# How fast does gravity propagate?

07 January 2014

## Question

Hi

If you could instantly switch off the sun's gravity, how long would it take for the earth to revert to a galaxy centered orbit?

During the 8 miniutes that the zero gravity wave travels to earth does the effect of the sun's gravity gradually diminish or will it be instant on arrival?

Peter Patay

Chris Smith discussed the answer with Dominic Ford and Mark Peplow...

Dominic - So yes, gravity is this force that binds everything in the universe together. There's this quite fundamental principle in physics that nothing can travel faster than the speed of light. And so, physicists have rather suspected on the basis of that that gravity must propagate at a finite speed - at the speed of light - because if it happened instantaneously, then wobbling something from side to side in the Universe would essentially be propagating information about how that thing was moving faster than the speed of light and violating this very fundamental principle.

It's been obviously quite hard to test because trying to find some experimental setup where you test whether gravity propagates faster than the speed of light, is really quite a challenge. But actually, in the last 10 years or so, we have done that with objects called pulsars which are very compact neutron stars. They are basically the mass of a star in the size of a mile or two across. Some of these things are very close to one another and spinning around each other very fast. Actually, Einstein's theory of general relativity which is the best description of gravity we have, predicts that when these things are orbiting very fast, they should produce what's called gravitational waves which are ripples of gravity, that travel out at the speed of light. If they're doing that, they should be gradually losing energy through these gravitational waves.

In fact, we have found pulsar binary pairs that seem to be gradually getting closer and closer together as if they're losing energy, at exactly the rate that Einstein predicts, if gravity travels at the speed of light.

Chris - Mark...

Mark Peplow - As the pulsars change their rate, you're sort of inferring the existence of gravity waves. What would it take to detect the gravity waves themselves?

Dominic - Well, there are a number of teams around the world who are trying to build detectors to detect these ripples of gravity moving through space. The sensitivity you need to that is absoloutely mind boggling. You're talking about distances of about a mile that you're sending light beams down and you're trying to see whether gravity is causing that distance to ripple by about the size of an atom. So, you've got an experimental setup a mile long and you're trying to detect something that's moving by the width of an atom. No one has yet detected those gravitational waves.

They are, I think, getting quite close incredibly. I'm always quite incredulous when I hear about these experiments because they sound bonkers to me. But I think in the next decade or so, we might actually start to detect these ripples in space time...