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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?

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- Thread starter nuby
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- #1

nuby

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Does gravity travel at the speed of light? If so, how can a black holes suck in photons?

- #2

rbj

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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.

- #3

JesseM

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- #4

George Jones

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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.

A quasar passed behind Jupiter. The announced results were controversial; see https://www.physicsforums.com/showpost.php?p=681816&postcount=12".

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- #5

DaveC426913

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Does gravity travel at the speed of light? If so, how can a black holes suck in photons?

- #6

nuby

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Gravitydoesn't travel at the speed of light,changesin gravity (in the form of waves) travel at the speed of light.

So the force of gravity is instantaneous, but changes in gravity move at the speed of light?

- #7

rbj

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So the force of gravity is instantaneous, but changes in gravity move at the speed of light?

think a bit about the meaningfulness of your question.

- #8

nuby

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- #9

off-diagonal

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So the force of gravity is instantaneous, but changes in gravity move at the speed of light?

may be you could think about a curvature of spacetime instead force of gravity :)

no if an object suddenly appear , it take some time that change in gravity will reach the earth.

- #10

DaveC426913

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Gravity is omnipresent; it is there in the first place (because it is is in fact the curvature of space-time).So the force of gravity is instantaneous, but changes in gravity move at the speed of light?

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'?

- #11

yuiop

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Hi Nuby,

Your spontaneously appearing object should feel the Earth's gravity instantly. This is easy to visualise using the (aprozimate) rubber sheet analogy with masses deforming the sheet to create gradients. You could call the map of those gradiants the spacetime "landscape". Your object would appear in this ready made landscape and instantly respond to the curvature of that landscape.

Now if half the Earth was to disappear then, yes it would take time for the object to "notice" because the landscape deforms at the speed of light. There is a caveat here, that it would be impossible for anything to suddenly appear or dissappear. For example if the Sun was somehow anhilated by some antimatter, it would turn into a bunch of photons and the energy of those photons would still behave as a gravitational mass. It would take about 8 minutes for the photons to radiate outwards past the orbital radius of the Earth from the Sun, after which the Earth would no longer continue in its normal aproximately circular orbit.

To more directly answer your original question "Does gravity travel at the speed of light? If so, how can a black holes suck in photons?" there is no requirement for something like gravitons from the black hole to "chase" after photons in order to draw them back. The photons simply move in the ready made spacetime landscape around the black hole.

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- #12

rbj

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think a bit about the meaningfulness of your question.

i really meant to say and should have said think about the

Gravity is omnipresent; it is there in the first place (because it is is in fact the curvature of space-time).

that's the direct answer i was groping for.

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'?

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. assuming they can accurately measure the wavespeed to a precision of much better than their swim speeds. and, they can't really measure their swim speeds except relative to each other. they can't measure their swim speeds against the water because they think they are drifting on it. so maybe a better analogy is that all of the swimmers are really just sitting around on very bouyant light rafts that skim across the water without any drag.

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- #13

Shackleford

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*gravitation

Gravity is localized gravitation.

Gravity is localized gravitation.

- #14

DaveC426913

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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 beit'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.

- #15

Nickelodeon

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- #16

DaveC426913

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So, I could set up a gravity-making machine and transmit singles across the diameter of the universe in zero time.

Huh.

Seems like one of those little facts that has escaped, oh say,

- #17

Nickelodeon

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Really?

So, I could set up a gravity-making machine

yes you could

and transmit singles across the diameter of the universe in zero time.

No you couldn't.

Seems like one of those little facts that has escaped, oh say,the entire collective body of physicists on the planet Earth.:uhh:

'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.

- #18

nuby

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Gravity is caused by mass. And mass is a potential form of energy, right? Interesting.

- #19

DaveC426913

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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.

But they do know that it doesn't travel at infinite speed. Which is why what you said makes no sense.

- #20

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If you violate such continuity you've stepped entirely outside the theory. You can suppose that you create for example monopolar and dipolar waves which cannot be done physically and the theory of how such should be have is pure theological speculation.

Related to the OP's question about black holes consider also that throwing negative charges into the black hole will not prevent their charges from being felt outside. You will find that the black hole will manifest the same electrical field (and gravity) as a larger star of the same mass containing these charges.

- #21

Nickelodeon

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But they do know that it doesn't travel at infinite speed. Which is why what you said makes no sense.

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.

- #22

DaveC426913

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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'.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.

- #23

rbj

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... 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.

that's fully incorrect. we

now, conceivably, if the speeds of propagation of different interactions were different, we

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 (as well as nuclear interactions, even though it doesn't make much difference for them, since the interaction is between particles very close to each other).

- #24

peter0302

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Yes, gravitational forces propagate at the speed of light according to all accepted theory and known experiments. If the hypothesized graviton (gravity force carrier) exists, it will travel at the speed of light like a photon.

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.

- #25

DaveC426913

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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 theYou 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.

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peter0302

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- #27

Phy6explorer

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I don't even think that gravity travels. I mean it's like asking "Does vacuum teavel?"

- #28

peter0302

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- #29

Phy6explorer

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That surely proves me wrong!

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Nickelodeon

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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.

'disturbances' of gravity is a different thing and I wouldn't dispute the idea that a disturbance would more than likely propagate at the speed light.

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.

- #31

DaveC426913

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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.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

- #32

rbj

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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.

sounds like a "gravity aether" to me. i don't think it exists and i don't think that is what is meant by this Minkowsky space-time.

- #33

RandallB

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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:

All you have shown is the discrepancy and incompatibility between General Relativity Astrophysics vs. Quantum Mechanics Particle Physics.

GR expects some extra dimensional curvature to account for gravitational field strengths or affects that according to the theory we might never directly observe.

QM and the standard model expect no curvature or fields but physical particles “gravitons” to account for gravity.

So does gravity travel in particles or do fields change in strength as the masses move and change the shape GR curves?

Until one theory can replace the other theory in that other’s domain, the best you can say is “I don’t know but from within the domain of each theory I have a tool that works”.

- #34

redtree

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Gravitydoesn't travel at the speed of light,changesin gravity (in the form of waves) travel at the speed of light.

In the case of a black hole, assume a change in the mass (perhaps from a large number of photons striking the black hole on the opposite side), what path do the "changes" in gravity follow traveling at c to escape the event horizon? How is that path different from the path a photon might travel (which is unable to escape the event horizon)?

- #35

Chaos' lil bro Order

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Gravitydoesn't travel at the speed of light,changesin gravity (in the form of waves) travel at the speed of light.

Is this proven Dave? That gravity travels in waves? I thought experiments like LIGO and LISA were meant to test this assumption. Is there more evidence for gravity being wavellike rather than point like?

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