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If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fl

If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2. https://en.wikipedia.org/wiki/Graviton https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton." It links to other resources from there.

raverbashing 8y ago
Yes but while we don't have the unification between QM and GR we don't have the full picture. We know what fits GM, but we don't know if it's the only solution.
codeflo 8y ago
Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.
- gpderetta 8y ago
  disclaimer: IANAP. as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field). In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge. Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field. At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).
marcosdumay 8y ago
> GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. IANAP, and don't get the full picture. But isn't that definition itself derived by the fact that gravity has a single polarity?
nonbel 8y ago
>"If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model." Somehow I doubt it. I bet as a save someone would propose "dark spin" or some elaborate calculations that shows a few factors happen to cancel out exactly but no one noticed before now.
- deytempo 8y ago
  Dark spin? It spins in a direction that nothin else can?
  - nonbel 8y ago
    I was thinking something like it spins along a dimension so small it can't be detected without a particle collider the size of the solar system.
feanaro 8y ago
Well GR is potentially very wrong both at very large scales (seemingly requiring dark matter and dark energy) and very small scales (due to known present incompatibilities with quantum mechanics). It's only been shown to be a highly accurate model everywhere in between.
- drjesusphd 8y ago
  I think both dark matter and dark energy are perfectly consistent with GR. Hitherto unobserved matter would contribute to the stress energy tensor and dark energy is (I believe - has this changed?) the cosmological constant.
  - feanaro 8y ago
    Yes, they are consistent. In fact, they are the result of the following line of thinking: given that we are observing unexpected results once we account for (using the GR paradigm) all the clearly apparent mass and energy, what else could we change so that GR doesn't have to change? In other words, the consistency with GR is already baked in. That's not to say that the approach doesn't have huge merit. One model for dark energy is indeed the cosmological constant, but there are other approaches, most notably quintessence (https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)).

raverbashing 8y ago

Yes but while we don't have the unification between QM and GR we don't have the full picture. We know what fits GM, but we don't know if it's the only solution.

codeflo 8y ago

Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.

gpderetta 8y ago
disclaimer: IANAP. as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field). In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge. Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field. At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).

marcosdumay 8y ago

> GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. IANAP, and don't get the full picture. But isn't that definition itself derived by the fact that gravity has a single polarity?

nonbel 8y ago

>"If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model." Somehow I doubt it. I bet as a save someone would propose "dark spin" or some elaborate calculations that shows a few factors happen to cancel out exactly but no one noticed before now.

deytempo 8y ago
Dark spin? It spins in a direction that nothin else can?
- nonbel 8y ago
  I was thinking something like it spins along a dimension so small it can't be detected without a particle collider the size of the solar system.

feanaro 8y ago

Well GR is potentially very wrong both at very large scales (seemingly requiring dark matter and dark energy) and very small scales (due to known present incompatibilities with quantum mechanics). It's only been shown to be a highly accurate model everywhere in between.

drjesusphd 8y ago
I think both dark matter and dark energy are perfectly consistent with GR. Hitherto unobserved matter would contribute to the stress energy tensor and dark energy is (I believe - has this changed?) the cosmological constant.
- feanaro 8y ago
  Yes, they are consistent. In fact, they are the result of the following line of thinking: given that we are observing unexpected results once we account for (using the GR paradigm) all the clearly apparent mass and energy, what else could we change so that GR doesn't have to change? In other words, the consistency with GR is already baked in. That's not to say that the approach doesn't have huge merit. One model for dark energy is indeed the cosmological constant, but there are other approaches, most notably quintessence (https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)).