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nuby
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Does gravity travel at the speed of light? If so, how can a black holes suck in photons?
rbj said:it is believed to and is assumed to in General Relativity.
i believe that there is some astronomical measure they did (wasn't it when that asteroid or comet crashed into Jupiter?) that determined to within 20% that GR was correct about it.
Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.nuby said:Does gravity travel at the speed of light? If so, how can a black holes suck in photons?
DaveC426913 said:Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.
nuby said:So the force of gravity is instantaneous, but changes in gravity move at the speed of light?
nuby said:So the force of gravity is instantaneous, but changes in gravity move at the speed of light?
nuby said:Let me know if this sounds as dumb as my previous question. If an object was to spontaneously appear within the Earth's atmosphere, would it feel gravity instantly? If the Earth's mass was split in half after the object appeared, would it take time for the object to feel the difference?
Gravity is omnipresent; it is there in the first place (because it is is in fact the curvature of space-time).nuby said:So the force of gravity is instantaneous, but changes in gravity move at the speed of light?
nuby said:Let me know if this sounds as dumb as my previous question. If an object was to spontaneously appear within the Earth's atmosphere, would it feel gravity instantly? If the Earth's mass was split in half after the object appeared, would it take time for the object to feel the difference?
rbj said:think a bit about the meaningfulness of your question.
DaveC426913 said:Gravity is omnipresent; it is there in the first place (because it is is in fact the curvature of space-time).
This is a dangerous analogy, but it's like a swimmer asking 'I see waves traveling at 100mph across the lake, but what is the speed of the lake'?
This is where the analogy breaks down, so we don;t carry it that far. All analogies break down at a certain level of detail. If they didn't, they would be models!rbj said:it's a good analogy, though, me thinks. but we have to add, that no matter how hard the swimmer is swimming nor in what direction, the speed of the waves going across the lake is the same for that swimmer. and equal to any other swimmers' measurements.
Really?Nickelodeon said:Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.
DaveC426913 said:Really?
So, I could set up a gravity-making machine
and transmit singles across the diameter of the universe in zero time.
Seems like one of those little facts that has escaped, oh say, the entire collective body of physicists on the planet Earth.:uhh:
Nickelodeon said:Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.
Nickelodeon said:yes you could
No you couldn't.
'the entire collective body of physicists on the planet Earth' haven't come up with any idea of what causes gravity so it's not really relevant making that comment.
DaveC426913 said:But they do know that it doesn't travel at infinite speed. Which is why what you said makes no sense.
Oh I see. You were saying something akin to 'scientists have narrowed down the speed of gravity to between zero and inifinity' - a tongue-in-cheek way of saying 'we just don't know'.Nickelodeon said:I may have gone over the top with the 'infinite' speed part but my point was that we shouldn't necessarily think of its speed being a universal constant or that it should be constrained by factors which dictate the speed of light.
Nickelodeon said:... we shouldn't necessarily think of its speed being a universal constant or that it should be constrained by factors which dictate the speed of light.
While well-aware of the precession of Mercury, and its explanation via the curvature of space-time a la GR, I'm not sure it is proof positive of the speed of gravitational waves.peter0302 said:You needn't even consider hypotheticals to understand this. Consider the planet Mercury. If you calculate the orbit using Newtonian mechanics (which assume gravitation is an instantaneous force) you will find that Mercury's true orbit is slightly different than what you calculate. The perihelion, the point at which the planet is closest to the Sun, actually moves over the course of time. This is explained by General Relativity due to the curvature of spacetime or, another way of looking at it, the fact that gravitational forces propagate at the speed of light.
rbj said:whatever property of space and time that makes disturbances of EM propagate at a finite speed is what makes disturbances of gravity propagate at the same finite speed.
I get that that's what you're claiming, I'm just not convinced that that's what Mercury's precession is from. I might be wrong. I'd like an authority to weigh in on that.peter0302 said:Why not? If you calculated the orbit assuming instantaneous attraction, you'd get the Newtonian result. If you calculated the orbit using virtual gravitons traveling at the speed of light, you should see e a slight lag in the pull from the star due to the finite speed of the gravitons
Nickelodeon said:To clarify things (hopefully) with an analogy - there are two fishes swimming some distance apart in a river. One fish claps its fins and creates a 'disturbance' which is heared by the other fish. This disturbance propagates at a speed governed by the properties of the water.
The speed of the flow of the non physical river is what we are talking about when discussing the speed gravity.
Nothing proven here.Phy6explorer said:I don't even think that gravity travels. I mean it's like asking "Does vacuum teavel?"That surely proves me wrong!peter0302 said:Well, you agree, don't you, that the electromagnetic force propagates at the speed of light, carired by the photon, right? So, same idea for gravity/gravitons.
DaveC426913 said:Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.
DaveC426913 said:Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.