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How fast is gravity?

by farmer
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Star Drive
#37
Jul5-10, 10:05 PM
P: 17
[QUOTE=Ich;2788034]Not a chance. Gravity waves, even of some decent astrophysical events (not a feeble H-Bomb) are very very very weak.


Good point.

However if two neutron stars in mutual orbit, merge on the far edge of the Milky Way, their gravity 'disturbance' would diminish by inverse square law over about a 100K light years.

Would the effect as received on Earth from the merger be more or less than that of an H-Bomb on the moon, which is tightly coupled to the Earth (relatively)?

Its all relative and I know its all in the numbers.

Thanks for your feedback.
B1ffB0ff
#38
Jan18-12, 07:56 AM
P: 2
I know its old, but then so is time... :-)

If Gravity, the affacts of a body in space time, are infinite (exerted on all points in the universe at the same time to a greater or lesser extent dendent upon distance from the body) , then the speed at which the affcets can be experinced cannot be limited to C! So hypertheticaly, we remove the moon instanateoulsy, will the tides stop instantly or after a minute or so in reation to C?

Cheers

JB
DaveC426913
#39
Jan18-12, 08:02 AM
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P: 15,319
Quote Quote by B1ffB0ff View Post
I know its old, but then so is time... :-)

If gravity's effect on a body in space time are infinite (exerted on all points in the universe at the same time to a greater or lesser extent dependent upon distance from the body)
Your supposition is false. While it does have an infinite extent, changes to the gravitational field are limited to propagating at c.

Quote Quote by B1ffB0ff View Post
, then the speed at which the effects can be experienced cannot be limited to c! So hypothetically, we remove the moon instantaneously, will the tides stop instantly or after a minute or so in relation to c?

Cheers

JB
You cannot remove a mass instantaneously. It doesn't work that way. The mass would be limited to movement below the speed of light.

However, even if it were not so, the change in gravitational force felt by the disappearance of the Moon would take time to propagate. It takes about 1.2 seconds.

Same with the sun. If the sun suddenly fell through a wormhole and disappeared, we would neither see it nor feel for 8 minutes.
B1ffB0ff
#40
Jan18-12, 08:37 AM
P: 2
So, that being the case we should be able to prove this then...

A variation of the other much older experiment with two large mass objects suspended next to each other and the attraction force between them. However.

Main difference we have a massive object suspended (100m) above a smaller weight placed on a very sensitive set of scales, capable of sampling in microseconds.

Allow the high mass object to fall on the much less mass object and the scales.

Film it with a high frame rate camera and time with v accurate clock.

Will need to be located in geologically stable area, maybe in a near vacume to eliminate affects of air movement.

We should see the measurement of the weight of the small mass object reduce slightly as the high mass object gets closer, further this effect should be measurable against C in proportion to distance, relative to the observer.

Thoughts?

JB
Ryan_m_b
#41
Jan18-12, 08:48 AM
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Quote Quote by B1ffB0ff View Post
So, that being the case we should be able to prove this then...
A much easier experiment would be to observe a large, distant mass such as Jupiter with both a telescope and something to measure gravity (gravitational interferometer?). If gravity is instantaneous the latter instrument should tell us it is further along its orbit than the former.
atyy
#42
Jan18-12, 08:58 AM
Sci Advisor
P: 8,389
Quote Quote by B1ffB0ff View Post
A variation of the other much older experiment with two large mass objects suspended next to each other and the attraction force between them. However.

Main difference we have a massive object suspended (100m) above a smaller weight placed on a very sensitive set of scales, capable of sampling in microseconds.

Allow the high mass object to fall on the much less mass object and the scales.

Film it with a high frame rate camera and time with v accurate clock.

Will need to be located in geologically stable area, maybe in a near vacume to eliminate affects of air movement.

We should see the measurement of the weight of the small mass object reduce slightly as the high mass object gets closer, further this effect should be measurable against C in proportion to distance, relative to the observer.
Quote Quote by Ryan_m_b View Post
A much easier experiment would be to observe a large, distant mass such as Jupiter with both a telescope and something to measure gravity (gravitational interferometer?). If gravity is instantaneous the latter instrument should tell us it is further along its orbit than the former.
Possibly relevant is George Jones's post #12 of http://www.physicsforums.com/showthread.php?t=562042. He links to Carlip's paper: "By analyzing the motion of the Moon, Laplace concluded in 1805 that the speed of (Newtonian) gravity must be at least 7106c." Carlip goes on to show why this is consistent with GR, in which gravitational waves travel at c. Evidence for GR's gravitational waves was obtained by Taylor and Hulse.


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