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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#51

Post by ars »

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…

That's not how you add speeds in a relativistic world.

If something is moving at the speed of light, then light (or gravity) is emitted from it also at the speed of light.

But if you add the two together, it doesn't equal twice the speed of light. Yes, I know it's weird, but that's where the whole time dilation comes in.

Re: What is the Speed of Gravity?

#52

Post by ars »

maeon3 wrote: post #18
cynicalkane wrote: post #17

Earlier quoted context omitted.

If the speed of gravity was the speed of light, then heavy objects passing near the earth would have more gravity than objects passing away. This is a problem because...?

problem because if you had a stable highly elliptical orbit of two bodies, the attraction would be greater during the approaching phase than the departure phase. The equation would not be balanced. Elliptical orbits would degrade due to the pulling being greater on arrival than departure. This is not the case. Maybe it is only a problem as you approach light speed.

As I mentioned in a different comment, that's not what happens.

Re: What is the Speed of Gravity?

#53

Post by ars »

cousin_it wrote: post #46
maeon3 wrote: post #18

Earlier quoted context omitted.

problem because if you had a stable highly elliptical orbit of two bodies, the attraction would be greater during the approaching phase than the departure phase. The equation would not be balanced. Elliptical orbits would degrade due to the pulling being greater on arrival than departure. This is not the case. Maybe it is only a problem as you approach light speed.

I believe the submitted article gives an example where orbits degrade in just the way you describe.

No, the article describes something totally different. It talks about cases where an orbit decays because the gravity is felt at a different location from where you expect.

Re: What is the Speed of Gravity?

#54

Post by binomial »

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…

I'm by no means well-versed in the special theory of relativity, but I believe one of its results is that light seems to travel the same speed at you no matter what your own speed may be (since it's all relative anyways, there's no absolute frame of reference for you to really measure your own speed), as a consequence of the Lorentz transformations.

I think this was confirmed in an experiment where scientists measure the speed of light from the sun both during sunrise and sunset, since the relative speed between the sun and the measurement devices are different due to earth's rotation (sorry, don't have a link handy for all this, I probably got some details wrong but you get the idea).

I guess the theory of general relativity would extend this to gravity.

Re: What is the Speed of Gravity?

#55

Post by thailandstartup »

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself?

So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself.

There's some evidence that gravity may change a little during a solar eclipse http://en.wikipedia.org/wiki/Allais_effect and also the Pioneer anomaly, http://en.wikipedia.org/wiki/Pioneer_anomaly

I am most definitely not a physicist!

Re: What is the Speed of Gravity?

#56

Post by ars »

randallsquared wrote: post #45
acqq wrote: post #38

Earlier quoted context omitted.

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

I was under the impression that tachyons (should they exist) would be expected to give off Cherenkov radiation in vacuum, but I'm not sure where I got that impression. I hadn't actually heard anything about Mallett for a few years, and I guess this is why. :)

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.

Re: What is the Speed of Gravity?

#57

Post by Tycho »

Luc wrote: post #49
philh wrote: post #27

Earlier quoted context omitted.

>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.

For fun: the moon has a radius of about 1730 km. At full moon we can see about half of the moon, so the distance from one side to the other is about pi * 1730 km. So how fast do you need to move a laser pointer across the moon surface to make the dot attain the speed of light? http://www.google.com/search?q=%28%281730000+*+pi%29+m%29+%2... A bit less than 2/100 of a second. That seems quite feasible really! (of cours…

So say the dot was fairly large and you had a vantage point on the moon near where the dot sweeps by... What would you actually see??

Re: What is the Speed of Gravity?

#58

Post by ewjordan »

wnoise wrote: post #44
ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

> Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predicted" and the direction of the force is adjusted accordingly - as long as a body is not accelerating, a purely attractive or repulsive force will be pointing at its current position, not its position 8 mins ago. Absolutely correct for E&M. This is because it's a vector field, and the velocity o…

Ah, that's interesting, I never realized the rank was what determined the amount of "prediction" that a field does. That's something that should have been obvious on degrees-of-freedom considerations, but I never thought about it in that way before.

That means my bootstrap-runaway argument must be flawed in some way, because a scalar field can't encode any velocity information, and as a classical field equation, it would cause a 1/r^2 force law just like everything else, but aimed at the retarded position.

Re: What is the Speed of Gravity?

#59

Post by ars »

Eliezer wrote: post #31
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 po…

Why should it move in a straight line? If no one is pushing it, it shouldn't move at all.

That graphic is just wrong, and I also hate it.

And gravity isn't the cause of the curvature! It's the effect. The cause is energy (often in the form of mass).

Re: What is the Speed of Gravity?

#60

Post by [deleted] »

ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

[deleted]
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