So this is a bit of a duplicate.

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In summary, the researchers say that if gravitons can be detected, then the universe is not as transparent to them as is being asserted in the article. They argue that the universe should have a cosmic gravitational wave background for the same reason that it has a cosmic electromagnetic wave background, and that we should be able to detect such a background radiation today if it exists.
  • #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.
 
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  • #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?
 
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1. What does "So this is a bit of a duplicate" mean?

"So this is a bit of a duplicate" means that the information or topic being discussed has already been mentioned or addressed before. It can also imply that the current discussion is similar to a previous one.

2. Why is "So this is a bit of a duplicate" often used in online forums or discussions?

This phrase is often used in online forums or discussions to acknowledge that the topic has been previously discussed and to prevent duplicate threads or posts. It can also be used to redirect the discussion to a previous one.

3. Is "So this is a bit of a duplicate" considered rude?

No, "So this is a bit of a duplicate" is not considered rude. It is simply a way to inform others that the topic has already been discussed and to avoid repetition in the conversation.

4. How can I avoid using "So this is a bit of a duplicate" in my discussions?

To avoid using this phrase, it is important to first do a quick search to see if the topic has already been discussed. If it has, you can refer to the previous discussion or add new information to it. If it hasn't been discussed, you can simply start a new thread or post.

5. Can "So this is a bit of a duplicate" be used in formal discussions or presentations?

No, "So this is a bit of a duplicate" is not appropriate for formal discussions or presentations. It is more commonly used in casual conversations or online forums. In formal settings, it is important to use clear and concise language without referencing previous discussions.

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