Gravity is, at heart, a mystery. Yes, we can talk about curvature of space-time and perhaps make analogies with stretched rubber sheets. But we don’t know why mass causes space-time to curve.

To put it another way, in our theory of gravity, matter is the scenery and space-time is both cast and stage crew. But matter’s behavior is described by quantum mechanics, which takes space and time as givens. For quantum mechanics, space and time are the stage in which matter puts on the best show ever. How do we get these two theories to put on just one play?