I will now start catching up on some of the papers that have accumulated over the summer. This time I'll say a bit about recent developments in quantum field theory and 4-dimensional topology. However, it's apparently not quite true. This is the basic idea of general relativity, which is nicely illustrated by the way in which the presence of the sun bends starlight that passes nearby.
Gravity is thus quite different from the other forces, at least to our limited understanding.
These are, of course, Einstein's equations. In fact it is no coincidence that we have no quantum theory of gravity. For most of the last 50 years or so physicists have been working very hard at inventing and understanding quantum field theories that rely for their formulation on a fixed metric. Mainly, though, we learned that we need to bite the bullet and figure out how to do quantum field theory without any background metric. This is a precise list of properties one would like a quantum field theory independent of any background metric to satisfy.