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If you held a handful of gravitons, would you be holding a handful of gravity, drawing things towards it, or do gravitons have to be exchanged or transmitted for gravity to take effect, in which case, a handful of stationary gravitons would not draw things towards itself?

Qmechanic
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Ben Warner
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At the present time, the graviton is a hypothetical particle, and we won't know its properties until we have a complete theory of quantum gravity. However, in most models the graviton is a massless particle like the photon. So, like the photon, it always travels at the speed of light. If these models are correct there is no such thing as a "stationary graviton". However, gravitons - again, like photons - carry energy and so contribute to the energy density of spacetime which causes the curvature of spacetime in general relativity. And it is this curvature of spacetime that we call gravity.

gandalf61
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  • Hypothetically, if a beam of gravitons passed the Earth, would we be pulled towards it? Let's say all the gravitons get extracted from Earth and moved to empty space, does that space now have Earth's gravity, or do the particles have to be inside actual mass to cause gravity? – Ben Warner Jan 19 '22 at 20:43
  • @BenWarner You have the wrong picture of graviton. The Earth emits gravitons (if they exist) but it does not contain them. In the same way, an LED emits photons but it does not contain them in some pre-emitted form. Just as you cannot extract all the photons from an LED, so you cannot extract all the gravitons from the Earth. – gandalf61 Jan 19 '22 at 21:06
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Talking about "gravitons" is inherently somewhat speculative, as there is no good quantum theory of gravity.

that said, John Archibald Wheeler worked out "geon" solutions to the classical Einstein equation, where a spacetime containing nothing but gravitational waves at early time ends in a collapse to a black hole at a late time, which indicates that gravitational waves (which, in a quantum theory, would be made up of many "gravitons") do, in fact, gravitate.

Note that this is a natural thing to expect from the Einstein equation, which is very nonlinear, which means that gravitational waves do couple with and interact with each other.

Zo the Relativist
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  • "gravitational waves do couple with and interact with each other" is not the same as "gravitons interact with each other. Is there a three graviton vertex in analogy with the three gluon vertex? – anna v Jan 19 '22 at 19:01
  • @annav: how could the classical theory self-interact but the quantum theory not? Also, your generic term in the Lagrangian is something like $g^{ab}g^{bc}g^{de}\partial_{e}g_{bd}\partial_{d}g_{ac}$, right? once you go and convert it into something perturbative, you will definitely end up with some odd-numbered exponents of components of $g$ in the perturbative Lagrangian, or at least if you don't it would require a lot of lucky cancellations. – Zo the Relativist Jan 19 '22 at 19:18
  • and I guess by "perturbative", I really mean, "background" plus "dynamical" part of the metric tensor, since $g_{ab}$ can't be a field itself, since a "zero field" is impossible, so you'd have to write something like $g_{ab} = \eta_{ab} + h_{ab}$, where $h_{ab}$ is the dynamical part, and $eta$ is some constant background metric. – Zo the Relativist Jan 19 '22 at 20:54
  • thanks. relevant question and answers here https://physics.stackexchange.com/questions/293873/do-gravitons-interact-with-each-other – anna v Jan 20 '22 at 04:24
  • So, basically "yes, there are graviton-graviton vertices because the lagrangian has terms with several factors of the metric tensor in it, and gravitational waves interact with each other, both of which are due to the same reason", then? – Zo the Relativist Jan 20 '22 at 18:20
  • seems to be the correct pov at the moment (pending definitive quantization of gravity) – anna v Jan 20 '22 at 18:34