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The nature of G waves vs tidal effects (and inflation)

  1. Feb 14, 2016 #1
    My simple description of gravitational waves is that they are due to the relativistic principle of locality, i.e. the fact that the action of gravity is not instantaneous, something that you can’t see from the tidal effects in Newtonian physics. Is it correct?

    I've also found a comment explaining them by "the principle of action equal reaction, which is true in GR too", so it would be " understandable that the acceleration of two bodies toward each other asks for and equal and opposite release of energy."
    Well, such an answer appears to be IMHO unnecessary complex in the context of distinguishing relativistic from classical physics.
    Anyway, one could simply answers that question about the nature of this release of energy, saying that it is mass, lost and transformed into gravitational waves. In fact it is the standard answer here. Is it true?

    But if you invoke the relativistic conservation of momentum, then it looks like you’re going to introduce mathematical complexity, pseudo-tensors and advanced staff like jet bundles… and finally one needs to introduce parallel transport so that you can get an “energy conservation law” in integral form. Look at Baez link and also this question. What about these concerns?

    A second point of my question (maybe this is really a distinct question, sorry!): it is also unclear to me why/whether gravitational waves would be initially a quantum effect, related to quantum fluctuations (as opposite to e.g inflation waves). They are predicted by GR, so I fail to fully understand the relation with quantum fluctuations... but I assume it would be some huge effect of dark energy in GR, like bursts of gravitational waves generating other gravitational waves (is it actually possible?)
  2. jcsd
  3. Feb 14, 2016 #2


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    Yes, this is a valid argument to show that gravitational waves must exist.

    Whether it's valid to say that "that they are due to" this is a different question. An argument that explains that something must happen is not the same as a cause for the thing to happen. Depending what you mean by "due to," you could say that gravitational waves occur because the Einstein field equations have wave solutions, or that they occur because a certain oscillating mass quadrupole existed.

    No. Why would you think this?

    This doesn't make much sense to me. Newton's third law doesn't even hold in E&M. It certainly can't hold in general in GR, for a variety of reasons.

    If "the principle of action equal reaction" means anything, it means what Newton's third law says. It doesn't have anything to do with energy.

    GR is a classical theory. Gravitational waves are a classical phenomenon.
  4. Feb 14, 2016 #3
    Perfect answer, even though very short. I would like to reply a lot of things for each point but I have to sacrifice them to priorities :-) I don't want to waste your time.
    So, the most important one to me is your "No. Why would you think this?". I hope this has been caused by my bad English wording...
    Incidentally I am surprised that you call GR a classical theory [in this forum the GR section is separated from the Classical one... but I understood what you mean: not Quantum]: so now I have to use another adjective "Newtonian", to refer to something that is not relativistic...

    Back to the point: I'm asking to confirm or correct the following:

    Earth tides (due to the moon, sun, the rotation, centrifugal and cohesive forces or whatever) are explained by newtonian physics or in other words they are not an effect of GR gravitational waves, are they?

    Thank you :smile:
    Last edited: Feb 14, 2016
  5. Feb 14, 2016 #4


    Staff: Mentor

    Yes; but newtonian physics is just an approximation of GR that is useful in the weak field, slow motion limit.

    No; but gravitational waves are certainly not the only effect of GR that does not appear in the Newtonian approximation.
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