What is the Speed of Gravity?

Do changes in a gravitational field propagate instantaneously, at the speed of light, or at a different speed altogether?

“The only problem with the speed of light, is it gets here too early in the morning.” -Danny Nevrath

One of the most common questions I get asked is whether gravity is instantaneous, or whether there’s a speed limit to how fast the force of gravity can travel. This is not as simple a question as it seems on the surface.

Image credit: NASA/JPL-Caltech, for the Cassini mission.

After all, we know how fast light travels, and if the Sun were to suddenly wink out of existence, we’d still receive light from it for just over 8 minutes after it disappeared! But what about gravity, and the Earth’s orbit? Would the Earth simply fly off in a straight line, like a twirled poi ball the instant a string broke?

Image credit: via Tom Harrison of New Mexico State, http://ganymede.nmsu.edu/tharriso/ast110/class25.html.

Or would it continue to move in its planetary orbit for some time, and perhaps suffer some more interesting effects? Believe it or not, this is one of the most severe differences between Newton’s old school theory of gravity and Einstein’s General Relativity. According to Newton, you have two masses separated by a distance, and that determines the force. You take one of those masses away, and the force goes away. Instantly. End of story.

Image credit: Wikimedia commons users Jarry1250 and Dna-webmaster.

But in general relativity, things are much more intricate, and very different from the simple Newtonian picture you likely learned in high school or college.

First off, it isn’t mass, per se, that causes gravity. Rather, all forms of energy (including mass) affect the curvature of space. So for the Sun and the Earth, the incredibly large mass of the Sun dominates the curvature of space, and the Earth travels in an orbit along that curved space, just like all the other bodies in…