pass that someone, for reasons that frankly escape me, would like to make the point that science doesn’t know everything. It doesn’t, of course, which is so obvious that the point hardly needs making. Equally obviously, science does know some things; when it comes to mundane features of the natural world, one hopes that existing puzzles will eventually be figured out.

One of the favorite anecdotes for the don’t-know-everything crowd involves the flight of the honeybee. As you may have heard, “bees shouldn’t be able to fly,” according to science as we know it. In fact, this idea goes back to French entomologists August Magnan and André Sainte-Lague, who in 1934 calculated that bee flight was aerodynamically impossible. Since bees have been observed to fly, the smart money has always been that Magnan and Sainte-Lague were, in scientific parlance, “wrong.” But that’s not the same as understanding how the darn insects actually do flit around.

Now we know. Bioengineers Michael Dickinson, Douglas Altshuler and colleages have analyzed the flight of the bumblebee (if you will), using a combination of high-speed photography and robotic models. The trick is that bees have flight muscles that have evolved differently from those of other insects — unintelligent design, I suppose. Consequently, they flap much faster than any other animal their size, and emply a unique rotation of their wings.

Chalk up another success for science. I understand that Dickinson and Altshuler will now start working on how to get experimental predictions out of string theory.