Does Gravity Travel at the Speed of Light?

Steve Carlip used to be my advisor in grad school and his style of writing shows his beautiful way of explaining (and thinking about) deep questions in physics.

One think that I believe is important to point out is that this short essay is focusing on the experimental aspect of the measurement of the speed of gravity. It gives an explanation on why this is even an intelligent question to answer and why the answer we commonly accept is that gravity moves at the speed of light. I will never have better words than him so I won't add anything to what he said.

What I would like to point out, is that this essay is not talking about whether or not theoretically would be possible to have a speed of gravity faster than the speed of light. General relativity and special relativity have been tested on several aspects, and they pretty much are in agreement with all experimental constraint. There is no other theory that can explain everything we see (some theories tend to explain a few things, but not all of them, or they are too vague).

A theory in which the speed of gravity is faster than the speed of light isn't unconceivable per se, but no one has been able to write a completely consistent one yet, mostly because it would have large consequences on what special and general relativity imply for cosmology and particle physics (and causality). If gravity moved faster than the speed of light then we'd have to 'fix' a lot of problems coming down to paradoxes and stuff like that.

So take this essay as a way to appreciate how elegant and fine some experimental questions on gravity (and physics) are.