Can gravity carry information out of a black hole?

[feynomite]

Here's a question I've had for some time.

I imagine that you can transfer information via gravity just as you can with light. For example, if I wave my hand back and forth (in any direction), and you had a sufficiently precise instrument, you could record the movement of my hand from a distance. I could transfer information to you by perhaps a binary system (left = 1, right = 0). Of course, you'd have to be sure you aren't measuring gravity of stuff other than my hand, like a bird flying by or something like that.

For the sake of this experiment, let's use something other than my hand, like a wrecking ball that moves up and down. You are trying to measure where it is from just 10 feet away. (I'm pretty sure there are gravitimeters(?) this precise... links appreciated). Now I can move the wrecking ball up and down and you could, knowing the mass and shape of the ball, know whether its 20ft or 20ft down or whatever.

Now, I'd like to know why this isn't possible for the ball to be inside a black hole, and you outside the black hole. If any particle with mass inside a black hole is moving around, then you should be able to detect its movements from outside the black hole, correct? Black holes do occupy a volume, correct? I at least know they have a radius. If the answer to my question (can you transmit information via gravity out of a black hole) is "no" then I assume that everything inside the event horizon cannot move, or is completely homogenous, otherwise you should be able to measure whatever has mass inside the black hole moving (which would be information escaping).

My intuition tells me this has been though of before and there's a reasonable explanation for it all.

Thanks

 

[rbj]

i think that this is an interesting question. it seems to me that at least the information of how much matter got sucked into the BH "escapes" it because, i think we can measure its mass by the "amount of gravity" it emits (or the curvature of space-time in its neighborhood).

 

[gendou2]

Interesting question indeed. The immediate problem I see is that gravitational waves propagate the same way light waves do, namely, at the speed of light. Since waves (electromagnetic, gravitational, or otherwise) cannot escape from inside the black hole, no information can come out as the wave its self. In other words, the internal mechanics of the area enclosed by the Event Horizon are hidden. Good thing too, because there's a nasty singularity in there.

However, just as Photon pairs leek Hawking radiation out of the Black Hole, so too might Graviton pairs leek out. As far as I can tell, all this calls for is Graviton Pair Production, which is of course highly speculative at best. Cool idea, though.

http://en.wikipedia.org/wiki/Pair_production
http://en.wikipedia.org/wiki/Graviton

 

[Hurkyl]

If any particle with mass inside a black hole is moving around, then you should be able to detect its movements from outside the black hole, correct?

No; you're accelerating away too fast, the information will never reach you. If you weren't accelerating, you'd fall into the black hole -- don't forget that from the perspective of someone hovering just over the black hole, the event horizon is also moving towards you at the speed of light.

 

[feynomite]

Interesting question indeed. The immediate problem I see is that gravitational waves propagate the same way light waves do, namely, at the speed of light. Since waves (electromagnetic, gravitational, or otherwise) cannot escape from inside the black hole, no information can come out as the wave its self. In other words, the internal mechanics of the area enclosed by the Event Horizon are hidden. Good thing too, because there's a nasty singularity in there.

However, just as Photon pairs leek Hawking radiation out of the Black Hole, so too might Graviton pairs leek out. As far as I can tell, all this calls for is Graviton Pair Production, which is of course highly speculative at best. Cool idea, though.

http://en.wikipedia.org/wiki/Pair_production
http://en.wikipedia.org/wiki/Graviton

Gravitational waves do not propagate the same way light waves do. For one, they aren't affected by gravity :). Light is electromagnetic waves, which travel at the speed of light, and are affected by gravity. Gravity is simply a part of space time, whose changes travel (coincidentally, as far as I can understand) at the speed of light.

See "How can gravity escape a black hole" http://www.faqs.org/faqs/astronomy/faq/part4/section-11.html"

 

[feynomite]

No; you're accelerating away too fast, the information will never reach you. If you weren't accelerating, you'd fall into the black hole -- don't forget that from the perspective of someone hovering just over the black hole, the event horizon is also moving towards you at the speed of light.

This applies to electromagnetic waves, but not gravity... otherwise how do black holes have mass?

 

[Naty1]

If any particle with mass inside a black hole is moving around, then you should be able to detect its movements from outside the black hole, correct?

It IS an interesting question, one regarding information. And how information emerges, whether it does, from a black hole is a major unanswered question in physics. I believe Hawking may have rescinded his strong belief that information is forever lost, that is hidden, but I'm unsure just where he stands. Charles Seife. Decoding the Universe,2006 which is largely an information study of physics, says information IS lost...this seems to violate conservation of information...all that is available is mass, angular momentum and charge... (Temperature can be inferred from these three). Other information appears inaccessible, but has it been destroyed? nobody knows. General relativity and quantum theory break down under the extreme conditions encountered.

 

[Jonathan Scott]

Gravitational waves do not propagate the same way light waves do. For one, they aren't affected by gravity :).

No, that's wrong. Gravitational waves propagate through space-time following the same path as light waves would, and traveling at the same speed.

 

[xantox]

Now, I'd like to know why this isn't possible for the ball to be inside a black hole, and you outside the black hole. If any particle with mass inside a black hole is moving around, then you should be able to detect its movements from outside the black hole, correct? Black holes do occupy a volume, correct? I at least know they have a radius. If the answer to my question (can you transmit information via gravity out of a black hole) is "no" then I assume that everything inside the event horizon cannot move, or is completely homogenous, otherwise you should be able to measure whatever has mass inside the black hole moving (which would be information escaping)

The short story from the classical theory is that spatial radius outside the black hole becomes time inside the black hole. From this follows that, while everything inside the event horizon can move normally, no information can exit since "to exit" from inside a black hole means "to go backwards in time". Moreover, no information can stay inside forever since it must fall in the singularity. The quantum story is much more rich and it is believed, though not fully proven, that most of the information comes out with the quantum radiation and that there is no singularity.

 

[Hurkyl]

This applies to electromagnetic waves, but not gravity... otherwise how do black holes have mass?

I don't see the connection. Why would this topic have anything to do with whether black holes have mass?

Incidentally, black holes also have charge, and have electromagnetic fields.

 

[gendou2]

Gravitational waves do not propagate the same way light waves do.

What I meant was, at the same speed.
Also, both are waves in Gauge fields.
It's no coincidence that all waves in Gauge fields propagate at the speed of light.

 

[Denton]

How is it that black holes cannot emit gravitational waves?

 

[gendou2]

How is it that black holes cannot emit gravitational waves?

Is it just me or was this already addressed?

Argument #1:
A black hole can't accelerate, nor can it change shape.
How can something emit gravitational waves without accelerating or changing shape?

Argument #2:
Say there is a gravitational wave, approaching the Event Horizon from the INSIDE (a strange thought).
It will climb the infinitely steep curvature of space and never get out of the gravity well. Ever.

 

[feynomite]

Is it just me or was this already addressed?

Argument #1:
A black hole can't accelerate, nor can it change shape.
How can something emit gravitational waves without accelerating or changing shape?

Argument #2:
Say there is a gravitational wave, approaching the Event Horizon from the INSIDE (a strange though).
It will climb the infinitely steep curvature of space and never get out of the gravity well. Ever.

I guess what I don't understand is given argument 2, how anything inside a block hole can gravitationally influence things outside the black hole. Say there's a black hole of 1,000 solar masses, and a 5 solar mass star falls into it. How does that black hole now have 1,005 solar masses if as soon as the star falls into it its gravity cannot reach outside of it?

Conversely, if the black hole loses mass due to Hawking Radiation (which I know next to nothing about) how is it that the black hole now exerts less gravitational influence (albeit by a minuscule amount)?

Also, in terms of black holes not being able to accelerate, what about binary systems of black holes and stars? Are the black holes not accelerating?

 

[gendou2]

I guess what I don't understand is given argument 2, how anything inside a block hole can gravitationally influence things outside the black hole. Say there's a black hole of 1,000 solar masses, and a 5 solar mass star falls into it. How does that black hole now have 1,005 solar masses if as soon as the star falls into it its gravity cannot reach outside of it?

It gets bigger.

Conversely, if the black hole loses mass due to Hawking Radiation (which I know next to nothing about) how is it that the black hole now exerts less gravitational influence (albeit by a minuscule amount)?

It gets smaller.

In terms of black holes not being able to accelerate, what about binary systems of black holes and stars? Are the black holes not accelerating?

What I mean is that one black hole doesn't accelerate all by it's self. It can't send a signal to you by jiggling the way I can like this *does a stupid dance*.

 

[Hurkyl]

I guess what I don't understand is given argument 2, how anything inside a block hole can gravitationally influence things outside the black hole. Say there's a black hole of 1,000 solar masses, and a 5 solar mass star falls into it. How does that black hole now have 1,005 solar masses if as soon as the star falls into it its gravity cannot reach outside of it?

The gravity doesn't have to "reach outside of it" -- the gravity was already 'outside of it'. Before the star crosses the event horizon, the system lies in a gravitational well of 1,005 solar masses^*. After the star crosses the event horizon, the system still lies in a gravitational well of 1,005 solar masses.

Changes in the gravitational field are caused by waves -- where are you going to find a gravitational wave that cancels out those extra 5 solar masses?

*: This is imprecise, but it should cover the idea.

Conversely, if the black hole loses mass due to Hawking Radiation (which I know next to nothing about) how is it that the black hole now exerts less gravitational influence (albeit by a minuscule amount)?

Hawking radiation is not a purely general relativistic effect -- it arises from (heuristically) combining GR's predictions with QM's predictions. When we leave the confines of GR, it's statements are no longer strictly accurate.

For the record, it's believed that Hawking radiation is purely thermal -- it should contains absolutely no information about what goes on inside the black hole.

 

[gendou2]

For the record, it's believed that Hawking radiation is purely thermal -- it should contains absolutely no information about what goes on inside the black hole.

From what I've read, many people still contest the thermal nature of Hawking radiation.
Since Hawking himself conceded his famous bet with Preskill, I'd say the community is at least divided on the subject.
For some interesting thoughts on alternative ways to solve the information paradox, read "The Black Hole War" by Leonard Susskind. He's a bit of a string nut, but the book is interesting.

http://en.wikipedia.org/wiki/Thorne-Hawking-Preskill_bet
http://en.wikipedia.org/wiki/Black_hole_information_paradox

 

[Jonathan Scott]

Some people still contest the existence of black holes on the grounds that Schwarzschild's original solution didn't have them and it was only when Hilbert made a different (mathematically plausible but physically questionable) assumption and changed the interpretation of the radial coordinate that the possibility of black holes arose.

I think the best explanation of this situation (and least over-stated) is an article by Salvatore Antoci and Dierck-Ekkehard Liebscher "Reinstating Schwarzschild's Original Manifold and its Singularity", also available at arXiv:gr-qc/0406090.

These ideas have been forcefully rebutted on-line in a thread "Flogging the Xprint" on sci.physics.research by "T. Essel", and also in a paper by Malcolm MacCallum at arXiv:gr-qc/0608033. However, although in both cases the "rebuttals" appear very authoritative, it appears that they are based on stating weak arguments very forcefully, which I don't find very convincing, and assuming that any fault with the opposing argument means that their own position is correct.

I believe that basically the situation is that at present there's no physical theory which allows us to determine the location of the mass point in the Schwarzschild vacuum solution. Schwarzschild assumed it to be at his original r=0, which is now R=2GM in the Schwarzschild radial coordinate defined by the proper areas of spheres. Hilbert assumed it to be at R=0 because it seemed to be mathematically valid to do so.

It was only a few years ago that Leonard S Abrams called attention to this difference of assumptions, which had apparently been ignored since Hilbert "corrected" Schwarzschild's original assumption. Some people have argued that since there are a lot of weird things in black hole theory, Schwarzschild's original assumption must have been correct. I'm inclined to sympathize with this point of view, but it's not a proof. On the other side, the people such as "T. Essel" who argue against them say that most of their arguments against Hilbert's assumption are invalid (which I'd probably agree with), and hence that Hilbert was right (which doesn't actually follow).

As far as I can see, Antoci and Liebscher's article referenced above gives a more balanced view, giving the background and explaining how the difference arose, and showing that Schwarzschild's original solution does appear to be better in many ways.

This situation could perhaps be resolved by experiment. There is some evidence in both directions, in that at least one super-massive black hole candidate appears to have a significant intrinsic magnetic field, which is not consistent with black hole theory, but objects which could according to black hole theory be near the borderline between neutron stars or black holes appear to show differences in X-ray emissions suggesting that there is some threshold being crossed (although Abhas Mitra has published a paper saying that this threshold doesn't necessarily imply a black hole, but could be some other form of phase change or similar).

 

[xantox]

For the record, it's believed that Hawking radiation is purely thermal -- it should contains absolutely no information about what goes on inside the black hole.

The original semiclassical Hawking radiation is indeed purely thermal, but there are strong indicators that the semiclassical theory is not applicable to this problem. Most attempts towards a purely quantum treatment (both in string theory and LQG), while still speculative, point generally to corrections to thermality and no information loss.

 

[JakeStan]

The gravity doesn't have to "reach outside of it" -- the gravity was already 'outside of it'. Before the star crosses the event horizon, the system lies in a gravitational well of 1,005 solar masses^*. After the star crosses the event horizon, the system still lies in a gravitational well of 1,005 solar masses.

Changes in the gravitational field are caused by waves -- where are you going to find a gravitational wave that cancels out those extra 5 solar masses?

I'm confused by this, if gravity cannot escape an event horizon the same way light does not, how would it exert a gravitational pull on an object outside the horizon? Or in relativity terms, how would gravity curve space outside the event horizon if it cannot escape the event horizon.

 

[gendou2]

How would [gravity] exert a gravitational pull on an object outside the [event] horizon?

The curvature of spacetime (read: gravity) extends smoothly outside the black hole event horizon.
However, a different animal known as gravitational waves (which are vibrations in the curvature of spacetime) do not travel outside the event horizon. Illustratively (not to be taken too literally) I might say that all is quiet on the event horizon.

 

[JakeStan]

I see the problem now. If "gravitational waves" are bound by the speed of light, and that speed provides the very definition and location of an event horizon, then the gravitational information (only carried by gravitational waves) would not escape it. It still begs the question of how the black hole curves spacetime outside itself. I would consider curved space-time to be information (i.e. there's a mass of this magnitude in this direction at this distance away) but the problem only arises in a change in information when it comes to black holes. Still if gravity is propagated by gravitational waves, moves at c and then cannot escape the event horizon then you shouldn't be able to be pulled into one. Unless there's a second part of gravity that allows for a pull outside the event horizon that isn't governed by c.

 

[Hurkyl]

It still begs the question of how the black hole curves spacetime outside itself.

You agree, I'm sure, that space-time around the black hole was curved before it formed. Well, space-time cannot spontaneously revert to flat -- such a change can only happen via a gravitational wave.

Still if gravity is propagated by gravitational waves

Gravity doesn't propagate. Changes in gravity propagate, and do so via gravitational waves.

 

[feynomite]

You agree, I'm sure, that space-time around the black hole was curved before it formed. Well, space-time cannot spontaneously revert to flat -- such a change can only happen via a gravitational wave.


Gravity doesn't propagate. Changes in gravity propagate, and do so via gravitational waves.

OK, so changes in gravity propagate via "gravitational waves" which travel at c. So then, an object very near a black hole is moving... how do the gravitational waves travel? With light, the frequency is lessened as they move away from the black hole. What happens with gravitation waves?

Another thing I don't understand, again, is if the black hole loses mass, how this "gravitation wave" that travels (eg: the slight "flattening" of spacetime) can escape the black hole. And conversely, when an object is entering the black hole, how the does spacetime become more "dented" when the object crosses the event horizon.

I'm very close to just giving up.

 

[feynomite]

The gravity doesn't have to "reach outside of it" -- the gravity was already 'outside of it'. Before the star crosses the event horizon, the system lies in a gravitational well of 1,005 solar masses^*. After the star crosses the event horizon, the system still lies in a gravitational well of 1,005 solar masses.

Changes in the gravitational field are caused by waves -- where are you going to find a gravitational wave that cancels out those extra 5 solar masses?

*: This is imprecise, but it should cover the idea.

So then let's say the black hole is 1000 solar masses, and there's a 100 solar mass star which collides with it head on. The 100 solar mass star will leave a "dent" right at point of the event horizon that it entered at? It's possible to deduce from what direction the star entered the black hole, because it's well will remain right there? I think this is false.

How then does the "dent" go inside the black hole? Assuming the waves propagate from the center of mass (or anything that mass) as soon as the star enters the black hole, it will no longer send gravitation waves to "update" that the dent is now inside the black hole.

Also, like in my other reply, what is the gravitational wave analog of "redshift," wherein light close to a black hole loses energy (frequency) as it travels outward.

 

[JustinLevy]

Some people still contest the existence of black holes on the grounds that Schwarzschild's original solution didn't have them and it was only when Hilbert made a different (mathematically plausible but physically questionable) assumption and changed the interpretation of the radial coordinate that the possibility of black holes arose.

I think the best explanation of this situation (and least over-stated) is an article by Salvatore Antoci and Dierck-Ekkehard Liebscher "Reinstating Schwarzschild's Original Manifold and its Singularity", also available at arXiv:gr-qc/0406090.

These ideas have been forcefully rebutted on-line in a thread "Flogging the Xprint" on sci.physics.research by "T. Essel", and also in a paper by Malcolm MacCallum at arXiv:gr-qc/0608033. However, although in both cases the "rebuttals" appear very authoritative, it appears that they are based on stating weak arguments very forcefully, which I don't find very convincing, and assuming that any fault with the opposing argument means that their own position is correct.

I believe that basically the situation is that at present there's no physical theory which allows us to determine the location of the mass point in the Schwarzschild vacuum solution. Schwarzschild assumed it to be at his original r=0, which is now R=2GM in the Schwarzschild radial coordinate defined by the proper areas of spheres. Hilbert assumed it to be at R=0 because it seemed to be mathematically valid to do so.

It was only a few years ago that Leonard S Abrams called attention to this difference of assumptions, which had apparently been ignored since Hilbert "corrected" Schwarzschild's original assumption. Some people have argued that since there are a lot of weird things in black hole theory, Schwarzschild's original assumption must have been correct. I'm inclined to sympathize with this point of view, but it's not a proof. On the other side, the people such as "T. Essel" who argue against them say that most of their arguments against Hilbert's assumption are invalid (which I'd probably agree with), and hence that Hilbert was right (which doesn't actually follow).

As far as I can see, Antoci and Liebscher's article referenced above gives a more balanced view, giving the background and explaining how the difference arose, and showing that Schwarzschild's original solution does appear to be better in many ways.

You keep pushing this point of view. If you are going to bring this up every time a black hole is mentioned, maybe we should address this directly. Can you please start a thread discussing specifically the "location of the singularity" in a black hole and your Schwarzschild vs. Hilbert claims?

Please note that even in the flat spacetime of Minkowski, the usual coordinates of an accelerated observer (Rindler coordinates) have a type of 'event horizon' as well. This is not a singularity. This is just a defect in the coordinate choice. Similarly with a black hole... there is no singularity at the event horizon, as spacetime is still locally flat there, and a freefalling observer will not notice anything special locally upon crossing the even horizon. You seem to be claiming spacetime is NOT locally flat at the Schwarzschild radius.

 

[Jonathan Scott]

You keep pushing this point of view. If you are going to bring this up every time a black hole is mentioned, maybe we should address this directly. Can you please start a thread discussing specifically the "location of the singularity" in a black hole and your Schwarzschild vs. Hilbert claims?

My main point is only to call attention to this interesting idea, as it could potentially eliminate all of the weirdness and paradoxes related to black holes. The previously referenced paper by Antoci & Liebscher provides all the information, including translations of the original papers by Schwarzschild and Hilbert. I would prefer people to read that paper (which I found very simple and clear) rather than asking me further questions, especially in threads which are not directly related to the existence of black holes. However, I'm happy to do my best to try to answer any reasonable questions arising from reading the paper, if I can.

Please note that even in the flat spacetime of Minkowski, the usual coordinates of an accelerated observer (Rindler coordinates) have a type of 'event horizon' as well. This is not a singularity. This is just a defect in the coordinate choice.

I have no problem with the "event horizon" in Rindler coordinates.

Similarly with a black hole... there is no singularity at the event horizon, as spacetime is still locally flat there, and a freefalling observer will not notice anything special locally upon crossing the even horizon. You seem to be claiming spacetime is NOT locally flat at the Schwarzschild radius.

With Schwarzschild's original assumption, the solution is unaffected outside the Schwarzschild radius, so space is locally flat as you approach it. However, in Schwarzschild's original solution, the center of the mass point is physically located at that radial coordinate (at the point r=0 in his original coordinate system, equivalent to the surface R=2GM in the "Schwarzschild coordinates" based on proper area), so you can't actually reach it without hitting the surface of the mass first. As the mass goes to infinite density within the coordinate system, the proper area of its surface goes to the usual expression for the finite area of the event horizon.

 

[Hurkyl]

My main point is only to call attention to this interesting idea, as it could potentially eliminate all of the weirdness and paradoxes related to black holes...

Did you miss the part about "start a new thread"? Hijacking the threads of others is not an appropriate way to have your discussion.

 

[Hurkyl]

OK, so changes in gravity propagate via "gravitational waves" which travel at c. So then, an object very near a black hole is moving... how do the gravitational waves travel? With light, the frequency is lessened as they move away from the black hole. What happens with gravitation waves?

Another thing I don't understand, again, is if the black hole loses mass, how this "gravitation wave" that travels (eg: the slight "flattening" of spacetime) can escape the black hole. And conversely, when an object is entering the black hole, how the does spacetime become more "dented" when the object crosses the event horizon.

This level of detail needed to answer these questions directly is beyond my ability. And I believe that the collision of a black hole with an object of significant mass (e.g. the collision of two black holes) is an extremely complex situation.

However, I can point out two things:

(1) There is an existing wave (albeit a very boring one) associated with the motion of the star -- it's exactly what makes the star's gravitational well travel along with it. So while the star has passed through the event horizon, parts of that wave are still about. It's certainly not obvious to me how to make any direct claims about how this wave will interact with the hole's gravitational well, -- but because we know the end result (a black hole with slightly larger mass), I would assume it would act to smooth out the gravitational field. And this leads to the next thing...

(2) I'm quite sure black holes are known to be able to deform and vibrate and stuff. So it makes perfect sense that there is initially a dent in the gravitational well which causes it to vibrate, and eventually settle into a smoother configuration.

Incidentally, not only can the black hole deform and vibrate -- you can actually extract rotational energy from it. Nothing is crossing the event horizon in any of these scenarios -- the gravitational field outside the black hole, but still near the horizon, is where the (relevant) angular momentum is 'stored', and it's also home to ripples involved in the deformation and vibrations of the hole.

(I hope I have that all right!)

 

[JakeStan]

But if a black hole can deform, how would you be able to measure that deformity without a change in gravitation (waves)? Unless the point is that it is the event horizon that is deforming which is causing the gravitational waves beyond the horizon which would then beg the question what is causing the black hole to deform if not events happening within the horizon.

 

[Jonathan Scott]

Did you miss the part about "start a new thread"? Hijacking the threads of others is not an appropriate way to have your discussion.

Sorry, I did not intend my response to start a discussion, but only to mention an alternative point of view, and then when JustinLevy responded to it I felt I should answer that too, but discourage further questions in this thread.

I've started a new thread on the subject of the radial coordinate assumption at https://www.physicsforums.com/showthread.php?t=272909.

 

[xantox]

So then let's say the black hole is 1000 solar masses, and there's a 100 solar mass star which collides with it head on. The 100 solar mass star will leave a "dent" right at point of the event horizon that it entered at? It's possible to deduce from what direction the star entered the black hole, because it's well will remain right there? I think this is false. How then does the "dent" go inside the black hole?

In most cases, a star colliding a black hole will be disrupted and form an accretion disk. In a direct merger, the infalling body creates a perturbation of the black hole gravitational field, setting up vibrational modes and emission and absorption of gravitational and electromagnetic waves until all deviations from spherical topology have been radiated (see Price's theorem). The final black hole is again surrounded by static fields whose imprints define only its mass, angular momentum and charge.

 

[feynomite]

Thank you all for answering my question. I will test out these things in a few years after my matter-antimatter space rocket is complete and I embark to the nearest gigantic black hole.