String theory is not the only theory that can accommodate extra dimensions, but it certainly is the one that really demands and requires it.

By dimension, we simply mean an independent direction in which, in principle, you can move; in which motion can take place. In an everyday world, we have left-right as one dimension; we have back-forth as a second one; and we have up-down as a third.

I do feel strongly that string theory is our best hope for making progress at unifying gravity and quantum mechanics.

Brian Greene Quotes