# Do gravitational waves propogate slower when blocked by mass

1. Oct 16, 2015

### NotJimmy

I'm not sure exactly how to phrase this question, but I was thinking earlier about electromagnetic waves being absorbed by atoms and 'slowing down' the speed of light.

Do gravitational waves propagate slower when blocked by, say, a really massive object? In the same way that light slows down when blocked by a medium like water or gas?

If not, why?

2. Oct 17, 2015

### Staff: Mentor

Gravitational waves are influenced by the spacetime geometry, so in general there is an effect. It is extremely tiny, however.

3. Oct 17, 2015

### NotJimmy

Is it noticeable at all when you're dealing with very large masses?

Like, let's say I had a 'transmitter' on one side of VY Canis Majoris moving around an object with great mass and generating gravitational waves. On the other side, I had a 'receiver' that filters out all of the noise and detects my transmitter's gravitational waves. How big would the discrepancy be between the speed of those gravitational waves and the speed of light?

4. Oct 17, 2015

### Staff: Mentor

You would get the Shapiro delay, something of the order of 1 millisecond for a very massive star.

5. Oct 17, 2015

### NotJimmy

What's the actual mechanism that slows down the gravitational waves though? I think I can understand how light waves slow down in a gravitational field, since they get bent on their path (correct me if that's the wrong interpretation), but how does that work with gravitational waves?

I'm just a non-physics student trying to sate my own curiosity, so sorry if I'm mangling some of the theory here.

6. Oct 17, 2015

### Staff: Mentor

The Shapiro delay is not a bending in space, it is an effect of (4D-)curved spacetime, the same in both cases.