Gravity dominates the large-scale structure of the universe, but only by default, so to speak. Matter arranges itself to cancel electromagnetism, and the strong and weak forces are intrinsically short range. At a more fundamental level, gravity is extravagantly feeble. Acting between protons, gravitational attraction is about 10^-36 times weaker than electrical repulsion. Where does this outlandish disparity come from? What does it mean?

These questions greatly disturbed Richard Feynman. His famous paper on quantizing general relativity,1 in which he first described his discovery of the “ghost particles” that eventually played a crucial role in understanding modern gauge field theories, begins with a discussion of the smallness of gravitational effects on subatomic scales, after which he concludes,

There’s a certain irrationality to any work on [quantum] gravitation, so it’s hard to explain why you do any of it. . . . It is therefore clear that the problem we [are] working on is not the correct problem; the correct problem is: What determines the size of gravitation?