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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#101

Post by lutorm »

Tycho wrote: post #12

Reminds me of a thought experiment that's always bugged me: if you had an incredibly long see-saw or lever, like the length of the galaxy, would movement at one end instantaneously be matched by movement at the other end? (I think the reason it bugs me is because the object itself is preposterous, but I still want to know)

Like others have said, any movement will propagate at the speed of sound through the rod. But moreover, as the end of the rod speeds up, it will experience relativistic mass increase from your point of view, so it would still take an infinite amount of energy to accelerate it to c, regardless of how long your lever was. (Though special relativity, dealing with inertial frames, is strictly not applicable to a rotating rod, but you can probably make use of a succession of infinitesimal inertial frames to figure that out.)

Re: What is the Speed of Gravity?

#102

Post by lutorm »

Andrew_Quentin wrote: post #100

I am not a physicist nor an astronomer. I wanted to ask however, when they explain gravity by the analogy of holding a blanket, first the blanket is on earth thus any object that is put on it is being pulled down by earth and the blanket is being held by two people, thus what is pulling the earth and sun down and who is holding the space blanket? Also, seeing some of the images on the site, it is clear that space is…

It's an analogy, and as such it has limited applicability. Noone is "holding" space, it's infinite. The blanket is two-dimensional and embedded in the three dimensions of our world, hence it has boundaries. Space is, well, space. It's not embedded in anything. This is quite nonintuitive and here's where the blanket analogy hurts more than it helps.

The best way I know how to think about curved spacetime is by analogy to the Earth. We are effectively two-dimensional on the Earth's surface, as the radius of curvature is much larger than we are. Yet you can imagine drawing straight lines (i.e., great circles) on the ground to make (large) triangles and discovering that the angles don't add to 180 degrees and hence deduce that the surface is curved.

It's sort of the same way with space. Since we are three-dimensional, we can't directly perceive the curvature as we can an embedded two-dimensional object. A straight line would always look straight to us, yet we could measure the curvature by observing that the fundamental theorems of Euclidean geometry don't hold.

The Earth does slowly spiral towards the Sun. There's no "friction" per se, but it loses angular momentum to other stuff, like gravitational waves and the dark matter hslo. It just operates extremely slowly.

Re: What is the Speed of Gravity?

#103

Post by Andrew_Quentin »

lutorm wrote: post #102
Andrew_Quentin wrote: post #100

I am not a physicist nor an astronomer. I wanted to ask however, when they explain gravity by the analogy of holding a blanket, first the blanket is on earth thus any object that is put on it is being pulled down by earth and the blanket is being held by two people, thus what is pulling the earth and sun down and who is holding the space blanket? Also, seeing some of the images on the site, it is clear that space is…

It's an analogy, and as such it has limited applicability. Noone is "holding" space, it's infinite. The blanket is two-dimensional and embedded in the three dimensions of our world, hence it has boundaries. Space is, well, space. It's not embedded in anything. This is quite nonintuitive and here's where the blanket analogy hurts more than it helps. The best way I know how to think about curved spacetime is by analogy…

Do we know that space is infinite?

Re: What is the Speed of Gravity?

#104

Post by lutorm »

Andrew_Quentin wrote: post #103
lutorm wrote: post #102

Earlier quoted context omitted.

It's an analogy, and as such it has limited applicability. Noone is "holding" space, it's infinite. The blanket is two-dimensional and embedded in the three dimensions of our world, hence it has boundaries. Space is, well, space. It's not embedded in anything. This is quite nonintuitive and here's where the blanket analogy hurts more than it helps. The best way I know how to think about curved spacetime is by analogy…

Do we know that space is infinite?

That depends on what you mean by "know", I guess... According to the current best estimates of the cosmological parameters, yes, it is infinite. The basic theory could be wrong, of course, and outside the observable Universe the properties could be different.

Re: What is the Speed of Gravity?

#105

Post by pavel_lishin »

sliverstorm wrote: post #98
benJIman wrote: post #91

Earlier quoted context omitted.

http://cdsweb.cern.ch/record/1122131/?ln=cs

I love the names they give to things nowadays. In this case, specifically "Spooky action"

I think that when it comes to the term "spooky action", "nowadays" would be around the time Einstein was working on all of this.

Re: What is the Speed of Gravity?

#106

Post by pavel_lishin »

eru wrote: post #92
ars wrote: post #56

Earlier quoted context omitted.

That would imply that tachyons interact with the electromagnetic force. But if they do we would have detected them long ago. But it's wrong anyway, it's not the moving particle that gives off the Cherenkov radiation, it's the matter that they move near. And there is no matter in a vacuum.

Only virtual matter.

Which should still give off Cerenkov radiation in the old, boring way, even without tachyons.

Re: What is the Speed of Gravity?

#107

Post by pavel_lishin »

lutorm wrote: post #104
Andrew_Quentin wrote: post #103

Earlier quoted context omitted.

Do we know that space is infinite?

That depends on what you mean by "know", I guess... According to the current best estimates of the cosmological parameters, yes, it is infinite. The basic theory could be wrong, of course, and outside the observable Universe the properties could be different.

I thought it was finite but unbounded.

Re: What is the Speed of Gravity?

#108

Post by pavel_lishin »

doki_pen wrote: post #66
ck2 wrote: post #40

Personally I think it's easier to tell people to stop thinking of gravity as a particle or wave - it's not like a beam of light or radio wave. Think of the space/time as the blanket the universe is wrapped in. If that blanket is stretched out like a trampoline, every object on it or moving around on it is causing flexing and ripples. That's gravity. When a girl bounces off a trampoline, the specific instance of her g…

Our reality was build on a 299 792 458 m / s processor. Newer realities are much faster.

Yeah, but at least ours is massively parallel.

Re: What is the Speed of Gravity?

#109

Post by pavel_lishin »

Tycho wrote: post #74
philh wrote: post #25

Earlier quoted context omitted.

If you move it, you're really moving some of the atoms at one end. They 'bump' into nearby atoms, which 'bump' into more atoms, and so on, and eventually the atoms at the far end get 'bumped', and the far end moves. Sound waves travel in the same way. So if you move one end, that movement propagates at the speed of sound (in whatever material the lever is made of).

Hmm, I'd take your word over mine but this doesn't fee quite right to me. What I'm thinking is, say you reverse the action so that instead of bumping one end of the see-saw, you release your grip on it having been holding it down. So in normal circumstances the other end would fall while the near end rises (of course now an external gravitational field has entered the thought experiment). But if movement is still due…

It's the same scenario, except in this case the force isn't you pushing down on your end, but gravity pushing down on the other.

Re: What is the Speed of Gravity?

#110

Post by SteveC »

kaffeinecoma wrote: post #88

The article asks what would happen to gravity if you suddenly "removed" the sun. My question is- is that even possible? How could you instantaneously "disappear" a giant blob of mass/energy? Is this kind of like trying to divide by zero? I'm not being facetious, just wondering if the whole idea is predicated on an impossibility.

You're thinking too deeply into that example. It's just a thought exercise to better understand how gravity works and easier for people to grasp.
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