Is the graviton believed to exist or is it purely hypothetical?
I only ask because my Physics teacher often talks about it as if it is the force carrier of gravity when from my understanding there is no evidence of its existence.
Posted Oct 9, 2016, 3:16 PM UTC
166
There's a lot of good reasons to think that it probably exists, in the sense that whatever the "correct" quantum theory of gravity will be, the theory will look effectively like a theory of gravitons at low energy. With that said, it's likely that the graviton description will be replaced by something which looks very different at high energies.
So when we refer to a "graviton," what we really are referring to is a massless spin-2 particle. It turns out that if you take Einstein's gravitational theory and attempt to quantize it, you get such a particle. Similarly, if you start with the quantum theory of a massless spin-2 particle, you can derive Einstein's general relativity if you take the classical limit. Gravitons seem to be the unique description of quantum gravity at low energies.
So unless something goes extremely wrong with quantum gravity already at very low energies, we expect that gravitons exist. But you're right that these are just very convincing theoretical arguments, and there isn't any experimental verification yet.
Finally, the theory of gravitons only makes sense at low energies. At high enough energy, the theory becomes unpredictive for reasons which have been explained on this forum before (here's me giving a stab at it). But this doesn't invalidate the theory at low-energies, there's a very long history of similar theories that only work well at low-energies but required us to find a new theory which fixed the high energy behavior.
Thanks for the insight!
The Standard Model of Particle Physics (essentially all of particle physics as we know it) being a great example! To expand on why this is the case, since our models are constructed based on observations, and measurements of higher energies/smaller distances are much harder to make, the data that we have is kind of a low resolution picture that glosses over higher energy details. That's why the Standard model is said to be an effective field theory, because it's thought to be a low energy approximation of a more complete theory.
Strictly speaking you could have free, non-interacting spin-2 particles although arguably that's gravity in the zero strength limit. IIRC there's a linear coupling that's consistent too.
Hello, I know this is an incredibly late comment, and I hope it finds you well.
I'm searching around because I remember reading on Reddit a couple of months ago that scientists made a breakthrough and found definitive evidence of the existence of the graviton, however, all Google searches I've conducted seem to indicate otherwise. Are we still in the theoretical realm here?
This is likely referring to last year's discovery of gravitational waves. This was a fundamental part of low-energy general relativity which had not been confirmed yet, and as I said before the graviton description gives low-energy GR so one could kinda say that gravitational waves are a prediction of gravitons (though not a new or quantum prediction).
I mentioned above that unless something goes really wrong with low-energy GR, gravitons seem to be the unique theory. But if there were no gravitational waves, GR would be incorrect at low-energies anyways which would really put a wrench in the idea of gravitons.
Finally, gravitational waves should be made up of gravitons in quantum gravity the same way that electromagnetic waves are made up of photons in quantum electromagnetism. So if you consider detecting light as detecting photons, then detecting gravitational waves is like detecting gravitons. But it's a little misleading, since these observations are already consistent with pure classical physics.
What a great response! Yeah, I was definitely thinking about gravitational waves. I had read scientists just assumed they were real but as of then not yet observed, so I guess knowing all this wasn't a real breakthrough but the puzzle piece we knew fit but had to be found underneath the couch cushion.
So are we just waiting for a brilliant scientist to dream up an iconic procedure along the lines of the double-slit experiment regarding one of the fundamental forces of the universe? Seems a bit tougher, but then again, imagine making the discovery that the invisible radiation that lights up your room actually exists as a particle (as well) to interact with physical matter! That probably wasn't consistent with physics then, either.