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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#111

Post by lutorm »

pavel_lishin wrote: post #107
lutorm wrote: post #104

Earlier quoted context omitted.

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.

That would have been the case if the universe was closed, i.e. if the matter density was above the critical density. However, the matter density is only about 20% of the critical, which means it's infinite. (The remaining 80% of the critical density seem to be the cosmological constant, which means the universe has flat geometry to within experimental error.)

Re: What is the Speed of Gravity?

#112

Post by jerf »

Luc wrote: post #79
Tycho wrote: post #57

Earlier quoted context omitted.

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??

I think philh already explained it pretty well. In your question you suppose the laser dot to be 'fairly large', which I think would complicate things (the shape of the dot 'smearing out' as it goes faster). For me, philh's explanation works best if you imagine the laser pointer to be a cannon of photons, emitting one photon after another at a very high rate and in a single direction, with the electrons of the moon's…

Actually, the shape of the dot is unchanged by going faster. Since it isn't a real object, it isn't affected by relativistic contraction. If you emit a circle, it'll be a circle.

It is of course changed by what it gets reflected off of, but that's not special to this situation at all. If you're at the center of an enormous hollow sphere and shine an astronomically-bright laser circle around, it'll always be a circle to you no matter how fast you move it.

Re: What is the Speed of Gravity?

#113

Post by fxj »

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…

Yes you are totally right. Furthermore: In relativity there is no "right now" moment. Two simultaneous events in one reference frame can be one after the other in another reference frame. (see: http://en.wikipedia.org/wiki/Relativity_of_simultaneity) This concept is even true in special relativity and has been tested and a basic principle why GPS is working.

Re: What is the Speed of Gravity?

#114

Post by Luc »

jerf wrote: post #112
Luc wrote: post #79

Earlier quoted context omitted.

I think philh already explained it pretty well. In your question you suppose the laser dot to be 'fairly large', which I think would complicate things (the shape of the dot 'smearing out' as it goes faster). For me, philh's explanation works best if you imagine the laser pointer to be a cannon of photons, emitting one photon after another at a very high rate and in a single direction, with the electrons of the moon's…

Actually, the shape of the dot is unchanged by going faster. Since it isn't a real object, it isn't affected by relativistic contraction. If you emit a circle, it'll be a circle. It is of course changed by what it gets reflected off of, but that's not special to this situation at all. If you're at the center of an enormous hollow sphere and shine an astronomically-bright laser circle around, it'll always be a circle…

If the laser emits all photons constituting the dot at the same time, in pulses... I was thinking of the photons that make up the dot arriving at random intervals, but I'm probably making things too complicated.

Re: What is the Speed of Gravity?

#115

Post by ars »

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.

Virtual matter can not give off photons - it would violate conservation of momentum (unless it was in pairs, but that would not work with Cherenkov radiation).

Re: What is the Speed of Gravity?

#116

Post by [deleted] »

sp332 wrote: post #71
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…

Actually, the atoms would move much more slowly, something like the speed of sound through the material.

[deleted]

Re: What is the Speed of Gravity?

#117

Post by guelo »

lutorm wrote: post #101
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 rotatin…

Good point. An unobtainium rod as JanezStupar calls it, would be the immovable object, stuck in the universe's gullet gumming up the works. I wonder if God would be able to move it after he created it.

Re: What is the Speed of Gravity?

#118

Post by zbyszek »

eru wrote: post #96
wnoise wrote: post #43

Earlier quoted context omitted.

Gravity absolutely is affected by gravity. The Einstein field equations are non-linear, so gravitational energy is itself a source of gravity. All forms of energy, and momentum, and even stress and pressure are combined into the "stress energy tensor" which determines the local curvature of space-time.

> The Einstein field equations are non-linear, so gravitational energy is itself a source of gravity. Non sequitur? I mean, non-linearity is not the same as feedback. Linear system can exhibit feedback (and still stay linear), and non-linear systems do not have to exhibit feedback.

The equations describing a field theory contain terms which are the product the field, the thing it couples to (could be another field) and the strength of the interaction. When a field couples to itself, there are terms non-linear in the field. So the non-linearity of GR indicates that the gravitational field couples to itself, i.e. that gravity gravitates.

Re: What is the Speed of Gravity?

#119

Post by Dylan16807 »

wnoise wrote: post #43
Dylan16807 wrote: post #35

Earlier quoted context omitted.

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.

Gravity absolutely is affected by gravity. The Einstein field equations are non-linear, so gravitational energy is itself a source of gravity. All forms of energy, and momentum, and even stress and pressure are combined into the "stress energy tensor" which determines the local curvature of space-time.

I knew it was a source of its own much smaller gravity, I guess I was wrong about how strongly coupled that is with the original field.

Still, that's not itself being 'normally' affected by gravity, and I've never seen anything discuss it.

Re: What is the Speed of Gravity?

#120

Post by eru »

ars wrote: post #115
eru wrote: post #92

Earlier quoted context omitted.

Only virtual matter.

Virtual matter can not give off photons - it would violate conservation of momentum (unless it was in pairs, but that would not work with Cherenkov radiation).

There seems to be one paper about Cherenkov radiation in a vacuum [1]. But I have only read the abstract. So you are probably right, that there's no Cherenkov radiation in a vacuum. Or does anyone has any better knowledge?

[1] "Cherenkov radiation in vacuum and plasma-filled microwave sources in the absence of guiding magnetic fields" by GS Nusinovich

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