11.11.2006 12:32

Imagine a particle moving under the unfluence of a central force. Then it's angular momentum is conserved, the particle moves on a two dimensional plane. Like the planets orbiting the sun. Nobody ever claims that you should quantify over these planes. Of course not that would be utterly silly. It's simply a constraint in the motion, induced by the fact that the force acts in a particular way.

If you have a closed relativistic string which moves in D dimensions and one of them is compact. Say you have a rubber band which moves very fast in a tube) then it turns out that there is a thing called T -duality which means essentially that your rubber band in the tube of radius R is equivalent to a rubber band in a tube of radius 1/R. Now you cut the rubber band and get an open string, you still want this T-duality to hold. I turns out, that the ends of your open string are confined to D-1 dimensions. They cannot move around the tube. Hence says the string theorist (at least that is how I understand the arguementation in Johnsons book) there is a thing, which is this D-1 dimensional object.

Caveat: I do a lot of string bashing here, because I have to read that book for a reading course here. The reason why I engaged in this was, that I thought it would help me to understand it a little better. The truth is it doesn't really. I just find it annoying. I have to give a presentation next week and I promised myself, it will be the last bit I do for it.
I have very strong prejudices against string theory which to a large part come from this incredibly over enthusiastic and uncritical attitude towards it. Together with the fact that there is no evidence for it to be true whatsoever. I don't have anything against people persuing it and it might turn out to be true eventually, but up to now it is completely misleading to talk in a way about as if it was well established. And that is what many string theorists do.
Remember this is an inductive science.
Also, I believe that string theory has already missed many of the goals it's proponents set out to achieve. Where have the elegance, the simplicity and especially the uniqueness gone? After leaving half a year in a string world, I wouldn't been amazed anymore if anybody claimed one day that in a certain string theory there are actually unicorns, and the strings are just the hair of their tails. I have the impression that string theory is so general or "rich" that in the end it cannot make any predictions at all. Whatever physicist will find out there in the world, there will be a string theory that describes it. And they will claim victory. But the more general a theory, the less interesting is it for obvious reasons, it doesn't exclude anything and hence doesn't make any predictions.
My feeling towards it is actually even worse: I am not even convinced that there is a possible world, where string theory is true.