How Fast is Gravity?

How long would it take for the gravitational well created by the Sun to disappear, and the Earth and the rest of the planets fly off into space?

In the very first episode of the Guide to Space, a clean shaven version of me, hunched over in my basement explained how long it takes for light to get from the Sun to the Earth. To answer that question, it takes light about 8 minutes and 20 seconds to make the trip.

In other words, if the Sun suddenly disappeared from space itself, we’d still see it shining in the sky for over 8 minutes before the everything went dark. Martians would take about 12 minutes to notice the Sun was gone, and New Horizons which is nearly at Pluto wouldn’t see a change for over 4 hours.

Although this idea is a little mind-bending, I’m sure you’ve got your head wrapped around it. We’ve sure gone on about it here on this show. The further you look into space, the further you’re looking back in time because of the speed of light, but have you ever considered the speed of gravity?

Let’s go back to that original example and remove the Sun again. How long would it take for the gravitational well created by the Sun to disappear.

When would the Earth and the rest of the planets fly off into space without the Sun holding the whole Solar System together with its gravity? Would it happen instantly, or would it take time for the information to reach Earth?

It sounds like a simple question, but it’s actually really tough to tell. The force of gravity, compared to other forces in the Universe, is actually pretty weak. It’s practically impossible to test in the laboratory.

According to Einstein’s Theory of Relativity, distortions in spacetime caused by mass – also known as gravity – will propagate out at the speed of light. In other words, the light from the Sun and the gravity of the Sun should disappear at exactly the same time from the Earth’s perspective.

But that’s just a theory and a bunch of fancy math. Is there any way to test this out in reality? Astronomers have figured a way to deduce this indirectly by watching the interactions with massive objects in space.

In the binary system PSR 1913+16, there’s a pair of pulsars orbiting each other within just a few times bigger than the width of the Sun. As they spin around each other, the pulsars warp the spacetime themselves by releasing gravitational waves. And this release of gravitational waves causes the pulsars to slow down.

It’s amazing that astronomers can even measure this orbital decay, but the even more amazing part is that they use this process to measure the speed of gravity. When they did the calculations, astronomers determined the speed of gravity to be within 1% of the speed of light – that’s close enough.

Scientists have also used careful observations of Jupiter to get at this number. By watching how Jupiter’s gravity warps the light from a background quasar as it passes in front, they were able to determine that the speed of gravity is between 80% and 120% of the speed of light. Again, that’s close enough.

So there you go. The speed of gravity equals the speed of light. And should the Sun suddenly disappear, we’ll be glad to get all the bad news at the same time.

Gravity is a harsh mistress. Tell us a story about a time gravity was too fast for you. Put it in the comments below.