# Gravitational waves speed in a medium

1. Feb 17, 2016

### Gerinski

Electromagnetic waves are known to travel a c in a vacuum, but at lower speeds in a material medium.
What about gravitational waves? They are also predicted to travel at c in a vacuum, but what about them traveling through material mediums? Do they get slowed down? by which factor?

TX

2. Feb 17, 2016

### andrewkirk

I doubt there has been much work done on that. The calculations I have seen regarding gravitational waves have all assumed that outside the source region there is negligible mass-energy, and accordingly assumes the stress-energy tensor outside the source to be zero. Since a medium has mass, the question of speed through a medium would be outside the scope of such calculations.

3. Feb 17, 2016

### bcrowell

Staff Emeritus
See Ingraham, "Gravitational waves in matter," GRG 29 (1997) 117. Ingraham suggests that the best chance for seeing $v<c$ might be in "gravitational wave propagation through molecular gas clouds of galactic or intergalactic size." Although n−1 would be incredibly small, he suggests that you might be able to see an effect accumulated over thousands or millions of light-years.

Note that the recent aLIGO result puts a very tight upper limit on dispersion.