I am not impressed by String Theory because it is extremely hard to test and in the end may be seen as unfalsifiable (How would one falsify that many versions of the same thing?). I also think that the best ideas in science tend to simplify things and make them easier to understand, not to be bewilderingly complex and complicate things.
I am impressed with it. It all stems from a single equation (Euler's formula?) which itself is derived from REVOKING an assumption about the nature of the fundamental particles of the universe. That assumption is that the fundamental particles can be modeled as point particles. This works at our vantage point, but actually CAUSES problems in the quantum world.
They changed the assumption to be a ring. This DRAMATICALLY complicated the maths, but led to something interesing.....
The rings can (and in fact should) have vibrational modes. The number of modes for a string is NOT infinite, but is fixed at a definite number. Oddly enough the number of vibrational modes is THE SAME as the number of fundamental particles known and predicted by the Standard Model. Furthermore, the modes appear to correspond to SPECIFIC particles.
Mode 1 = photon
Mode 2 = electron
Mode 24 = (drumrole) THE GRAVITON
Gravitons have not been observed, but are predicted to exist. The Standard Model predicts SOME properties of the Graviton, but not all. String Theory predicts properties that the Standard Model does not. String Theory does indeed make predictions, and is testable. It's just that the theory is rather new, and rather different from normal theories. It's taken a while for people to understand it sufficiently to MAKE predictions. It's like Quantum Theory, right after it was created...a lot of head scratching and trying to get ones head around it.
M-Theory is an extension to String Theory, that is necessary, because it was realized that MOST vibrational modes are OPEN, meaning the string is attached to something. That 'something' is spacetime. However, a few modes are CLOSED. The Graviton is a closed-mode particle. This is important! It leads to a SECOND prediction of String Theory! -- The reason that the gravitational force is so profoundly weak compared to the other forces, is because it is NOT bound to our spacetime.
Gravity might be EXACTLY as strong as the other forces. It just 'leaks' off our spacetime plane. Only gravitons that just happen to travel parallel to our plane produce an effect that we can measure.
The ONLY other closed-mode particle would be a so-called "Sterile Neutrino" which would be a neutrino in every way, except that it also would not be bound to our spacetime. Experiments looking for these are also underway, and initial results indicate THAT THEY EXIST. (This has not been confirmed, one lab found something like a 2% excess of neutrinos, others have denied the results, but the positive results are scientifically sound enough that further experiments were IMMEDIATELY enacted)
THIS CAN BE TESTED. The LHC will try to find the graviton, but other experiments can detect it also. Particularly tests involving very large masses placed very close to each other. In this case, gravity should appear VERY MUCH stronger that it does normally, because the proximity of the masses allows fewer gravitons to escape.
If this is verified, it DIRECTLY implies 1) a multiverse and 2) Dark Matter might (at least in part) be gravitational attraction FROM MATTER IN OTHER UNIVERSES.
Pretty cool, eh?