String theory (and its ilk) rely on the possibility of some greater than 4 n-dimensional space with the extra spatial dimensions curled up pretty small. I’ve liked the elegence of the theory and have personally been thinking that dark matter would turn out to be stuff travelling preferentially differently oriented in these extra dimensions (thus unobservable to us other than via gravity). I’ve even been expecting that to be where all the antimatter went to - ie there was no preference for matter afterall, all the CP violation and excess of matter is really just how antimatter orients in n-dimensional space. Now this as reported in May 14 Science apparently blows the whole concept out of the water.
All the experiment does is rule out large (macroscopic) extra dimensions. The scale of the puported extra dimensions postulated by string theory are many orders of magnitude smaller than this experment was able to probe.
There is speculation that gravity is so weak because it can leak into other dimensions that other forces and particles cannot. The only upper limit on size is a size where it can be shown that gravity no longer follows the inverse square law, which due to experimental constraints was recently almost as large as a human hair. This experiment pushes the upper constraints on the size of the extra dimensions down a few orders of magnitude, but string theory wasn’t even remotely threatened by this.
It’s such a nebulous and non-worked out theory anyways, it would be hard to concretely attack it with current technology. I feel that it is more like a religion almost than a real theory- a religion based upon mathematics instead of old superstitions like normal.
Personally, I feel it is more likely that Einstein was slightly wrong, and the speed of light is more variable than we thought. Then all the super-small-scale incompatabilities between quantum physics and relativity dissapear without having to resort to constructing 11 dimensional strings that cannot be seen or tested.
There’s been a lot of attention paid to loop quantum gravity in the past few years, which has produced some interesting results. Loop quantum gravity is an alternative to string theory, in that it’s a very different approach to trying to combine general relativity and quantum mechanics. To put it simply, loop quantum gravity is a model of space itself while string theory is a model of particles. Much of the recent interest in loop quantum gravity is because it makes a prediction which ought to be testable in the near future; that as a result of the discrete nature of space, the speed of light should vary slightly with frequency.
Even if this turns out to be true, it won’t necessarily kill string theory. As far as I know, we haven’t worked out the mathematics of either theory in enough detail to be certain wether or not the two models are inconsistent.
After a sunny Sunday afternoon with Stephen Hawking’s “The Universe in a Nutshell” (well written and with pretty pictures!) I’m hardly the cognescenti, but I can point out that String Theory is a little old fashioned since there are five types of string (I, II, IIA, IIB, E) with no way to tell which one is “correct”, and since it seems to break down at lower energies because its observable large scale, macroscopic consequences don’t fit as well as those from super-symmetry and super-gravity hypotheses.
The new, unifying approach is M Theory (sometimes called “brane” theoy: a membrane can either be left flat or rolled up into a string), which encapsulates all five types of string and supersymmetric supergravity.
The idea is tha as-yet-unobserved “regions” of the universe have different dimensionality - “somehwere” our everyday third dimension is “rolled up” so that only two dimensions are visible (which would seem to preclude complex life), “elsewhere” the fourth spatial dimension is unfolded (again precluding life since gravity would be weaker and stars couldn’t form). We live, like Goldilocks, in the “just right” 3-brane of 3 dimensions of space and one of time (whatever that is), and cannot live elsewhere any more than we could live on Jupiter.
The experiments to and fro, supporting some aspects of M Theory and attacking others, and this process looks set to continue for decades if not centuries. However, M Theory does have testable consequences and so is falsifiable in principle. It’s just that the apparatus might cost billions of dollars.