Quantum gravity vs. general relativity

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Discussion Overview

The discussion revolves around the nature of gravitons, their detectability, and the implications for distinguishing between gravitational fields and acceleration. Participants explore theoretical frameworks, including quantum gravity and general relativity, while addressing concepts such as virtual gravitons and the Unruh effect.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that if gravitons exist, a perfect detector would not be able to detect virtual gravitons in a static gravitational field, similar to how a photon detector does not register virtual photons in a static electric field.
  • Others argue that the inability to detect virtual gravitons does not imply they do not exist in static fields, raising questions about the completeness of the graviton concept.
  • A participant mentions the Unruh effect, suggesting that an accelerating observer would measure thermal photons, which parallels the discussion on detecting gravitons.
  • Some contributions emphasize that gravitons may only be relevant in dynamic scenarios, such as gravitational waves, rather than static fields.
  • There is a discussion about the implications of detecting gravitons for the principle of equivalence in general relativity, with some suggesting that it could challenge the notion that local experiments cannot differentiate between acceleration and gravity.
  • Questions are raised about the size and wavelength of gravitons, particularly in relation to static gravitational fields, with references to astrophysical phenomena.
  • One participant discusses the complexities of quantum chromodynamics (QCD) and quantum electrodynamics (QED) in relation to static fields and their underlying gauge theories.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature and detectability of gravitons, with no consensus reached on whether static gravitational fields can be associated with gravitons or how they should be understood in the context of quantum gravity.

Contextual Notes

Limitations include unresolved definitions of virtual versus real gravitons, the applicability of quantum field theories to gravity, and the complexities involved in measuring static versus dynamic fields.

  • #31
That seems to be rather interseristing for special cases in QM. But how does it affect my statement "even in well-understood theories like QCD the plane waves are a very poor approximation ..."?
 
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  • #32
Plane waves are the initial approximations for PT calculations. My simple QM example may serve to estimate the meson masses from PT calculations. Concerning three-quark baryons, it is probably harder but I think it is still possible.

Of course, the plane waves without PT corrections are poor approximations.

Bob.
 
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  • #33
kashiark said:
Let's suppose gravitons exist, and you have a machine that is 100% effective at detecting them. If you were in a room with no windows, and there is an apparent gravitational field, then would using this machine let you tell if you were in a gravitational field or accelerating?

I'm being (somewhat) whimsical here, but a simple bathroom scale can be used to detect gravitons. If there is a reading on the scale, you have gravitons, if not, you don't. The interesting thing about this (whimsical) situation is that if the scale is accelerating, it turns out to display a reading, which would imply that acceleration generates gravitons!
 
  • #34
I think we are mixing different things:
a) are gravitons only small quantum exitations of the gravitational field?
b) or is a static field composed of gravitons?
c) if b) does acceleration generate gravitons?

I think that gravitons in the sense of a) are a very special concept that has been shown not to produce a viable theory; therefore I think b) is the rigth answer. And that means that I have to admit that c) is correct, too.

Neverthelesee, I still think we that don't agree what gravitons really are.
 
  • #35
tom.stoer said:
I think we are mixing different things:
a) are gravitons only small quantum exitations of the gravitational field?
b) or is a static field composed of gravitons?
c) if b) does acceleration generate gravitons?

I think that gravitons in the sense of a) are a very special concept that has been shown not to produce a viable theory; therefore I think b) is the rigth answer. And that means that I have to admit that c) is correct, too.

Neverthelesee, I still think we that don't agree what gravitons really are.

I forgot the name of it, but there is a theory that proposes acceleration is quantized. It is used to explain dark matter, and the Pioneer annomoly. So if gravity is equivalent to acceleration, then are gravitons a form of quantized acceleration in general?
 
  • #36
Are you talking about MOND (= modified Newtonian dynamics)? It seems to explain DM, but acceleration is not quantizied, as far as I know.
 

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