Speed of Gravity

TriskettheKid

Graduate Poster
TriskettheKid
Joined
Oct 4, 2007
Messages
1,431
So I've seen all of these arm-chair "physicists" come around talking this and that about relativity, and how they think it works. Accompanied, of course, by the fairly rapid smackdown courtesy of people on these boards who know what they're doing. Hopefully, same people can help me out with the upcoming inquiry.

All of this talk of relativity has piqued my interest in something. I've searched the forums, but as I did not do a thorough search, apologies in advance if this has been asked and answered a dozen times before:

What is the speed of gravity?

Take the Sun, for example. If the sun "disappeared", we wouldn't know on Earth for about 8 minutes, as we should all know it takes about 8 minutes for the light of the Sun to reach Earth.

But if the Sun "disappeared", how long would the gravitational effects take to reach Earth? Are they instantaneous?
 
TriskettheKid said:
So I've seen all of these arm-chair "physicists" come around talking this and that about relativity, and how they think it works. Accompanied, of course, by the fairly rapid smackdown courtesy of people on these boards who know what they're doing. Hopefully, same people can help me out with the upcoming inquiry.

All of this talk of relativity has piqued my interest in something. I've searched the forums, but as I did not do a thorough search, apologies in advance if this has been asked and answered a dozen times before:

What is the speed of gravity?

Take the Sun, for example. If the sun "disappeared", we wouldn't know on Earth for about 8 minutes, as we should all know it takes about 8 minutes for the light of the Sun to reach Earth.

But if the Sun "disappeared", how long would the gravitational effects take to reach Earth? Are they instantaneous?
Click to expand...
You already answered 8 minutes.
 
~enigma~ said:
You already answered 8 minutes.
Click to expand...

So, if the sun vanished the earth would still be held in it's gravitational field and maintain it's orbit for 8 minutes before flying off on a tangent?

Never thought about it before but it's an interesting question.
 
TriskettheKid said:
But if the Sun "disappeared", how long would the gravitational effects take to reach Earth? Are they instantaneous?
Click to expand...

The sun can't disappear according to our best theory of gravity, because energy is conserved, and mass is a form of energy.

But if you modify your question a little - if some giant bomb went off inside the sun, badly distorting it, how long would it take for the gravitational effects of that event to reach earth - the answer is precisely the same time it takes for light to reach earth (about 8 minutes).
 
So if it is c, does it behave any differently than light?

I mean, beside the whole "light is a wave and a particle" thing.
 
TriskettheKid said:
So if it is c, does it behave any differently than light?
Click to expand...

Sure - very differently. Gravity acts on energy rather than electric charge, for one thing. And it's significantly more non-linear, so it can do things like make black holes. A gravity wave is something you could detect with some collection of suspended masses (which will move slightly when the wave passes), while a light wave you detect with an antenna or photodetector (or your eyes).

I mean, beside the whole "light is a wave and a particle" thing.
Click to expand...

That part is probably the same (although no one knows for sure).
 
OT: I love the idea that if the sun were replaced by a black hole of the same mass, solar system orbits wouldn't change.

Of course, what would happen to our atmosphere without the sun to keep it from freezing solid...
 
We got into this only three weeks ago in this thread. It started getting good when Zeuzzz mentioned Tom Van Flandern in post #13.
 
Gravity must be timeless or we would spiral in a ever bigger orbit, it can't have a 8 minute lag. Put your thinking caps on all not as it seems.
 
Old Bob said:
Gravity must be timeless or we would spiral in a ever bigger orbit, it can't have a 8 minute lag. Put your thinking caps on all not as it seems.
Click to expand...

There's effects in gravity (and electromagnetism, but to a lesser extent) that 'correct' for the time delay of propagation - we're pulled towards where the sun is 'now', not where it was 8 minutes ago.
 
Uh... it's instantaneous in the sense that gravitoelectric and gravitomagnetic effects conspire to keep the force toward the Sun's true position rather than apparent visual (delayed) position. You're right in that if that were the case, our orbit would not be stable, as aberration would produce a force component in the direction of Earth's motion, increasing kinetic energy. However, the "speed of gravity" refers to something quite different.
 
You must log in or register to reply here.

Back
Top Bottom