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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#81

Post by othermaciej »

achille wrote: post #62
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…

Wait, so you are saying we would feel the effects immediately, but the light would not disappear for eight minutes? So let's assume once can cause the sun to vanish instantly somehow. If that's the case, would it then be possible to setup a 'gravitational wave' communication system that travels faster than light? Ie, I would put a whole bunch of stars together, and then have them disappear at certain intervals. My bu…

No, the sun disappearing would not be constant velocity or constant acceleration motion, so the info would indeed only propagate at the speed of light.

Re: What is the Speed of Gravity?

#82

Post by sp332 »

thailandstartup wrote: post #76
sp332 wrote: post #75

Earlier quoted context omitted.

> can a sufficient amount of light 'pull' objects towards itself? Yup. As the article says, all forms of energy cause gravitational attraction. Even light! But, as you can see from E=m * c^2, it takes an insane amount of energy to make even a little mass-equivalent. C=3 * 10^8 m/s, so c^2 = 9 * 10^16 or 90000000000000000 m^2/s^2. So 1kg of mass at rest is equivalent to 9 * 10^16 joules of energy. It's about equal to…

Mmmm. Okay, but a star puts out a lot of energy, over a long period of time. Is it enough that we calculate the gravitational effect of all those photons, rather than just the mass of star itself when calculating say the acceleration of the pioneer crafts?

http://www.wolframalpha.com/input/?i=3+cubic+au+times+682+ki...

so 2.5 * 10^14 kg inside the earth's orbit, or equivalent to 254 cubic kilometers of water - on an astronomical scale, insignificant. Still, bigger than I expected.

Re: What is the Speed of Gravity?

#83

Post by Eliezer »

ars wrote: post #59
Eliezer wrote: post #31

Earlier quoted context omitted.

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).

> If no one is pushing it, it shouldn't move at all.

Thank you, Aristotle. Objects not being pushed on continue with constant velocity, they do not stay motionless.

Re: What is the Speed of Gravity?

#84

Post by wnoise »

lutorm wrote: post #77
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…

I don't think it's incorrect. But for constant velocities it is canceled out because the relative motion causes things to also appear in a different direction. This happens with ordinary light, also. Stars appear in a different place to us because of our relative motion (they appear to be located more "forward" in our direction of motion). As as example, assume there is a star directly perpendicular to our motion (an…

> And your charge example is not so good. For Galilean invariant theories it's only relative motions that matter.

E&M is Lorentz invariant though, not Galilean.

This causes an issue with your starlight aberration example as well. Velocities do not add linearly (though that is a reasonable approximation for low velocities).

Re: What is the Speed of Gravity?

#85

Post by sliverstorm »

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…

I'm not certain your thought experiment applies. Your use of charges implies that we are discussing EM attraction or repulsion, which we are not.

Considering gravity does not have an inverse (a gravitational repulsion) that I am aware of, I'm not certain we can extend your thought experiment to gravity.

Re: What is the Speed of Gravity?

#86

Post by lutorm »

wnoise wrote: post #84
lutorm wrote: post #77

Earlier quoted context omitted.

I don't think it's incorrect. But for constant velocities it is canceled out because the relative motion causes things to also appear in a different direction. This happens with ordinary light, also. Stars appear in a different place to us because of our relative motion (they appear to be located more "forward" in our direction of motion). As as example, assume there is a star directly perpendicular to our motion (an…

> And your charge example is not so good. For Galilean invariant theories it's only relative motions that matter. E&M is Lorentz invariant though, not Galilean. This causes an issue with your starlight aberration example as well. Velocities do not add linearly (though that is a reasonable approximation for low velocities).

Yes, I know that it's not exact for high velocities. My point was just that you don't need relativity for this effect to happen.

Re: What is the Speed of Gravity?

#87

Post by achille »

gnok wrote: post #69
achille wrote: post #62

Earlier quoted context omitted.

Wait, so you are saying we would feel the effects immediately, but the light would not disappear for eight minutes? So let's assume once can cause the sun to vanish instantly somehow. If that's the case, would it then be possible to setup a 'gravitational wave' communication system that travels faster than light? Ie, I would put a whole bunch of stars together, and then have them disappear at certain intervals. My bu…

Out of curiosity, why do you think this is not possible?

You can't transmit data faster than the speed of light.

Re: What is the Speed of Gravity?

#88

Post by kaffeinecoma »

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.

Re: What is the Speed of Gravity?

#89

Post by mryall »

wnoise wrote: post #61
ggchappell wrote: post #48

Earlier quoted context omitted.

> For gravity, it goes one better -- this is a tensor field (hence the notion of gravitons having to be "spin-2" if they were described by a quantum theory), and the acceleration is encoded as well, .... Well, that's mind blowing. Where could I go to learn more about this?

Other than "graduate textbooks", I'm not really sure. Jackson does cover the E&M case pretty well (EDIT: in chapters 11 and 14, note especially sections 10.11 and 14.1), but I'd combine it with the treatment in Taylor & Wheeler's highly readable _Spacetime Physics_. For E&M, the standard way to develop this is to explain magnetism as Lorentz-transformed static electrical attraction/repulsion. Take two wires, and run…

I think my brain just exploded. Thanks for the references!

Re: What is the Speed of Gravity?

#90

Post by ars »

Eliezer wrote: post #83
ars wrote: post #59

Earlier quoted context omitted.

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).

> If no one is pushing it, it shouldn't move at all. Thank you, Aristotle. Objects not being pushed on continue with constant velocity, they do not stay motionless.

So?

Electrostatic force also causes objects to move. Just making an object move does not make gravity special.

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