The next logical step for the intrepid theoretical physicist is to bring the fourth and last fundamental interaction, gravity, under the same sort of quantum field umbrella as the other forces. Unfortunately, we have no real successes to report in this effort. There are a lot of reasons for this, but here we will describe just two big ones.
One major problem is that we already have an excellent theory for explaining how gravity works, a theory that goes far beyond Newton’s remarkable insights. It is called “general relativity,” and Albert Einstein gets most of the credit for coming up with it. This theory has been very successful at predicting and explaining all sorts of observations about gravity near and far, especially under extreme conditions such as near black holes and the like. The bad news is that general relativity is nothing at all like a quantum field theory. It is a purely geometric account of the gravitational interaction, and if we want to quantize it somehow, we will have to quantize space and time themselves, whatever that means.
The second major problem is on the experimental side. All of the theoretical work we’ve been talking about has been guided and verified by lab experiments, usually the scattering of high energy particles at accelerator labs. The gravitational interaction between subatomic particles is so incredibly small, even when compared with the weak interaction, that its effects are completely impossible to detect in such experiments. In order to detect any appreciable effects of gravity, your measurements must be made on larger, macroscopic scales. And as we have seen many times, large scale is not the place to look for quantum effects. We’ll take a closer look at general relativity and possible quantum theories of gravity.
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