Gravity: Speed of Light & Black Holes

[nuby]

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

 

[rbj]

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.

 

[JesseM]

See Does Gravity Travel at the Speed of Light? and How does the gravity get out of the black hole? from the Usenet Physics FAQ.

 

[George Jones]

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.

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

 

[DaveC426913]

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

Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.

 

[nuby]

Gravity doesn't travel at the speed of light, changes in 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?

 

[rbj]

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.

 

[nuby]

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?

 

[off-diagonal]

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 :)

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?

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

 

[DaveC426913]

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

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'?

 

[yuiop]

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?

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.

 

[rbj]

think a bit about the meaningfulness of your question.

i really meant to say and should have said think about the meaning of the question. it's more that the meaning of the two were inconsistant.

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.

 

[Shackleford]

*gravitation

Gravity is localized gravitation.

 

[DaveC426913]

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.

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!

 

[Nickelodeon]

Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.

 

[DaveC426913]

Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.

Really?

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, the entire collective body of physicists on the planet Earth.

 

[Nickelodeon]

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.

'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.

 

[nuby]

Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.

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

 

[DaveC426913]

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.

 

[jambaugh]

One must be very careful in GR of thought experiments where things "appear instantly out of no-where". Built into the theory is the continuity equations which prevent such from happening. The source of gravity is stress-energy. There's no way for energy (mass) to appear at a point without there being some energy-momentum current transmitting it there.

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.

 

[Nickelodeon]

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.

 

[DaveC426913]

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.

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'.

 

[rbj]

... 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 should think of both EM and gravity as well as all other fundamental interactions as propagating through a vacuum with this same speed (it's not a function of which interaction, they're all "trying" to be instantaneous, it's a property of space and time that these ostensibly instantaneous interactions do not have effect instantly on distant object, as viewed by someone who is equi-distant from both source and destination object), and since we can only measure dimensionless quantities, there is no meaning to this speed being a different value. if it were'nt "constant', we wouldn't be able to tell.

now, conceivably, if the speeds of propagation of different interactions were different, we could detect that (and they're trying, there was one astronomical measurement that claims to have measured the speeds of gravity vs. light to be equal to within 20%). but with other quantitative predictions of GR being confirmed in experiment, with the fact that we have measured G (in terms of our meter sticks, cesium clocks, and kilogram prototypes) to 5 digits, if the speed of gravity was much different than c, that constant in the Einstien field equation (G/c[sup]4[/sup]) would 4 times more different, and the quantitative results would be different than what was measured.

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).

 

[peter0302]

Wow. How many non-answers can a single thread generate.

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.

 

[DaveC426913]

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.

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]

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 versus the momentum of the planet in the orthogonal direction, because the planet is tending to move away from the star ever so slightly. Granted, this is just a hueristic. It would be interesting to do the math and, ignoring the GR field equations, just use a Feynman "virtual graviton" model to see what the orbit looks like given the lag from the speed of gravity.

 

[Phy6explorer]

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

 

[peter0302]

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.

 

[Phy6explorer]

That surely proves me wrong!

 

[Nickelodeon]

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.

 

[DaveC426913]

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

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.

 

[rbj]

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.

 

[RandallB]

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

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.

That surely proves me wrong!

Nothing proven here.
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”.

 

[redtree]

Gravity doesn't travel at the speed of light, changes in 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)?

 

[Chaos' lil bro Order]

Gravity doesn't travel at the speed of light, changes in 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?

 

[Phy6explorer]

But I still don't get! I read in a reliable website that even light cannot escape from a black hole and a black hole sucks stuff due to extremely strong gravitational field. And according to general theory gravity travels at c. So how does a black hole suck light?

 

[Nickelodeon]

And according to general theory gravity travels at c.

It's a pity we don't have Einstein here to ask if he really said that.

In answer to your question I think we have to assume that, whatever the cause of gravity, it has to be either traveling faster than light or that it modifies the mechanics of EM radiation to such an extent that it can no longer propagate.

 

[RandallB]

(And according to general theory gravity travels at c.)
It's a pity we don't have Einstein here to ask if he really said that.

It is an entirely different perspective to use gravity fields of GR vs. graviton particles to visualize how gravity works.
I think the problem comes in trying to use the same “sense of seeing” for both theories and that does not work on fields so easily.

I believe Einstein would say something to the effect that the fields are fixed potentials set by the GR masses establishing the extra dimensional GR curves. Thus, just as it is impossible to actually perform the hypothetical examples often put forward such as “What if the Sun ‘just disappeared’ what would the Earth do?“. We can only expect to displace or move the sun at a speed of c. Further even if we suddenly move the sun at the speed of “c” we should not expect that rate of displacement to be translated to an instant new curve at distances away from the sun to realign those fixed fields to the new position of the sun. Any change in the gradient of curve that needs to propagate out to distant gravitational fields should be expected to take some time.

What might that speed of propagation be? Although it is not quite the same thing as gravitation particles moving in some form of plenum, it hard to see any explanation use anything other than something very close to “c”.
But the idea that changes in curve shapes (not dependent on particle movements like gravitons in GR) could accumulate from multiple masses to establish larger and larger ‘curves in space’ should not be that hard to visualize. Even curves so steep that real particles such as light would need speeds faster than “c” to climb over.

Visualizing changes in GR curves within the GR theory is simplified since it is not dependent on the propagation of “gravitons” just the propagation of the gravity field change information.

It might be true that QM in expecting gravitons in the Standard Model “Cannot Work” here, But QM does not predict Black Holes GR does. GR “does not work” in the microscopic domain of QM either, that’s why we use both theories in their appropriate place.

 

[Hans de Vries]

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.

Correct

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.

But Mercury's orbit is a different effect caused by the curvature of space-time yes,
but not due to the speed of light.The direction of the gravitational (or electric) force coming from a moving object is not
in the direction of where the object was, but where the moving object is if it continues
to move in a straight line during the time needed for the propagation.

For electric forces you can check this by using the Lienard Wiechert potentials which
are derived by assuming that V propagates with c. The gradient of V, the electric field,
points to a "Phantom position" which is the position where the source was plus the vector
"speed x duration".

The mathematical derivation is for instance given in chapter 2 of my book.

http://physics-quest.org/Book_Chapter_EM_LorentzContr.pdf

specifically sections 2.2, 2.8 and 2.10 Regards, Hans

 

[DaveC426913]

But I still don't get! I read in a reliable website that even light cannot escape from a black hole and a black hole sucks stuff due to extremely strong gravitational field. And according to general theory gravity travels at c. So how does a black hole suck light?

The light does travel at c, even near a black hole. The gravity bends its path into a circular orbit, so that it never escapes - it doesn't actually stop light in its tracks.

 

[redtree]

The light does travel at c, even near a black hole. The gravity bends its path into a circular orbit, so that it never escapes - it doesn't actually stop light in its tracks.

A photon emitted exactly normal to the "surface" of a black hole, which direction will it orbit? How will its path be bent?

 

[DaveC426913]

A photon emitted exactly normal to the "surface" of a black hole, which direction will it orbit? How will its path be bent?

(By surface, we'll talk about its Schwarzschild Radius.)

It won't. It will head straight out. But it will be almost infinitely red-shifted - its frequency will approach zero and its wavelength will approach infinity.

 

[redtree]

And will it escape the event horizon?

 

[Phy6explorer]

Are you asking whether the light will pass through the event horizon or go on without passing the event horizon? I mean, if the light passes through the event horizon, that's it, isn't it? The observer cannot see anymore of it? I don't think it will go on without passing through it ?

 

[Nickelodeon]

(By surface, we'll talk about its Schwarzschild Radius.)

It won't. It will head straight out. But it will be almost infinitely red-shifted - its frequency will approach zero and its wavelength will approach infinity.

The frequency could probably stay the same - the wavelength approaching infinity would do the trick.

This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?

 

[dst]

The frequency could probably stay the same - the wavelength approaching infinity would do the trick.

This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?

The constant denoted c is as you say, THE universal constant. It places a limit on how fast anything capable of conveying information can go. Since gravity can convey information, it's limited as such.

 

[DaveC426913]

The constant denoted c is as you say, THE universal constant.

Yes, as dst eloquently points out, c is THE limit, not just on light.

 

[Janus]

This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?

To build on what has already been said:

The speed of light is considered a constant in GR because it is postulated as such in SR, and SR is just a limited subset of GR. Also, every observation to date has upheld that postulate.

Also, once you establish that a given speed is constant for all observers, it follows logically that this speed is also the ultimate speed limit for information.

 

[redtree]

Are you asking whether the light will pass through the event horizon or go on without passing the event horizon? I mean, if the light passes through the event horizon, that's it, isn't it? The observer cannot see anymore of it? I don't think it will go on without passing through it ?

I mean if a photon travels away from a black hole with a non-zero velocity, then at some point in time, how would the photon not cross the event horizon? Particularly since, as the photon traveled further from the black hole's center of gravity, the gravitational dilation (or curvature) of space-time would diminish.

 

[DaveC426913]

I mean if a photon travels away from a black hole with a non-zero velocity, then at some point in time, how would the photon not cross the event horizon? Particularly since, as the photon traveled further from the black hole's center of gravity, the gravitational dilation (or curvature) of space-time would diminish.

Yes, the photon will escape the black hole! Saying that light can't escape a black hole is a bit sloppy, so there's no paradox.