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The Speed Of Gravity


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#1 Christopher

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Posted 18 January 2003 - 10:09 AM

Kevin Street, on Jan. 18 2003,10:32, said:

Now, this is service.  Just last night I signed up here and suggested adding a science & tech forum, and now here it is this morning. :alien:

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And I have to wonder - how does the Solar System hold together if Gravity is limited to c? How can gravity be thought of as a curve in space when it's subject to light-speed delay?

Well, I think the idea is that a change in that curvature can only propagate at c.

As for how the solar system holds together -- you're right, many of the objections to lightspeed gravity are based on the premise that it would cause a planet to be drawn to where the sun was several minutes in the past rather than where it is now, which would create instabilities that would fly the system apart.  But the response is that there's an effect which causes the pull on a moving body to be skewed sideways by just the right amount to cancel out that effect and make it behave as though it is being drawn toward the instantaneous position of the sun rather than its past position.  Sounds contrived, I know, but it has been demonstrated to exist in electromagnetic fields, and the math is basically the same with gravity fields.

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#2 Christopher

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Posted 18 January 2003 - 09:50 PM

Kevin Street, on Jan. 18 2003,19:36, said:

Is this some kind of action at a distance? Very interesting! What's the name of this effect? I'd love to read up on it, if the math isn't too far over my head.  :hehe:
No, it's not "spooky action at a distance," not an FTL effect at all.  You can read about it at http://math.ucr.edu/...grav_speed.html -- at least, theoretically you can; I get a "forbidden to access on this server" message when I try it.  Anyway, it's a paper by Carlip, Wiener and Landis.  They describe it thusly (I'm reading from an article that quotes it):

"...one finds that the 'force' in GR is not quite central--it does not point directly towards the source of the gravitational field--and that it depends on velocity as well as position....  The net result is that the effect of propagation delay is almost exactly canceled, and general relativity very nearly reproduces the Newtonian result."

(The Newtonian result is instantaneous propagation of gravity.)

So it's like, the gravity the Earth is feeling right now is from the point the Sun was at 500 seconds ago, rather than the point where it is right now -- but the effect of our motion skews the direction it pulls us just a bit, so that it feels like we're being pulled toward where the Sun is right now.  No FTL "action at a distance" of any kind, just the illusion of an instantaneous effect.  All that's really happening is a change of direction, like aiming ahead of a moving target in order to hit it dead-on.

"You don't use science to show that you're right, you use science to become right." -- xkcd

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#3 Christopher

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Posted 19 January 2003 - 08:24 AM

uncle sid, on Jan. 18 2003,22:42, said:

So does this mean that if we were able to say, very rapidly decrease the velocity of the Sun in relation to the rest of the galaxy, that the planets could "miss" and fly out of their orbits because their predictive response to gravity was incorrect?
Hmm, certainly an abrupt change in the Sun's motion would seriously perturb the orbits of the bodies around it in any case.  I don't think there's any need to invoke the Carlip effect (for want of a better term) there.
"You don't use science to show that you're right, you use science to become right." -- xkcd

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#4 Kevin Street

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Posted 18 January 2003 - 05:32 AM

Okay, here's a first post to start the forum off with a bang. It may be old news to most of you by now, but it's still pretty neat.

Two scientists have measured the speed of gravity, and have found that it is indeed equal to the speed of light. (Einstein, please take a bow.) They did it by measuring the gravitational lensing effect that Jupiter produced when it passed in front of a quasar, which gave them a good measurement of the strength of Jupiter's gravitational field - then they reformulated general relativity so it expressed the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. All factors except the speed of gravity were then known, and they computed that.

Simple - not!  :cool:

Anyway, this validates General Relativity, but it raises some interesting questions. According to the article, it constrains the "brane world" theories, which "suggest the Universe has more spatial dimensions than the familiar three."

And I have to wonder - how does the Solar System hold together if Gravity is limited to c? How can gravity be thought of as a curve in space when it's subject to light-speed delay?

And now another scientist is suggesting that their reasoning is flawed.

What do y'all think?

Per aspera ad astra

#5 Kevin Street

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Posted 18 January 2003 - 07:36 PM

Quote

Well, I think the idea is that a change in that curvature can only propagate at c.

Ahhh...

Quote

...the response is that there's an effect which causes the pull on a moving body to be skewed sideways by just the right amount to cancel out that effect and make it behave as though it is being drawn toward the instantaneous position of the sun rather than its past position.  Sounds contrived, I know, but it has been demonstrated to exist in electromagnetic fields, and the math is basically the same with gravity fields.

Is this some kind of action at a distance? Very interesting! What's the name of this effect? I'd love to read up on it, if the math isn't too far over my head.  :hehe:

Per aspera ad astra

#6 Uncle Sid

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Posted 18 January 2003 - 10:42 PM

Christopher, on Jan. 18 2003,21:50, said:

All that's really happening is a change of direction, like aiming ahead of a moving target in order to hit it dead-on.

So does this mean that if we were able to say, very rapidly decrease the velocity of the Sun in relation to the rest of the galaxy, that the planets could "miss" and fly out of their orbits because their predictive response to gravity was incorrect?

I can picture in my mind a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey


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