• Question: What, essentially, is string theory? And is it a feasible ToE?

    Asked by stormwhite to Arttu, Ceri, James_M, Monica, Philip on 21 Jun 2011.
    • Photo: James M Monk

      James M Monk answered on 21 Jun 2011:

      I would argue that string theory isn’t science because it can’t make predictions (I say “string theory is bobbins” as a joke 🙂

      I did do a three lecture course in it years ago – I remember there are lines in spacetime that can either be closed loops or open strings, and when those lines move through spacetime they sweep out a 2d surface. You then do all of the field-theory maths on that 2-d co-ordinate system – the x,y,z,t of normal spacetime becomes a field on that 2-d “world sheet.”

      What really confused me is that string theory introduces 26 space-time dimensions because the sum of all the positive integers should be -1/12 . This is not as mad as it sounds if you remove the infinities from that sum using renormalisation.

      So string theory doesn’t have a lot to do with experimental particle physics right now. I’m *told* that if you supersymmeterise string theory then you get gravity, which is why people think it might be a candidate for a grand unified theory.

      There was a (now not so well known) theory in the 19th century that different atoms were different types of knots. It had some nice features, but ultimately turned out to be wrong.

    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 21 Jun 2011:

      There some truth in James’s criticism. In principle string theory makes predictions, but most of them are extremely difficult to test. Some obvious predictions, for example that spacetime must have 10 dimensions, are clearly wrong, and therefore one needs to modify the theory to make it compatible with nature. However, it is the only even remotely feasible theory of everything we have, meaning a theory that describes all known interactions: electromagnetic, weak, strong and gravitational. Of course, that does not mean that it is true, but at least it allows us to see how that kind of unification could work.

      String theory is a strange theory in many ways. It is often described as a theory of little pieces of strings, but that is actually just an approximation of the full theory, which we call M-theory. In fact, there are several different ways in which M-theory can be approximated by different types of little strings. The problem is that we don’t really know what M-theory is (and some would say we don’t even know if it exists), so we cannot actually write it down, but we know that in M-theory spacetime is 11-dimensional.