At first sight, the problem of constructing a quantum theory of gravit — Gary Horowitz

"A hundred years ago our view of space and time was dramatically changed by the introduction of special relativity. Ten years after that, Einstein made spacetime dynamical in his general theory of relativity. It has long been expected that quantum gravity will require an even more radical change in our view of spacetime. String theory is a promising approach to a consistent quantum theory of gravity. In the past few decades a new picture of spacetime has been emerging from this theory. While this picture is far from complete, it is already clear that spacetime has many different features than it does in relativity."

"Ten years ago, it was common (and correct) to distinguish the two main approaches to by saying that string theory ... was perturbative, and background dependent while the other approach ... was non-perturbative and background independent. In light of this, it is not surprising that most relativists were not interested in string theory. … One of the main things that has changed over the past decade is that we now know that string theory does not just involve strings. Higher (and lower) dimensional objects (called s) play an equally fundamental role. Using these branes, convincing evidence has been accumulated that all five of the perturbative string theories are just different limits of the same theory, called . (There is no agreement about what the M stands for.) There is yet another limit in which M theory reduces to eleven dimensional ."