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ralqs
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Are there any theories of quantum gravity that don't incorporate gravitons?
bcrowell said:No, there aren't any theories of quantum gravity that don't incorporate gravitons, but that's just because there aren't any theories of quantum gravity
atyy said:How about if we consider quantum gravity as an effective field theory (just like QED)? Something like http://arxiv.org/abs/gr-qc/9512024 or http://relativity.livingreviews.org/Articles/lrr-2004-5/ ?
bcrowell said:No, there aren't any theories of quantum gravity that don't incorporate gravitons, but that's just because there aren't any theories of quantum gravity
bcrowell said:That part was a joke, hence the emoticon. I was emoting, really ... visualize me as William Shatner and that'll be about right.
bcrowell said:Basically there are pretty generic arguments to the effect that it's not possible to couple classical and quantized fields to each other. E.g., Bohr wanted the atom to be quantized and the electromagnetic field to be classical, but that really doesn't work; you'd be able to violate conservation of energy because without quantum-mechanical correlations more than one atom could absorb the energy from the same light wave.
martinbn said:What are the arguments for the case of the gravitational field? For the electromagnetic field, they are relatively easy and clear, but for gravity I have not seen anything convincing. I even asked the question elsewhere but didn't get any useful answers. In fact I was accused of being a non-believer in quantum gravity)
Are you arguing that a graviton may not need to exists (gravitational field doesn't need to be quantized), or that we need to look at quantization differently?Fra said:Given this difference, it's not clear that it's conceptually sound to think of bonafide quantization of gravity the same way as we do with other fields. The formal similarity when you quantize a perturbation doesn't IMO mean it makes sense.
ralqs said:Are you arguing that a graviton may not need to exists (gravitational field doesn't need to be quantized), or that we need to look at quantization differently?
Fra said:The obvious difference is that EM, weak and strong are defined relative to a background geometry/space.
But since gravity "is" space/geometry this same trick only works when you make an artificial split between what one may call asymptotic geometry and local geometry which is treated as a perturbation on the asymptotics. This works when you study a small subsystem, and you have a controlled environment which effectively encodes the asymptotic properties. This subsystem assymmetry we always have in cases where SM is tested. The interaction domain is small (< molecular scale).
Now, partly one can imagine this trick also for stuff like microscopic black holes, which are then defined as local distortions relative to the ambient field. But this does introduce an artificial split between background and disturbance.
Given this difference, it's not clear that it's conceptually sound to think of bonafide quantization of gravity the same way as we do with other fields. The formal similarity when you quantize a perturbation doesn't IMO mean it makes sense.
/Fredrik
A graviton is a hypothetical particle that is predicted by quantum gravity theories to be the carrier of the gravitational force. It is believed to be the smallest unit of gravity and is thought to interact with other particles through the exchange of virtual gravitons.
Determining the existence of gravitons is crucial for our understanding of the fundamental laws of nature. It would provide evidence for the existence of a quantum theory of gravity, which is currently one of the biggest mysteries in physics. It could also potentially lead to a unified theory that combines all of the fundamental forces of nature.
Gravitons, if they exist, are extremely difficult to detect due to their extremely low mass and weak interaction with other particles. Currently, there are no experimental methods that can directly detect gravitons. Scientists are working on developing new technologies, such as gravitational wave detectors, that may be able to indirectly detect the presence of gravitons.
While the existence of gravitons is predicted by most quantum gravity theories, there are some alternative theories that do not rely on the existence of gravitons. These include theories such as loop quantum gravity and string theory, which propose different mechanisms for how gravity works at the quantum level.
Currently, there is no definitive proof of the existence of gravitons. However, scientists are making progress in developing new experimental techniques and theoretical frameworks that may provide evidence for their existence in the future. It is a complex and ongoing area of research in the field of quantum gravity.