Just a random, baseless question I'm throwing out there; but how do we know that we haven't discovered the graviton? How do we know we haven't just labeled it as something else? i.e. how do we know for instance the Higgs boson isn't a graviton? If we found it, and it wasn't like what was predicted, would we know to label it as a graviton?

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    $\begingroup$ Saying that we haven't discovered the graviton is like saying we haven't discovered the photon in 1850. This sentence means that "we have not yet detected quantum aspects of the gravitational wave". $\endgroup$ Aug 20 '18 at 0:55
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    $\begingroup$ It has not been proven that gravity is quantum. There may be no gravitons at all. $\endgroup$
    – safesphere
    Aug 20 '18 at 5:03

The properties of the graviton include:

  • it has spin 2
  • it is massless (so the force it carries is long range)
  • it couples to the stress-energy tensor, which basically means everything should be able to emit and absorb gravitons
  • it has zero electric charge and zero color charge

These are extremely distinctive properties, so it's really unlikely that we've already seen the graviton and not noticed. For example, the Higgs has spin 0 and it is massive.

In fact, since the graviton is massless and can be produced by anything, we hypothetically produce tons of them all the time; you produce them whenever you move. The issue is not that we've seen it and not noticed, but that a graviton is too feeble to be seen at all.

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    $\begingroup$ It also has zero isospin charge (unlike the Higgs). More generally, it has no non-trivial internal charges, because the metric has no internal indices. $\endgroup$ Aug 20 '18 at 1:12
  • $\begingroup$ Neutrino almost fits the bill, but it has spin 1/2 instead of 2. Would a composite of four neutrinos work? $\endgroup$ Aug 20 '18 at 10:53
  • $\begingroup$ @JohnDvorak the Weinberg Witten Theorem rules out composite gravitons in a local QFT $\endgroup$ Aug 20 '18 at 11:28
  • $\begingroup$ @JohnDvorak Neutorinos have masses. $\endgroup$ Aug 20 '18 at 12:40

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