There’s a slightly snarky Review of Leonard Susskind’s book on string theory (The Cosmic Landscape: String Theory and the Illusion of Intelligent Design) in the New York Times this week. Predictably, Peter Woit is all over it.
The central issue of the book, and the review, and Woit’s whole blog is what’s referred to as the “Landscape” problem in string theory. This is a topic that seems to consume a remarkable amount of intellectual energy for what’s really a pretty abstract debate. It also leads to a remarkable amount of shouting and name-calling for something that just doesn’t seem like that big a deal to me.
The central issue, in my outsider’s view, is that the current version of string theory does not appear to make a definite prediction about the nature of the universe, in terms of particle masses and interaction strengths and whatnot. Instead, it allows a dizzying array of equally likely possible universes, each with slightly different masses and interactions and all that– something like 10500 possible universes, or a one followed by more zeroes than I care to write out.
To string skeptics like Woit, this is proof that the whole enterprise is a bunch of crap. If the theory doesn’t predict a single set of particle properties, or at least a small number of possibilities, then it’s worthless. Susskind, on the other hand, appears to think that this is the greatest thing since sliced bread. If there are infinite possibilites, he reasons, then anything is possible. More than that, everything is inevitable. Then he starts talking about the Anthropic Principle, and the whole thing drifts off in the general direction of late-night dorm-room bull sessions.
Personally, I can’t quite see what all the fuss is about. (More after the cut…)
I mean, the essence of the problem here seems to be that the theory doesn’t predict a unique set of values for the fundamental constants of nature. At least, that’s what I think is meant by the enumeration of universes– there are 10500 different combinations of fundamental particle masses and interaction strengths that are possible in a mathematically valid formulation of string theory.
If that’s what they mean (and it’s entirely possible that the universe tally could refer to something completely different), then I really don’t understand what the problem is. Or, more precisely, I think I see what the issue is, but I’m not really bothered by it.
If our universe is one out of ten bazillion possible universes, then it would mean that theorists would have to sort of hand-select theories that happen to give parameters close to the ones we observe in the real worlld. That strikes some people as a crisis, but my immediate reaction is, “Welcome to my world.”
I mean, whenever I go to do a calculation or write an exam problem, I have to look up a bunch of constants that are just… constants. The mass of an electron is 511 keV/c2, give or take a bit. Why is it 511 keV/c2, and not 512, or 510, or 347? Shut up and calculate.
Yeah, fine, it’s inelegant and aesthetically unappealing. Life sucks, get a helmet.
Even if you were able to generate a single theory, or even a limited number of theories that look more or less like what we observe, I don’t think that has much probative value. It’s really easy to come up with theories that “predict” what we already know. A theory isn’t interesting until it predicts something beyond what we already know– some new particle, or some physical effect that doesn’t arise out of the current models of the universe. And it’s not right until somebody detects that particle, or observes that effect.
Now, as I said, it’s possible that I’m misunderstanding the issue– even the pop-science descriptions of this stuff are pretty obscure. It could be that they’re referring to 10500 different universes that all predict the parameters we observe, and differ only in the extra particles and effects that are possible (I don’t think that’s what they mean, based on the Anthropic Principle stuff). That’s a slightly bigger problem, but I’m still not particularly bothered. If they differ in ways that can be measured, then we can (potentially) sort them out with experiments, and if they differ only in ways that can’t be measured, well, wake me when you start doing science.
(I should note, just so I’m not only hacking on string theorists, that I have a similar opinion of the debates over interpretations of quantum mechanics. The question of whether the Copenhagen Interpretation or the Many-Worlds Interpretation, or some other Interpretation is the right one is interesting on an abstract level, but I can’t see investing a great deal of intellectual energy in it, because they all “predict” exactly the same thing. If some really smart person comes along and invents a sort of meta-theory version of Bell’s Inequality that lets you distinguishi experimentally between the various Interpretations, then this will become a really interesting issue. Absent that, it’s an interesting diversion, but not worth getting worked up over.)
Whatever the real Landscape issue is, I don’t really understand the name-calling. It’s vaguely amusing as spectacle, but it’s a little hard to take seriously.