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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#21

Post by nfg »

maeon3 wrote: post #10
Locke1689 wrote: post #6

Earlier quoted context omitted.

Wait, what? Did you read the article? The propagation of the curvature of spacetime happens at the speed of light. If it didn't you could use large amounts of energy to propagate information faster than the speed of light, which screws the hell out of causality (using the light cones). If you have a good math background you can solve for it using the equation. I see gravity as more of a property of the universe, ever…

Maybe data could travel faster than light if we find a property of the universe which goes faster than light. I imagine a bowling ball rolling down a blanket stretched tight at half light speed. The bowl shape of the blanket in front of the bowling ball would be very different from the rear. I'm just pointing out that if gravity goes light speed, then something coming toward the earth at half light speed will have a…

There are experiments currently underway to detect gravitational waves, see for example http://en.wikipedia.org/wiki/Gravitational_wave_detector#Spe...

Re: What is the Speed of Gravity?

#22

Post by JanezStupar »

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)

Oh - yes, the unobtainium rod thought experiment :)

Well - I guess that when you move an object - you firstly move (push away) - due to electromagnetic force the closest atoms first - which in turn push away the next... etc. until the whole object moves. I guess that what happens is actually an oscillation between atoms.

So you would notice the far end moving in the time a photon would take from one end to other - in best case scenario.

You know that information cannot travel faster than light? Even if you try to send it via wiggling of unobtainium rod :)

Re: What is the Speed of Gravity?

#23

Post by JanezStupar »

adamtj wrote: post #20
1053r wrote: post #16

This is all well and good, but what the article is talking about is the speed of the propagation of gravity waves. I've never heard anyone who wasn't a crank argue that gravity waves travel at anything other than the speed of light. As the article confirms, observation of pulsars has confirmed the speed of gravity waves to very close to the speed of light. However, I think the larger objection goes something like thi…

What is the technical definition of "information"? I know what information is in a colloquial sense, and I know something about it's technical definition in the field of computer science, but I don't know what physicists mean when they use the word. Is information the same thing as light? Is it analagous to light? Is it a property of light? Matter?

I guess that information is change of state that is clearly distinguishable from some "random" change of state and caries a "meaning".

Eg. If you had an entangled pair of particles on this side of galaxy and another on other side of galaxy. Now you "wiggle" (change its state) one then the other one "wiggles" too - but you didn't send any information - since the other observer cannot know if the particle changed state because of your message - or it changed state "of its own accord". Thus you would still need to notify him of you wiggling the first particle eg. via photon - thus information only moves at the speed of light :)

Bear with my awful analogy - since I really don't know anything about physics :)

Re: What is the Speed of Gravity?

#24

Post by Luc »

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)

If it were a physical object (say, a long steel bar), the atoms would communicate at the speed of light, so your object would bend or squish, with a wave travelling along it.

Now, let's say you move an ideal laser across an ideal screen some distance away. The further the screen is, the faster the laser dot will move. At some point the dot will move faster than c... But the dot is not an object. If you think in terms of individual photons, no speed limit is being broken.

IANAPhysicist...

Re: What is the Speed of Gravity?

#25

Post by philh »

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)

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).

Re: What is the Speed of Gravity?

#26

Post by splat »

gigafemtonano wrote: post #11
ccarpenterg wrote: post #7

I've always thought what we call 'speed of light' is the indirect observation of a structural property of (this) the universe. A property that has something to do with movement in general and not only the movement of light (the movement of space itself?). Another interisting property is the 0º Kelvin (absolute zero) http://en.wikipedia.org/wiki/Absolute_zero

I'm pretty sure that Einstein intuitively "guessed" that the speed of light would be the fastest possible speed and used that fact to work out the rest of the equations. E = mc^2 was the bizarre discovery that those equations yielded, and it was so weird that he felt like he had made a mistake at first.

That's not quite right. Einstein (and several other physicists of the time) inferred that the speed of light might be invariant in all reference frames from the structure of the Maxwell Equations where the speed of light enters as a true, fundamental physical constant of the universe.

Re: What is the Speed of Gravity?

#27

Post by philh »

Luc wrote: post #24
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)

If it were a physical object (say, a long steel bar), the atoms would communicate at the speed of light, so your object would bend or squish, with a wave travelling along it. Now, let's say you move an ideal laser across an ideal screen some distance away. The further the screen is, the faster the laser dot will move. At some point the dot will move faster than c... But the dot is not an object. If you think in terms…

>Now, let's say you move an ideal laser across an ideal screen some distance away. The further the screen is, the faster the laser dot will move. At some point the dot will move faster than c...

I'm pretty confident that you're wrong about this, but I don't really understand the scenario you're describing. Could you clarify?

edit: I eat my words and stand corrected.

Re: What is the Speed of Gravity?

#29

Post by jockc »

I hate that curved space graphic. Now you have some mysterious gravity pulling both the sun and earth down. I wish someone would come up with a better example to show gravity as curved space.

Re: What is the Speed of Gravity?

#30

Post by doki_pen »

sliverstorm wrote: post #28

It always seems rather low-class to take shots at Newton as if he was a fool for coming up with Newton's theory of gravity instead of Einstein's.

Where was there at shot taken a Newton? AFAICT, he explains how Newton's theory addressed the problem, but there is no shot taken. I've never heard of such a problem as people taking shots at Newton..
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