Gravity: Speed of Light & Black Holes

[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.

 

[mitesh9]

@ OP and others

I'm not sure if u people are aware of this, but, if in case not, a paper published in physics letters A (Publisher: Elsevier Sciences), the speed of gravity is 2*10^10 c, where c is of course, the speed of light. I posted it's link in my previous post, but has been deleted, and I've been warned not to post it again. However, the citation details of that article are,

Tom Van Flandern, Physics Letters A 250:1-11 (1998)

This may not be the ultimate truth, but is a publication in a peer-reviewed journal none the less...

Regards,

Mitesh Patel

 

[atom888]

The only number with order of magnitude so large that I respect is Plank's constant with -34 orders. He got a valid equation to back him up. Now the next constant with more than 5 orders going to put doubt in me until I examine the derivation.

 

[Hans de Vries]

Tom Van Flandern, Physics Letters A 250:1-11 (1998)

This may not be the ultimate truth, but is a publication in a peer-reviewed journal none the less...

Tom Van Flandern also claims that Mars was inhabited by humans or human like beings...

Maybe you understand why your link (to his website) was removed. As far as the 1998
paper concerns, it is wrong. It has been discussed here multiple times as well as on
sci.physics.research (with http://xxx.lanl.gov/abs/gr-qc/9909087" and John Baez)Where does Tom Van Flandern go wrong?

He assumes that the direction of the force, gravitational or electric, is always pointing
to the place where the source was at the moment that the force field was emitted.

This assumption, as we now know more than a century (!), is wrong. The electric field
of a moving charge points to the place where the charge will be if it continuous
moving in the same direction during the time the force field needs to propagate.

The same is true for gravitation. In practice this means for the dynamics of the solar
system that the force is towards the location where the planet or sun is at that moment.

This can be measured and Van Flandern erroneously concluded that this means that
gravity must be instantaneous. Because the force is directed to the location where
the object is and not to where it was. His conclusion is a beginners error which
unfortunately made it into a peer reviewed journal.For the math in case of the electric field, see for instance the links to my book in the
post I made on this thread https://www.physicsforums.com/showpost.php?p=1717519&postcount=39".
Regards, Hans.

 

[redtree]

Tom Van Flandern also claims that Mars was inhabited by humans or human like beings...

Where does Tom Van Flandern go wrong?

He assumes that the direction of the force, gravitational or electric, is always pointing
to the place where the source was at the moment that the force field was emitted.

This assumption, as we now know more than a century (!), is wrong. The electric field
of a moving charge points to the place where the charge will be if it continuous
moving in the same direction during the time the force field needs to propagate.

The same is true for gravitation. In practice this means for the dynamics of the solar
system that the force is towards the location where the planet or sun is at that moment.

This can be measured and Van Flandern erroneously concluded that this means that
gravity must be instantaneous. Because the force is directed to the location where
the object is and not to where it was. His conclusion is a beginners error which
unfortunately made it into a peer reviewed journal.


For the math in case of the electric field, see for instance the links to my book in the
post I made on this thread https://www.physicsforums.com/showpost.php?p=1717519&postcount=39".



Regards, Hans.

One question. I apologize if I've missed something. Is the exchange of information between a point in space-time and the center of gravity limited by c? I assume that you're not suggesting that information is exchanged instantaneously.

 

[Hans de Vries]

One question. Is the exchange of information between a point in space-time and the center of gravity limited by c?

Yes.


Regards, Hans

 

[rbj]

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.

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

... but, just to dot t's and cross i's, the consequence is that c is THE limit, because to overuse a metaphor, in the fabric of space and time, all fundamental interactions propagate at this finite speed c in a vacuum. these fundamental interactions don't propagate at a speed slower than c (which ostensibly wouldn't violate the concept of a limit of c) nor do they propagate at a speed faster than c. since some kinda physical interaction is needed to convey information (i nudge at the string on my end and the string nudges you at your end), the speed limit of c comes about because the speed of propagation of any fundamental interaction in a vacuum is c. and this finite speed of propagation is a property of space and time, not of the interaction.

now, if i understand this right (which may not be the case), because, in a non-vacuum, this interaction does interact with matter that comes in between (if the interaction is EM, charged particles disturbed by the propagating EM wave move around and, in that motion, themselves create an EM wave where, all combined, results in an EM wave with group velocity that is slower than c), the physical disturbance that conveys information may end up moving at a slower speed.

 

[rbj]

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?

i think it's because this speed of propagation is not a function of which beast is creating a disturbance. whether it's me moving electric charge around (with this device we call an "antenna") causing a disturbance in electric charges in your antenna, or something much bigger than me moving the masses of stars around (like supernovae) causing disturbances in matter that are difficult to measure, it's not an issue of which beastly interaction is doing the disturbing (they all "want" to be instantaneous). it's an issue of the nature of space and time that if you are equal distant from some point of cause "A" and some other point of effect "B", when you observe the cause at A, you will see the consequence of that cause as a disturbance at B at a later time that is proportional to the distance that B is from A. doesn't matter what the cause is; gravity, E&M, weak nuclear, or strong nuclear. (not that we will ever observe a distant interaction of the latter two.)

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.

but that's true for information conveyed with E&M? (if all you established is the speed of light is the same for all observers.) but what if gravitational disturbances were instantaneous? then, conceptually, one could convey information instantaneously by triggering supernovae with morse code. it's the speed limit (both upper and lower) for all physical interactions traversing a vacuum that makes it the speed limit for conveying information unless someone postulates a means of conveying information that requires no physical interaction.

 

[dude222]

A non-field form of electromagnetism can be seen at johnwilliams22dothi5dotcom, posted anonymously. The same theorem can be used for Newton's gravity law, but it needs to be corrected by a factor to account for the fact that the force is proportional to the total energy of the mass that is being acted. This factor is the lorentz factor of the source transformed from a stationary reference frame to that of the influenced mass.

Observations of the adherence of gravity to this formula and those which also assume that the propagation is equal to c (Einstein's) might be used to verify that gravity propagates at the speed c, but only in situations when the source of the gravity changes speed can relevant observations be made. This is because for a uniformly moving source, the formula so derived does not depend on the gravitational force actually moving at c. It only uses a hypothetical speed equal to c. It is, however, assumed that the speed is c.

According to VSL techniques, the speed c is not constant. Light exiting a black hole would stop. The local speed of light at that location would be the one used in the Lorentz factor, so extreme relativistic effects would be seen by an outside observer.

 

[dude222]

A mistake was made in the previous post. All speeds would slow proportionally, so extreme relativistic effects wouldn't necessarily be see.

 

[nuby]

Could it be possible that gravity propagates at the angular velocity of an electro-magnetic wave?

 

[gtimespace]

In his book The Fabric of the Cosmos, Brian Greene writes, "Einstein was able to work this out, " and he found that the speed at which gravity can transmit it's influence is exactly the speed of light. "...the warps and ripples-gravity, that is-do not travel from place to place instantaneously, as they do in Newtonian calculations of gravity." Greene then gives an interesting example of what would happen if aliens removed the moon from its orbit. It would take one and a half seconds for the tides to be affected. Exactly the time it would take for us to see that the moon had vanished.

 

[HarryWertM]

This thread has been interesting reading. Here is my best understanding:

The "speed of gravitational information" is c. The "speed of electrical information" is also c. Both can easily escape a black hole. But nothing inside the black hole can generate any "waves". Only "normal" fields escape, kinda like the poster who mentioned infinitely long red shifting.

So: Charged particles fall into a black hole. The charge on the black hole changes. This this electric field change propagates out at the speed c. But if charges are somehow moving around inside the black hle - [[An entirely different question: Can they?]] - then no apparent effects from the moving charges would go outside the hole. Likewise, if massees fall into the hole, the increase in gravity goes out at c, but if masses are spiraling around inside the hole, no "gravity waves" would exit the hole.

Right?

-Harry Wertmuller

 

[dude222]

The speed of gravity isn't constant though. It is different at different locations. I'll give you a link if you want.

 

[Chaos' lil bro Order]

The speed of gravity isn't constant though. It is different at different locations. I'll give you a link if you want.

Umm. You should give that link, because its currently accepted as a constant. Are you referring to the theory of MOND?

 

[HarryWertM]

Would surely appreciate a reliable reference explaining where speed of gravity is different.

 

[dude222]

I can give you a link for a uniform gravitational field (or uniform acceleration). I'm thinking of posting a more complete account. I post on hi5, username johnwilliams22.

 

[dude222]

sorry, user name john williams

 

[stone1]

I have been curious to find an answer to that question too.

Suppose that at 12pm one marks the position of the Sun (as it was ~8 min in the past) and also at 12pm one marks the position of the gravity pull (I don't know whether there are instruments that can do that yet). Would the two positions be the same?

A similar question is whether the Earth rotates around the actual position of the Sun or around the position of the Sun as it was ~8 min in the past.

 

[DaveC426913]

I have been curious to find an answer to that question too.

Suppose that at 12pm one marks the position of the Sun (as it was ~8 min in the past) and also at 12pm one marks the position of the gravity pull (I don't know whether there are instruments that can do that yet). Would the two positions be the same?

Yes. Light and gravity both lag by 8 minutes.

A similar question is whether the Earth rotates around the actual position of the Sun or around the position of the Sun as it was ~8 min in the past.

"Actual" is a tricky term here. Since nothing can travel faster than c, it's kind of moot to talk about "actual" position.