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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#31

Post by Eliezer »

jockc wrote: post #29

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.

But gravity isn't curved space, it's curved spacetime. Think of a graph that shows both space and time, so that it shows constant motion as a straight line (four units of space per one unit of time with the slope constant). Then acceleration is a curved line. Now curve the graph paper - not through some higher dimension, but by stretching it and compressing it in ways that change the shortest distances between two points - so that the Earth orbits the Sun in a straight line. Not a straight line in space, mind you, but a straight line in spacetime: no acceleration. After all, since no one is pushing on the Earth, it should move in a straight line, right? Got it?

Re: What is the Speed of Gravity?

#32

Post by doki_pen »

philh wrote: post #27
Luc wrote: post #24

Earlier quoted context omitted.

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.

It seems he is correct. If you imagine a laser pen light being swung on an angle, the further away the screen is, the faster the dot will "move". If the screen is sufficiently far away, the dot will appear to move faster then the speed of light.

Re: What is the Speed of Gravity?

#33

Post by kscaldef »

philh wrote: post #27
Luc wrote: post #24

Earlier quoted context omitted.

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.

He's right.

Say you have a laser pointer and point it at a screen some distance away. Now, you rotate the pointer such that the tip is moving at, say 1/10th the speed of light. Simple geometry tells you that the spot on the screen will move much faster than the speed of light.

The important point, though, is that no physical object and no informaaation is actually moving faster than light. Each photon moves at exactly c in a straight line from the pointer to the screen.

Re: What is the Speed of Gravity?

#34

Post by doki_pen »

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)

I think it would snap. It would take a tremendous amount of force to move it and no material could withstand that force.

Re: What is the Speed of Gravity?

#35

Post by Dylan16807 »

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…

You don't have to go 'faster than the speed of light' to escape a black hole if you're not affected in the usual manner by gravity. And I'm not aware of any suggestion that gravity itself is affected by gravity.

Re: What is the Speed of Gravity?

#36

Post by meric »

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 just define it for myself as "If it can transmit information, it could also be used given enough technology to send any kind of data, such as a Microsoft word document."

Re: What is the Speed of Gravity?

#37

Post by sliverstorm »

doki_pen wrote: post #30
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..

There's no single remark, it just seemed somewhat condescending

Re: What is the Speed of Gravity?

#38

Post by acqq »

randallsquared wrote: post #13
acqq wrote: post #3

The point of the article is in Clifford M. Will "The Confrontation between General Relativity and Experiment" (2001) http://relativity.livingreviews.org/Articles/lrr-2001-4/ "Gravitational wave damping has been detected in an amount that agrees with general relativity to half a percent using the Hulse-Taylor binary pulsar," I also have something to say to the first question in comments there, to which the article aut…

Something that puzzles me about "the speed of light in X" discussions is exactly the confusion over whether the speed of light is "different" in a medium. On the one hand, it seems like it isn't, since no actual photons are moving more slowly. On the other, things like Cherenkov radiation seem to indicate that the apparent speed of light matters for things that you'd expect to be based on the constant speed of light…

But AFAIK Cherenkov radiation occurs in medium, so there isn't any contradiction to the generally accepted limits, and Ronald Mallet is highly unlikely to be trusted, see the wikipedia entry:

http://en.wikipedia.org/wiki/Ronald_Mallett

Re: What is the Speed of Gravity?

#39

Post by gigafemtonano »

splat wrote: post #26
gigafemtonano wrote: post #11

Earlier quoted context omitted.

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.

From the actual paper: http://www.fourmilab.ch/etexts/einstein/specrel/www/

We will raise this conjecture (the purport of which will hereafter be called the "Principle of Relativity") to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell's theory for stationary bodies.

Re: What is the Speed of Gravity?

#40

Post by ck2 »

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 gravity is gone but the ripples from that instance continue on for some time until everything "settles".

But apparently the goo that is space/time really does not tolerate anything going faster than the speed of light for some reason. Doesn't mean things aren't trying to go faster than that, they just cannot achieve it.

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