Gravity: How Fast Does it Travel?

[Salman2]

Reading this old thread..but..what is the correct answer to the OP question ?:

OP Question: How fast is gravity?

==

Does gravity have a "speed" ? Is it fast or slow ?

It does not seem correct to me that we can say "gravity" is fast or slow. Seems to me it would be the object of motion (particle and/or wave) that is fast or slow. Thus, a fast particle/wave is one that moves much in a short period of time, slow particle/wave moves little in a long period of time. Some particles/wave (such as photons) always move the same distance in any period of time and are thus neither fast or slow, they move at c = speed of light. What am I missing in my understanding ?

 

[bcrowell]

Salman2, this thread has been dead since 2007. The speed being referred to by the OP was the speed at which gravitational waves propagate, not the speed of material particles.

 

[Salman2]

Salman2, this thread has been dead since 2007. The speed being referred to by the OP was the speed at which gravitational waves propagate, not the speed of material particles.

OK thanks.

What was the conclusion of the discussion--what is the speed at which gravitational waves propagate--is it c, the same speed that photon wave propagates ?

 

[bcrowell]

What was the conclusion of the discussion--what is the speed at which gravitational waves propagate--is it c, the same speed that photon wave propagates ?

I haven't read the whole discussion, but that is the correct answer.

 

[Star Drive]

Here is a thought experiment about a way to perhaps detect gravity waves.

All around the Earth at strategic points, attach transducers to solid bedrock. These transducers would output electrical waves in response to mechanical waves in the Earths crust, much like a seismometer. However, the sensitivity would be greater and the response would be faster. All transducers would be data linked and digitally time phase adjusted back to a common data processing center for simultaneous processing.

With very close time coordination, remote detonate about a 100 Megaton H-Bomb directly against the Moons surface.

Could we expect to pick up a "ping" and possibly "ringing" in the Earths crust from gravity wave coupling all the way from the moon?

I don't have a clue for how to analyze this.

I'm not a physicist, so go easy on me.

 

[Ich]

Could we expect to pick up a "ping" and possibly "ringing" in the Earths crust from gravity wave coupling all the way from the moon?

Not a chance. Gravity waves, even of some decent astrophysical events (not a feeble H-Bomb) are very very very weak. You need a very high quality resonator with the correct resonance frequency to see, well, still nothing, but no by these orders of magnitude.

 

[Star Drive]

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]

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]

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.

, 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]

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 vacuum 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]

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]

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 vacuum 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.

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 https://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 7×10⁶c." 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.