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Does the graviton exist? If so, why has it never been detected? How does this fit with Einstein's belief that gravity is actually the curving of space/time?
Submitted by Nick from the USA

First, what's a graviton?

On the microscopic level, the other fundamental forces in the universe work by exchanging "carrier" particles. The photon is the carrier of electromagnetism. Microscopically speaking, static cling in your laundry is your socks tossing photons back and forth. The zap in a freshly dried sock contains an enormous number of photons, but there are more delicate experiments where single photons are evident. The weak nuclear force has carriers called W and Z bosons, which didn't get detected until more recently because they're massive and thus hard to make in accelerators. The strong nuclear force is carried by gluons, which haven't been directly detected on their own and probably never will be because of the way they combine. But the matter particles are arranged in groups exactly like you'd expect from the combination rules for gluons, which is regarded as solid indirect evidence that gluons exist.

Since the other forces work by exchanging carrier particles, we assume that gravity does too and call the carrier the graviton. From the macroscopic properties of gravity you can work out what the graviton should be like. For example, it should be massless since the range of gravity is infinite. It doesn't do combinatorics like the gluon, so it should be out in the open and detectable in principle.

But in practice, it'll be a long time before anyone detects a graviton. You need lots of them to make any visible effect because gravity is the weakest of the fundamental forces. Since gravity holds you down all your life that might sound strange, but it's true on a microscopic level. Any two electrons are pushed apart by electricity far more strongly than they're pulled together by gravity. So you only detect things involving huge numbers of gravitons, where the collective effects swamp the microscopic effects which indicate the existence of individual particles.

So how does this fit with Einstein's idea of gravity being geometry? There is some suggestion that the graviton itself is a collective effect that shows up when you tweak the underlying structure of spacetime in a certain way. But the whole question of how quantum mechanics (which includes the business with the carrier particles) fits with general relativity is still wide open.