Unlike the smooth curvature of gravity, quantum mechanics consists of small fluctuations that take place between particles. It describes an uncertain world in constant motion, in which no piece of matter can ever be in one spot. This can be difficult to understand or actually see, especially in the case of something large, like a person standing motionless in a room. But in the case of a tiny electron moving around an atom, it becomes more apparent.
But there's a problem: The basic tenets of these two theories clash. General relativity says that space/time is curved; quantum mechanics holds that space/time itself is inherently uncertain. Professor Strominger describes it this way: For the past thousand years, people have used the space/time dimension as a starting point to describe the laws of physics - like going to a blackboard and drawing a collection of lines that describe particles and how they intersect. Throw quantum mechanics into the mix, and the blackboard can't be written on anymore, because it's waving around. The problem, he says, is both mathematically and conceptually difficult.
Morever, today's fundamental laws of physics are more like a laundry list than a complete outfit of clothing. That is, each makes sense on its own, but there's no neat way to put them all together. Physicists long for that kind of unification. "It shouldn't be laws of physics, it should be the law of physics," says Strominger. "There should be one law and it should be the nicest one around." String theory, he says, could be all-encompassing law that physicists seek.