I've heard that gravitational waves travel at the speed of light, and have some parallels to electromagnetic waves.

EM waves slow down as they pass through matter (speed of light in glass is slower than in vacuum, for instance). Do gravitational waves also slow down as they pass through matter?

If so, are there any effects like Cherenkov radiation when matter passes through a medium at a speed greater than the velocity of gravity in that medium?

Do large masses like stars or Jupiter act as ball lenses for gravitational waves?

  • $\begingroup$ I guess a good answer would explain why light slows down when it passes through matter $\endgroup$
    – endolith
    Sep 8, 2020 at 13:43

2 Answers 2


Unlike electrostatic charges, mass is always positive, so that there is no dipole density to deflect gravitational waves as they pass through a material. So the answer is no, not in an analogous way.

The main effect is the gross retardation of gravitational waves by matter the same way that they retard light, by focusing. This is just the bending of light/gravitational-waves associated with gravitational lensing.

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    $\begingroup$ That "dipole density" is irritating me. Refraction of eg visible light is not due to dipoles. Its just the electrons not moving "instantly" with the changing electric field. A (theoretical) collection of electrons in vacuum with similar density as in a glass, would have the same refractive index without any dipoles, wouldn't it? $\endgroup$
    – Georg
    Nov 18, 2011 at 10:25
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    $\begingroup$ @endolith: it may not sound right, but it is right. If you fire photons and gravitons together, their path is the same. The gravitational lensing will slow down photons and gravitons the same way. But it isn't a material property, like an index of refraction, because it is not properly extensive like dipole scattering summed over many many atoms is. $\endgroup$
    – Ron Maimon
    Nov 21, 2011 at 4:42
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    $\begingroup$ @Ron, could you answer Georg's question? Would light passing through an electron gas slow down? $\endgroup$ Nov 5, 2012 at 4:44
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    $\begingroup$ @JessRiedel: I misunderstood Georg's comment, that's a good question. Light doesn't pass through an electron gas. If you keep the electrons together like this with a low potential region to confine them, you get metallic total reflection, you don't get slowing down. Slowing down is a property of dipole formation in response to field. You propagate light with index through a material with permittivity. $\endgroup$
    – Ron Maimon
    Nov 5, 2012 at 5:11
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    $\begingroup$ I realize I am far too late to be likely to get any response, but I am not so sure that Ron's last comment is correct. Shouldn't there be a plasma frequency of such a confined system? In that case, I would expect that light below this frequency is reflected as he said, but light above this frequency is able to propagate through the confined electrons. Am I missing something? $\endgroup$
    – Rococo
    Nov 30, 2015 at 20:36

You ask:

Do gravitational waves also slow down as they pass through matter?

and an answer has been provided above.

If so, are there any effects like Cherenkov radiation when matter is traveling faster than the speed of gravity?

To be clear the speed of gravitational waves is supposed to be c, the speed of electromagnetic light.

The recent publication of the OPERA superluminal neutrino results brought out a study by Andrew G. Cohen, Sheldon L. Glashow that answers the question positively. Matter moving superluminally is expected to radiate a ring of pair production about its path, according to them.

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    $\begingroup$ I mean if the speed of gravity is slower than c as it passes through matter, are there effects caused by objects traveling faster than that speed (through matter)? $\endgroup$
    – endolith
    Jan 15, 2012 at 5:47
  • $\begingroup$ @endolith the speed of gravity is always c, it does not have an index of refraction effect. read Ron's comment. The speed of the gravitational wave is not affected by matter, its path can be changed by strong gravitational fields. $\endgroup$
    – anna v
    Jan 15, 2012 at 8:31

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