# How fast is gravity

1. Dec 18, 2008

### jefswat

So suppose we are on earth and the sun moves 10 miles farther away in one second. When do we feel the changed effects of the gravitational attraction here on earth. Is it immediately or after at least ~8 minutes(time it takes light to get here)?

2. Dec 18, 2008

### mgb_phys

Gravity as such doesn't take any time - it's a property of the local space time around you.
Changes in gravity (like you describe) propagate at the speed of light - they must since you can use them to send information.
So if the sun dissapeared we wouldn't notice any change in our orbit for 8mins.

3. Dec 18, 2008

### jefswat

Ok, thats what I figured but I don't remember reading anywhere that explicitly said that
thanks

4. Dec 18, 2008

### Jonathan Scott

Note that it's very difficult to create a gravitational "surprise" because of conservation of energy and momentum. The most sudden thing the sun could do gravitationally, even in theory, is explode into two parts which separate in opposite directions. Even then, the gravitational field at the Earth would not change until the parts had separated far enough for the sum of the potential due to the two parts to be noticeably different from the original total effect.

5. Dec 18, 2008

### Nickelodeon

How do you do that?

6. Dec 18, 2008

### mgb_phys

Well there is the information that the Sun is still there!
In theory you could move a large mass and detect the change in gravity at a distance (I didn't say it was easy) if you could measure the gravitational effect instantly on say Pluto you could use it to rig interplanetary horse races before the TV pictures of the winner arrived.

7. Dec 18, 2008

### jefswat

What changes in gravity propagate faster than the speed of light? I think you answered my original question though

8. Dec 18, 2008

### Nickelodeon

I was thinking that the gravitational response would be slower than c. If the sun disappeared instantly then we would see that it had gone a short while before the Earth headed off at a tangent. It must be reasonably easy to test with a torsion balance, or, hopefully, a more sophisticated equivalent, a large block of lead and an interferometer and away you go.

9. Dec 18, 2008

### mgb_phys

No changes propagate faster than light - what I meant to say was.
1, gravity doesn't travel - it's already there
2, changes in gravity do travel - at c

10. Dec 18, 2008

### jefswat

ok, you had me confused for a minute there

11. Dec 20, 2008

### Naty1

Don't all four forces propagate at the speed of light....and how about the theoretical Higgs ocean..that too?? It seems to be a hint at least that they were all unified at some point...