We all consider that gravity travels at the speed of light. Light travels at the speed of light except when it is in a medium ,say glass, where it travels slower. What happens when gravity passes through a distributed mass. Will it still travel at the speed of light or will each atom absorb and emit gravity as it passes. I realize this is not the way anyone would like to think about gravity, but an answer to this question may be interesting.
Asked
Active
Viewed 178 times
2 Answers
8
In principle a gravitational wave will travel slower than $c$ when passing through matter, but in practice the reduction in speed is absurdly small.
Consider first a light wave passing through a dielectric. You can explain what happens using either classical or quantum approaches, but we'll use a classical description since that's all we have available for gravity. The oscillating electric field associated with the light wave makes the electrons in the dielectric oscillate, and those oscillating electrons reradiate an electromagnetic wave. In most cases the frequency will not match a natural oscillation frequency of the dielectric, so the phase of the reradiated wave lags the incident light wave. When you sum up the incident and reradiated wave the result is a wave travelling slower than $c$. (If you're interested, the process is discussed in detail in the answers to Why do prisms work (why is refraction frequency dependent)?.)
Now consider the gravitational wave. The gravitational wave will induce a quadrupolar oscillation in the matter it is passing through (this quadrupolar oscillation is what LIGO has been looking for). The oscillation in the matter will then reradiate a gravitational wave, and as with light there will usually be a phase lag so summing the incident and reradiated waves will give a wave travelling slower than $c$.
But gravitational waves interact much, much less strongly with matter than light does with a dielectric. The induced oscillation of the matter is so small that no-one has ever managed to measure it, and the gravitational wave reradiated by this undetectably small oscillation will be vastly smaller again. So while in principle the interaction with matter will slow down the gravitational wave, in practice any reduction in speed is utterly negligable.
John Rennie
- 367,598
-3
The acceleration of gravity is that itself. The bigger the mass the more gravity pulls on the object. Especially with a force. This could be simplified into an equation we learned in Intro-Physics, F=ma or f=mg
