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They travel at the same speed, why? Maybe they are the same thing, but seen from different perspectives.

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marked as duplicate by Kyle Kanos, Alfred Centauri, ACuriousMind, JamalS, Qmechanic Mar 28 '15 at 14:11

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  • $\begingroup$ Afaik. light consist of photons, which is the particle of electromagnetic interaction, while gravitational interaction has its dedicated particle: gravitons. So I doubt there is any relation between light and gravitational waves. $\endgroup$ – inf3rno Mar 28 '15 at 12:38
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    $\begingroup$ hi we notice light waves all the time, say from the sun, or a torch, but as yet we have not detected gravitional waves, so no is my answer. Could you edit your question to say from what perspective are you referring to? $\endgroup$ – user74893 Mar 28 '15 at 12:39
  • $\begingroup$ @inf3rno: note that gravitons are still hypothetical. $\endgroup$ – Kyle Kanos Mar 28 '15 at 12:39
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    $\begingroup$ possible duplicate of If gravitational waves exist are they technically just another form of light/electromagnetic wave? $\endgroup$ – Kyle Kanos Mar 28 '15 at 12:40
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    $\begingroup$ All excitations of massless fields travel at the speed "of light", which is really the speed of massless objects, see also this Q&A. $\endgroup$ – ACuriousMind Mar 28 '15 at 13:18
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Could gravitational waves and electromagnetic waves/light be the same thing?

I think most physicists would say they aren't, in that gravitational waves are thought to be quadrupole waves, whilst electromagnetic waves are thought to be dipole waves. However it's worth noting that the photon can be considered to be a singleton electromagnetic wave, and that it has an non-zero "active gravitational mass". As per Einstein's E=mc² paper, the photon conveys inertia between the emitting and absorbing bodies. It conveys energy, and it conveys a gravitational effect. And one thing that really struck me was Einstein talking about field theory in 1929. He was talking about gravitational fields and electromagnetic fields, and said this:

"The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds..."

So I think there may be a sense where the photon isn't just an electromagnetic wave, but is a gravitational wave too, of sorts. Not the only sort, but enough to throw a spanner in the works for things like quantum gravity and LIGO.

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  • $\begingroup$ "the photon can be considered to be a singleton electromagnetic wave" [citation needed] The precise connection between a classical EM wave and the quantum object that is the photon is notoriously elusive. $\endgroup$ – ACuriousMind Mar 28 '15 at 13:59
  • $\begingroup$ See for example this wherein "Maxwell's equations exactly define the propagation of a lone photon in free space". The photon has a wavelength λ and a frequency f as per the expressions E=hf and E=hc/λ. $\endgroup$ – John Duffield Mar 28 '15 at 15:32

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