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Why Quantum Gravity?

One of the hot research topics in theoretical physics today is quantum gravity. While physicists already figured out how to treat the rest of the basic forces in nature quantum mechanically, gravity is proving to be more difficult in this respect. One may ask, why bother? Why can’t we just live happily with our existing classical description of gravitation, namely general relativity? After all, there is no experimental evidence that gravity acts differently on small scales.

I’ll give my view on this in a minute, but please take it with a grain of salt, me being a mere undergrad and all

Most problems nature presents us with are too complicated for us to solve. If we are ever to solve anything, we must make our models as simple as possible: Take into account only those aspects of the system that are absolutely essential to describe the phenomena we are interested in. Otherwise, the models will lie beyond that tiny region of mathematics that we know how to handle. Deciding which parts to keep and which to throw out is a guessing game, and sometimes we get it wrong. One of the clues that we got it wrong is the appearance of a singularity: A region of the problem where the model breaks down, and fails to provide a solution.

There are several examples. One is the ultraviolet catastrophe in blackbody radiation, which provided evidence that classical physics was insufficient, and which also showed that quantization might be a good way of looking at things. Another example, I believe, is from hydrodynamics, where viscosity (internal fluid friction) is sometimes introduced to get rid of singularities.

So what does this have to do with quantum gravity? Well, our classical description of gravity, general relativity, contains singularities. One such singularity occurs at the beginning of time — the big bang. Einstein’s equations allow us to calculate what happened right after the big bang, but they don’t tell us what went on prior to it. They sort of break down.

Judging from history, this suggests that general relativity is not a complete description of gravity–that something is missing. And betting on quantum gravity as that something seems to me a pretty good bet, seeing how we were able to solve similar problems we had with the other forces using this method.

I’d like to thank prof. Oded Agam from the HU, whose discussion of singularities in physics during a recent colloquium led me to this post