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It stands to reason if gravity changes the color of light then the color will be affected by gravitational waves. My question is, when the waves pass, will the color change be permanent or will the light always return to its original color? It seems to me that if gravitational waves have energy and momentum and can impart this upon objects it could do so also to photons – changing their frequency.

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    $\begingroup$ Gravity admits three vertex graviton interaction, so one can have an energy change in the photon-graviton scattering, like in the Coulomb scattering in the limit of $m \rightarrow 0$ arxiv.org/abs/1410.4148 $\endgroup$
    – Pipe
    Commented Sep 17, 2020 at 5:50

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My question is, when the waves pass, will the color change be permanent or will the light always return to it's original color?

One has to keep clear in discussing light , its particle nature, and its classical electrodynamics wave nature. The classical electrodynamic wave can be mathematically shown to emerge from the underlying quantum mechanical. In simpler words the light we see is a confluence of an enormous number of photons. In analogy, it is usually assumed that gravitational waves are a confluence of an enormous number of gravitons.

Photons and gravitons are quantum mechanical particles. Once they scatter there can be energy transferred and this means that the photon frequency will change . This change collectively will show effects only in the $dt$ time and dV volume where there is the crossing of the two classical waves, when the particles interact. The waves before and after are unaffected.

There cannot be a permanent change in the color of the classical light , because new photons of the original frequency are continually traveling in the direction of classical light. Only at the crossing point there will be changes in the frequency of the photons if the scattering is inelastic. Considering the coupling constants involved in the interaction between photons and gravitons this would be hardly detectable, as most would not interact.

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  • $\begingroup$ What do you mean by hardly detectable? Sounds like you are implying there is a change. $\endgroup$ Commented Sep 17, 2020 at 23:30
  • $\begingroup$ Note " hardly detectable, as most would not interact." . At the crossover volume, if one believes in quantum mechanics as a theory for gravitation also, the couplings allow for scattering inelastically and thus changing the energy of the photon and the graviton, But the couling constants make it a tiny number, undetectable with our experimetal means . hyperphysics.phy-astr.gsu.edu/hbase/Forces/couple.html $\endgroup$
    – anna v
    Commented Sep 18, 2020 at 3:29
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Why would the photons change their state forever? As the gravitational waves approaches, the space-time in their vicinity is distorted. As the waves pass, the space-time will revert to its original state. If you were able to observe the photons as the waves come in and pass, then you will momentarily observe a change in the paths of the photons. After the waves pass there is no reason for this effect to last forever. So the photons will not change their frequency forever (this would also violate the law of conservation of energy, wouldn't it? Assuming there is nothing more complex happening like "graviton-photon scattering").

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  • $\begingroup$ It would not violate the law of conservation if the energy comes from the gravity waves. It seems likely if gravity waves have energy and momentum they can do work. We know photons respond to and create gravity, so if the gravity waves can impart energy on matter why not light? $\endgroup$ Commented Sep 17, 2020 at 23:32
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It changes the color of photons temporarily. That state won't last forever

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