The discussion centers on the role of the graviton in the context of General Relativity (GR) and its relationship with quantum physics. Participants explore the conceptual differences between classical and quantum theories of gravity, the nature of gravitational interactions, and the implications of these theories for understanding spacetime geometry.
Participants express differing views on the necessity and role of gravitons in relation to GR and quantum gravity. There is no consensus on how to reconcile these theories or the implications of string theory for understanding gravity.
Some participants highlight the limitations of current theories, noting that the quantum field theory of gravity is non-renormalizable and that the relationship between quantum concepts and classical curvature remains unclear.
The "graviton" doesn't have any role at all in GR. The "graviton", or any "bosons" for that matter, is an artifact of Quantum Physics, not GR. Reconcileing GR with Quantum Physics is the main problem of Physics at the present time.La Guinee said:If I understand things correctly, GR says that gravity isn't really a force at all, gravity is just particles moving in the geometry of spacetime. Given this, I don't understand the role of the graviton. I thought that gauge bosons were force mediators, that they somehow communicated and transmitted a force to other particles. So why the need for a graviton?
jtbell said:Similarly, photons have no role at all in classical electrodynamics. They are part of quantum electrodynamics. The difference with gravitions is that we do not yet have a successful theory of "quantum gravitodynamics."
You were also told that gravitons are not a part of GR. Those are the two best answers to your questions. There isn't a whole lot more to be said. I can think of a couple of things though:La Guinee said:My whole point was that QED is completely different because the photon mediates the EM force between particles, but in the case of gravity, it seems like there's no need for a mediator because the geometry of spacetime is the force.
I know this gets into quantum gravity, but I think there's a better answer than, "We don't have a theory of quantum gravity so nobody knows."
La Guinee said:My whole point was that QED is completely different because the photon mediates the EM force between particles, but in the case of gravity, it seems like there's no need for a mediator because the geometry of spacetime is the force.
jtbell said:In classical electrodynamics, the electric and magnetic fields "mediate" the electromagnetic interaction, and photons do not exist. In quantum electrodynamics, photons "mediate" the electromagnetic interaction, and classical electric and magnetic fields do not exist, except as an "effective theory" in the non-quantum limit (very large numbers of photons).
In "classical" (non-quantum) GR, the curvature of spacetime mediates the gravitational interaction, and gravitons do not exist. In a (so far hypothetical) fully quantum theory of gravitation, gravitons (or something similar) will mediate the gravitational interaction, and spacetime curvature will presumably appear only as an "effective theory" in non-quantum limit (very large numbers of gravitions).
La Guinee said:My whole point was that QED is completely different because the photon mediates the EM force between particles, but in the case of gravity, it seems like there's no need for a mediator because the geometry of spacetime is the force.
Indeed and in addition GR represents EM forces and all other forces as waves not as particles.HallsofIvy said:My point was that the statement " GR says that gravity isn't really a force at all, gravity is just particles moving in the geometry of spacetime." is wrong. General relativity represents gravity as the curvature of spacetime, not as particles.
I suppose this is a little off topic, but does string theory provide an answer to how curvature arises from the graviton?
I think it does. I don't know enough about string theory or quantum field theories of gravity to be certain about what I'm saying here, but I'll describe the connection between gravitons and curvature as I understand it (or possibly misunderstand it):La Guinee said:I suppose this is a little off topic, but does string theory provide an answer to how curvature arises from the graviton?
I read an interview with Edward Witten where he said (roughly) that it's obvious to those who understand string theory really well that it is background independent even though the language they're using is background dependent.
Do you think you can write this form for all non-stationary spacetimes?Fredrik said:You write the metric as g_{ab}=\eta_{ab}+h_{ab} and only quantize h_{ab}. This means that a graviton is a quanta of "non-Minkowskiness" for lack of a better word. A lot of that "non-Minkowskiness" is curvature.
I have no idea. Is there any reason why we shouldn't? I mean, whatever g is, we should always be able to write g=\eta+(g-\eta) and define h=g-\eta, right?MeJennifer said:Do you think you can write this form for all non-stationary spacetimes?
I just assumed that the separation of g into two pieces is done like this: You start with a quantity g, which is assumed to be a tensor. Then you just express it as g=\eta+g-\eta where \eta is defined as the tensor that has components \eta_{\mu\nu} in some coordinate system. That makes sure that both \eta and h are tensors.Mentz114 said:I'm not so sure now but I do know that for gmn to be a covariant tensor, then under transformation from x^\mu to x^\bar{\mu}
g_{\bar{m}\bar{n}}=g_{mn}A^n_{\bar{n}}A^m_{\bar{m}}
where
A^n_{\bar{n}}=\frac{\partial x^n}{\partial x^\bar{n}}
I don't think \eta_{mn}+h_{mn} satisfies this rule.
I could be wrong, maybe someone can clarify ?
when I try to google for it, this thread comes up in the search results even though It's only been a few hours since I posted the words that matched the search.
there is no conceivable mechanism by which particle interaction could result in the perfectly smooth "bending" of light as it passes a massive star.