Idiot’s Guide to String Theory

SciAm network bloggers George Musser and John Horgan just had a physics reality debate:

But strings may forever be beyond the scope of science, as John points out. His objection is that strings are too small to detect with any conceivable experiment.

George (who is the author of The Complete Idiot’s Guide to String Theory) had a great answer: that smallness is not specific to strings. Any theory that unifies all forces of nature will have to include gravity. But gravity is extremely weak compared to the other forces, which implies that detecting its quantum behavior requires experimenting with energies that are completely outside of our reach. Any test of a quantum theory of gravity “will have to be indirect and none of them is going to be decisive.”

This is another way of saying that the quantum gravity problem has already been solved.

One thing that fascinates me about the focus on phenomena versus noumena is that in my view, on a very different philosophical level, it is beautifully paralleled in the formal structure of mathematics. Modern math is entirely rooted in the theory of sets, in the sense that any mathematical object and construction can be defined starting from sets. But in any description of set theory that I’ve seen, you never say anything about what the sets ultimately are made of. There’s never a set that contains anything except other sets.

In mathematics, set theory is axiomatized via ZFC, and all math is reduced via logicism. The analogy is weak. Set theory really does give all of math. String theory does not give any of physics.

An interview of Ed Witten, the chief string theory guru, says:

A common criticism of string theory is that the theory has not provided so far a single prediction to be tested at experiments, namely that it is not falsifiable, or is it?

For now, we have to be satisfied with the fact that string theory automatically generates quantum gravity while the pre-string framework of physics makes this impossible (as far as we understand). This is a key point since after all gravity and quantum mechanics are both part of nature and they need to be combined somehow. String theory is the only real idea about combining them and has proved to be remarkably powerful. This is the main reason that people are excited about it.

Let us also remember that supersymmetry, which we search for at the LHC, is an idea that emerged in large part because of its role in string theory.

String theory only generates quantum gravity in some abstract theoretical world that can never be tested.

The latest word from the LHC is that the most widely conjectured supersymmetric particles have been shown not to exist, with 90% confidence. We will know with greater confidence by the end of the year.