Black holes and causality

[Pacifist]

I was thinking, how can black holes possibly exist?
Information has to travel faster than the speed of light to escape a black hole right. And it's impossible for information to travel faster than the speed of light (causality).
So how is a black hole about to transmit information about its mass in the form of gravity to the outside world?

[von.Gunst]

Originally Posted by Pacifist

I was thinking, how can black holes possibly exist?
Information has to travel faster than the speed of light to escape a black hole right. And it's impossible for information to travel faster than the speed of light (causality).
So how is a black hole about to transmit information about its mass in the form of gravity to the outside world?

I didn't know that information could escape black holes, however heat apparently can - when in doubt, blame quantum theory. I tried reading a bit more to help you out, but my brain started melting - stupid physics-less science degree.

[hlokk]

Radiation can be emitted by Hawking radiation from a black hole, so they eventually disappear.

As for the information of the mass, the black hole, like any massive object (used in the actual sense of the word) distorts spacetime, which then gives the illusion of gravity. There is not some invisible hand that reaches out and pulls something down, the objects are just in warped spacetime.

Visualise it like a bowling ball on a trampoline, and you have a remote control car. The bowling ball is like the sun and distorts the fabric of the trampoline. The car going in an apparant straight line will deviate from the expected line, as light is deviated as it passes by the sun (ever so slightly). Now, a black hole is like a incredibly dense, small object, so for the trampoline, just imagine someone has taken a point and attached it to the floor using a bolt or something. Say we reverse the car down into the hole, at some point the car wont have enough power/speed to stop itself slipping. The transition point is the event horizon. Now, just because the car cant escape from below that line, doesnt mean that the car is not affected by the warped spacetime.

Changing the mass might be a little different, gravity waves and all that.

[Pacifist]

Originally Posted by hlokk

As for the information of the mass, the black hole, like any massive object (used in the actual sense of the word) distorts spacetime, which then gives the illusion of gravity. There is not some invisible hand that reaches out and pulls something down, the objects are just in warped spacetime.

Yeah but regardless of how the gravitational information is transmitted it is still going faster than the speed of light to escape the black hole (i'm guessing it travels instantly considering the information is the warping of space-time itself).
If i was to move a mass around and someone could detect the gravitational effects of that mass moving around then that would be a faster than light form of communication.
But my whole life i've been taught faster than light communication = time travel (link) so i'm really having trouble getting my head around this.

[Zylatis]

Information has to travel faster than the speed of light to escape a black hole right. And it's impossible for information to travel faster than the speed of light (causality).
So how is a black hole about to transmit information about its mass in the form of gravity to the outside world?

Quantum Theory to the rescue! Basically what happens is the gravitational field loses energy via virtual pair produciton (and gravity waves but only if its a kerr black hole ie rotating or changing somehow).

In QM there is the well known Heisenberg uncertainty principle which manifests itself most prominently in the position momentum uncertainty equation which says that

(change in x)(change in p) >= hhbar/2 However there is also the energy-time uncertainty relation which is of a similar form, although the RHS is different.

Basically this allows for virtual particles. Now dont be fooled, they do actually exist and can be detected in scattering experiments so virtual is a pretty poorly picked name if you ask me. ALl it means is that as per the ETU relation (rar acronyms ftw) they can only exist for a certain amount of time based on their energy. I cant remember the specifics as i did particle physics a while ago but basically if you get them having negative energy they are virutal, i think. Anyway people often say that virtual particles just pop into existence from nowhere and then annihilate each other. This isnt strictly true as 'from nothing' would violate thermodynamics. In a vacuum you have vacuum energy to provide for it. Around a black hole you have its gravitational field and all the energy contained therein (its mass basically).

So, what happens is the gravitational field loses a bit of energy and in doing so creates virtual particles (always produced in matter/antimatter pairs afaik). There is a chance that one of the pair is created on one side of the event horizon and the other on the other side. So one falls into the black hole and the other can escape which is what we see as Hawking radiation. This process seems to undo the 'virtualness' of the particles, as to why im not entirely sure, i think it has to do with measurement postulates or something.

Anyway, we detect this hawking radiation and say ooh ah but more importantly the gravitational field has lost some energy. This means the BH has lost some mass, thus the BH will slowly 'evaporate', which i think is pretty friggin cool to be honest =)

[hlokk]

Originally Posted by Pacifist

Yeah but regardless of how the gravitational information is transmitted it is still going faster than the speed of light to escape the black hole (i'm guessing it travels instantly considering the information is the warping of space-time itself).
If i was to move a mass around and someone could detect the gravitational effects of that mass moving around then that would be a faster than light form of communication.
But my whole life i've been taught faster than light communication = time travel (link) so i'm really having trouble getting my head around this.

Ok, back the trampoline example. If you hit a trampoline, the resulting waves will travel at some speed. (Lets assume the trampoline is practically infinitely big, so the distortion doesnt apply more tension to the edges). Now, you cannot transmit information faster than these waves (analogous to the speed of light). If we drop a bowling ball on there, waves will propagate out.
If we leave the ball on the trampoline, the trampoline stays distorted around the ball. So, the 'gravity does not need to travel to the object everytime to influence it'. Say, if you had some pulsating mass, then there would be a time delay, as gravitational waves travel at the speed of light, but you dont get gravitational waves with a non-changing mass (rotating or mass change).

Due to the speed of the waves, moving a mass around would not result in FTL information transfer (providing you could detect it).

The gravity properties of a black hole is such that it behaves as a normal mass, with diameter outside the event horizon with whatever mass it has (in other words, the gravitational field is identical, outside of the event horizon to an non-black hole object of the same mass)

[Pacifist]

Originally Posted by Zylatis

Quantum Theory to the rescue! Basically what happens is the gravitational field loses energy via virtual pair produciton (and gravity waves but only if its a kerr black hole ie rotating or changing somehow).

In QM there is the well known Heisenberg uncertainty principle which manifests itself most prominently in the position momentum uncertainty equation which says that

(change in x)(change in p) >= hhbar/2 However there is also the energy-time uncertainty relation which is of a similar form, although the RHS is different.

Basically this allows for virtual particles. Now dont be fooled, they do actually exist and can be detected in scattering experiments so virtual is a pretty poorly picked name if you ask me. ALl it means is that as per the ETU relation (rar acronyms ftw) they can only exist for a certain amount of time based on their energy. I cant remember the specifics as i did particle physics a while ago but basically if you get them having negative energy they are virutal, i think. Anyway people often say that virtual particles just pop into existence from nowhere and then annihilate each other. This isnt strictly true as 'from nothing' would violate thermodynamics. In a vacuum you have vacuum energy to provide for it. Around a black hole you have its gravitational field and all the energy contained therein (its mass basically).

So, what happens is the gravitational field loses a bit of energy and in doing so creates virtual particles (always produced in matter/antimatter pairs afaik). There is a chance that one of the pair is created on one side of the event horizon and the other on the other side. So one falls into the black hole and the other can escape which is what we see as Hawking radiation. This process seems to undo the 'virtualness' of the particles, as to why im not entirely sure, i think it has to do with measurement postulates or something.

Anyway, we detect this hawking radiation and say ooh ah but more importantly the gravitational field has lost some energy. This means the BH has lost some mass, thus the BH will slowly 'evaporate', which i think is pretty friggin cool to be honest =)

But then could the virtual particles be used for faster than light information transfer? It seems they could if they can be created on each side of the infinite space-time distance that is the event horizon.

Edit: Also it seems scientists say virtual particles do not count as information transfer? The mass of the black hole is information. The virtual particles tell me about the mass from the other side of the event horizon. That's information isn't it?

[Pacifist]

Originally Posted by hlokk

Ok, back the trampoline example. If you hit a trampoline, the resulting waves will travel at some speed. (Lets assume the trampoline is practically infinitely big, so the distortion doesnt apply more tension to the edges). Now, you cannot transmit information faster than these waves (analogous to the speed of light). If we drop a bowling ball on there, waves will propagate out.
If we leave the ball on the trampoline, the trampoline stays distorted around the ball. So, the 'gravity does not need to travel to the object everytime to influence it'. Say, if you had some pulsating mass, then there would be a time delay, as gravitational waves travel at the speed of light, but you dont get gravitational waves with a non-changing mass (rotating or mass change).

Due to the speed of the waves, moving a mass around would not result in FTL information transfer (providing you could detect it).

The gravity properties of a black hole is such that it behaves as a normal mass, with diameter outside the event horizon with whatever mass it has (in other words, the gravitational field is identical, outside of the event horizon to an non-black hole object of the same mass)

Put it this way. Why doesn't light escape using the same method?
ie. light is always propagating but as far as it's concerned there is an infinite distance from one side of the event horizon to the other. The same should hold for gravity waves unless they don't move at the same speed as light.

[Bastard Child]

I'm not sure if you're asking the same thing (and I don't think you are), but there was a show about this a while ago:

http://en.wikipedia.org/wiki/Black_h...mation_paradox

[Zylatis]

Thats a bit different, that was about information being destroyed. Im not sure that virtual particles cant be used as information transfer but like i said i think in this process they become 'real'

[Kow]

Information /particles can travel faster than the speed of light, at least in the quantum world. check out these biatches right here :P

http://en.wikipedia.org/wiki/Tachyon * may or maynot actually exsist yet :P

[Whisper]

The guys who really know their stuff are still arguing about this.

So don't expect an answer here, one doesn't really exist, I'm not even sure they have something that amounts to a theory yet. Whatever is below a theory is what they are arguing about.

[Zylatis]

Well afaik Hawking Radiation and the way its produced is fairly concrete, as far as these theories go.

[DaveHimself]

a: no 100% proof blackholes do or don't exist.
b: no 100% proof we cannot go faster than light....yet.
c: 100% proof that this shit is so damn cool to think about.

[Sniper]

There such a thing as dark matter as well.