So this is a bit of a duplicate: Is the universe fully transparent to gravitons?

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Discussion Overview

The discussion centers around the concept of whether the universe is fully transparent to gravitons, as suggested by a recent article. Participants explore the implications of graviton detection, the nature of gravitational waves, and the theoretical frameworks surrounding these concepts, including quantum gravity and cosmic gravitational wave backgrounds. The scope includes theoretical implications, conceptual clarifications, and speculative reasoning regarding the early universe and the nature of transparency in physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Philosophical

Main Points Raised

  • Some participants question the meaning of "transparent to gravitons," noting the lack of a theory for gravitons and suggesting that the universe may not be transparent to them.
  • One participant proposes that if a graviton emitter and detector exist, a black hole could obstruct the detection, implying limited transparency.
  • Another participant clarifies that while gravitational waves have been detected, gravitons have not, and argues that the universe may not need to be fully transparent for a cosmic gravitational wave background to exist.
  • Concerns are raised about the implications of detectability and whether detection implies change, with analogies drawn to light passing through glass.
  • Participants discuss the limitations of information that could be gleaned from the early universe and the implications of the Bekenstein Bound on information content.
  • There is a philosophical debate about the nature of detectability and change, questioning whether detecting something necessarily alters it.
  • One participant suggests that if a cosmic gravitational background is detected, it could challenge the inflation theory, which posits that such a background should be diluted to undetectability.

Areas of Agreement / Disagreement

Participants express differing views on the transparency of the universe to gravitons and gravitational waves, with no consensus reached on the implications of detectability or the nature of transparency. The discussion remains unresolved regarding the theoretical frameworks and interpretations of the concepts involved.

Contextual Notes

Limitations include the absence of a clear theory for gravitons, the speculative nature of the arguments presented, and the unresolved mathematical aspects related to the early universe and cosmic gravitational wave backgrounds.

  • #31
Paul Colby said:
could one reframe the OPs question in terms of low energy gravitons?
As I already pointed out in post #3, the OP question can and should be reframed in terms of classical gravitational waves, not gravitons. Nothing in the OP's question actually depends on any putative quantum aspects of gravity.
 
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  • #32
PeterDonis said:
Where are you taking that away from?
the final statement of the reply.

“But it's not all lost. As I tried to explain in the last paragraph, the problem of non renormalizability is actually an issue of high energies. The theory remains predictive at the energies we can reach in the collider. However, at energies larger than ##M_p## we have no clue.”
 
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  • #33
Paul Colby said:
the final statement of the reply.
Should not be taken as a statement about an actual quantum theory of gravity, since that would imply that we have a collider that can run experiments at or near the Planck energy--which we don't, by many orders of magnitude. We certainly do not have collider experiments involving gravitons, and we don't expect to. Where gravity is concerned, the "low energy limit" is classical GR; to the extent the non-renormalizable spin-2 quantum field theory of the "graviton" plays any role, it is purely an abstract one, that the field equation for this quantum field turns out to be the Einstein Field Equation, so the theory is consistent with classical GR in the low energy limit.
 
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  • #34
PeterDonis said:
Should not be taken as a statement about an actual quantum theory of gravity, since that would imply that we have a collider that can run experiments at or near the Planck energy--
Fermi’s theory of weak interactions was not renormalizable yet it yielded answers that could and were, tested. If there are low energy gravitational quanta, they may well be beyond detection, as all the cross sections end up being super small.
 
  • #35
Paul Colby said:
Fermi’s theory of weak interactions was not renormalizable yet it yielded answers that could and were, tested.
So what? I am not saying that no non-renormalizable theory can ever be tested.

Paul Colby said:
If there are low energy gravitational quanta, they may well be beyond detection
You're missing the point. We can detect low energy classical gravitational waves already, so obviously there are processes that, if we insist on modeling them using the spin-2 quantum field theory at low energy, do not have cross sections that are too low to detect. It's just that, for those processes, the predictions of this low energy quantum field theory are exactly the same as those of the classical theory of gravitational waves; so there is no way to test the quantum theory by experiment in this way.

What we do not have any prospect of testing, now or in the foreseeable future, is any scenario in which the quantum theory of gravity would make different predictions from the classical theory. That is what we would need a Planck energy accelerator for.
 
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  • #36
PeterDonis said:
You're missing the point.
Perhaps I am. Maybe you could explain further. In the usual naive gravitational field theory, energy is exchanged with mater in quanta, ##E=\hbar \omega##. In a low energy test, an atom could absorb a graviton quanta. Now, I'd be the first to agree that the cross section of such a reaction is exceedingly small making it impractical to detect. However, the process is allowed and not classical as far as I can tell.
 
  • #37
Paul Colby said:
In a low energy test, an atom could absorb a graviton quanta.
Yes, but we have no feasible way of testing for this, now or in the foreseeable future. Or for any other process where the quantum theory would make a different prediction from classical GR, which is the classical limit of the quantum theory.

But now consider a process such as the detection of a gravitational wave by LIGO. We could, in principle, try modeling this using the non-renormalizable spin-2 quantum field theory. But we would get the same prediction as for classical GR, which is the classical limit of the quantum theory. So we can't use this kind of test to test the quantum field theory.

Paul Colby said:
the process is allowed and not classical as far as I can tell.
Yes. I have not said otherwise. I was not saying that there can't be any non-classical processes involving gravity.
 
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  • #38
Vanadium 50 said:
Now place a black hole between them. I would expect -= and again, there's no theory so no way to do the calculation - the detector to stop seeing the emitter.

Please forgive the question, but can gravitons escape a black hole? If not, then how do black holes attract other objects?