Originally Posted by

**DrRocket**
Susskind claims that there are something like 10^500 string theories, based on allowable physical laws. I don't doubt that someone could cook up a version in which the defining constants change in time. That strikes me as a liability rather than a virtue.

Heck, there doesn't even seem to be agreement as to whether supersymmetry is required for string theory. I think most string theorists are of the opinion that it is, but Susskind has written differently.

Will the real string theory please stand up ?

It doesn't matter how many string theories there are as long as they agree where observable phenomena are concerned.

For example, think of electromagnetic potential. The actual value of the electromagnetic potential V(x) is not in itself a physical quantity. What's physical is the gradient. So you have a situation in which there are an uncountable number of ways of describing a physical system, but each of these ways predicts the same physics.

To give a more interesting example, if you have a bosonic string propagating on a closed circle of radius R, then the physics is essentially unchanged if you replace R with 1/R. The physically meaningful quantity, the partition function of the system, is in fact invariant under this transformation. This observation, called T-duality, has led to the discovery of much more profound dualities in string theory, such as mirror symmetry.

So having more than one string theory is not a bad thing so long as the underlying physics is the same.

As far as supersymmetry goes, my sense of things is that string theory is not much of a subject without it. I think actually that I'd be more surprised if supersymmetry was false than if string theory were false. One of the things I find compelling about supersymmetry is that a lot of beautiful and interesting mathematics (example, Hodge theory, index theory, Morse theory) just kind of falls naturally into your lap via the supersymmetry point of view.