# What is the Speed of Gravity?

1. Dec 10, 2012

### ParamTv

I am confused some people are saying Gravity has speed of light and some are talking about infinite speed.
Friends tell me what exact is speed of gravity.
If you have any way to demonstrate it please share with me.
ok
and my new question is why matter has mass??
tHANKS

2. Dec 10, 2012

### Staff: Mentor

From the Frequently Asked Questions section of our relativity forum:

https://www.physicsforums.com/showthread.php?t=635645 [Broken]

Last edited by a moderator: May 6, 2017
3. Dec 10, 2012

### Barry_G

Many, if not all, solar system simulators assume instantaneous speed of gravity, and apparently that just works. Like this one for example: http://www.orbitsimulator.com

However, author of that simulator himself said on his forum that's not completely accurate, even though he also claims the simulator stays stable/accurate for thousands if not millions of simulated years. Hmmm. On the other hand I've seen on some Wikipedia-talk page some people supposedly made similar simulation but limited speed of gravity to the speed of light and that simulation then became unstable and planets flew away. Response to that was that they failed to account for many other effects given by SR/GR that would counter the error and that if you take all the effects into account they would balance out so that simulation taking into account all the SR/GR effects would produce the same result as if you don't account for any them and just assume the speed of gravity is instantaneous. Go figure!

You tell me why would any matter, or space even, or anything at all, exist in the first place, and then I'll tell you why matter has mass. Ok?

4. Dec 10, 2012

### Staff: Mentor

Finite propagation of gravitational influence does change the orbits a bit, but the dominant effect (emission of gravitational waves) is already negligible for planets (~200 W for earth I think). The modified effective potential is more interesting, as it causes a measurable perihelion shift of mercury - something you won't get in a (proper) simulation with Newtonian gravity.

5. Dec 10, 2012

### Barry_G

All I know about it is what I wrote above, so please tell me more about what you just said. Like about that "modified effective potential", does that include finite gravity propagation is or is it just modification to Newtonian gravity? Also, if you could explain what is the difference about Mercury orbit with and without that correction.

6. Dec 10, 2012

### Staff: Mentor

7. Dec 10, 2012

### Barry_G

It is not clear to me is that correction related to finite gravity propagation or something else. The article only says: "In general relativity, this remaining precession... is explained by gravitation being mediated by the curvature of spacetime."

However other articles on the internet seem to suggest the correction is about Lienard-Wiechert retarded potentials, which I'd think belongs more to Special than General relativity, but in either case it's about finite propagation of gravity rather than curvatures of space-time, unless curvatures of space-time somehow imply finite propagation of gravity.

8. Dec 10, 2012

### Whovian

Basically, people noticed that some objects took more force to accelerate. They then decided to define mass as "how hard it is to accelerate something."

Until Newton and Galileo came along, the theories of motion, despite being somewhat wrong, still used the same definition. It remained the same after Newton and Galileo.

So, I guess you're asking why some stuff is harder to accelerate? No one really truly knows why (though there would be a few mathematical weirdnesses otherwise,) but the bottom line is that every experiment or observation so far agrees with the current theories of mass. All physical theories are, in a sense, guesses; just because one experiment agrees with it doesn't mean all will. (An interesting parallel would be that the sequence 2, 4, 6, 8, ... doesn't necessarily have to have 10 next, it could be the sequence of even numbers that don't have a 0 as their last digit.)

The current theories are just absurdly well-tested; no observations thus far contradict them. No one really knows "why" they're true, we just know (or, rather, are really, really, really close to being absolutely sure) they're true. Let's imagine there are two answers to the question you asked. I doubt there would be any experiment that could distinguish between them.

9. Dec 10, 2012

### Khashishi

It's probably better to ask what the speed of gravity waves are. Asking what the speed of gravity is is like asking what the speed of electromagnetism is.

10. Dec 10, 2012

### Staff: Mentor

I think that question is meaningless. Both the finite propagation speed and the modified potentials are results of GR.

SR alone gives wrong results for orbiting objects. This should not be surprising, as SR is the special case of GR without gravity (and without expansion/contraction of space).

11. Dec 10, 2012

### cosmik debris

In the case of one body orbiting another there is nothing propagating so speed is meaningless. The gravitational field is just there, just as an electric field is just there around a charged body. Changes in the field is a different story, in both cases changes in the field propagate at c.

12. Dec 11, 2012

### superdave

This can be conceptualized as two people swinging around each other holding a tense rope.

The tension in the rope is felt by both parties all the time they swing. But if one person tugs or let's go, it will take some amount of time for this to be felt by the person on the other end. Now, for short ropes the changes will seem instant because our perceptions are limited, but if you watched on high speed film, you could actually see the change propagating down the rope.

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