Re: How do gravitons escape a black hole?
 

[ QUOTE ]
Gravitons exist?

[/ QUOTE ]
They haven't been directly detected yet (due to limitations on experimental technology), but if they don't exist something is seriously wrong with our understanding of physics.

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
[ QUOTE ]
3. Is there anything special about the event horizon?

[/ QUOTE ]Not locally -- you wouldn't notice anything special if you were in a box falling across the horizon.

[/ QUOTE ]

Perhaps in the ideal case, but there would be a gigantic tidal force due to the gravitational gradients in a box with a non-negligible height. Also, at some point in such a compartment, the event horizon would be between the floor and ceiling, which could produce some interesting effects.

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
This property, often called tunneling, is responsible for a phenomenon known as Hawking radiation, which is basically a radiation of photons every black hole gives off. I'd imagine a similar thing could happen for gravitons.

[/ QUOTE ]

I could be wrong about this, but I thought Hawking radiation occurred as follows (please forgive my simplification; I understand this stuff mostly from books like "A Brief History of Time" and not from real astrophysics studies):

It is known that at the smallest space and time scales, matter-antimatter pairs can pop into existence and immediately annihilate afterwards. Hawking radiation occurs when this interaction happens at the event horizon. A pair pops into existence, and one of the pair is sucked into the black hole. The other one is given a boost of energy that is enough to kick it away from the black hole, and voila, you get Hawking radiation.

Is it possible that this same thing is occurring with gravitons?

~MagicMan

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
3. Is there anything special about the event horizon?

[/ QUOTE ]Not locally -- you wouldn't notice anything special if you were in a box falling across the horizon.

[/ QUOTE ]

Perhaps in the ideal case, but there would be a gigantic tidal force due to the gravitational gradients in a box with a non-negligible height.

[/ QUOTE ]
The tidal forces are there regardless of the horizon (both inside and outside the horizon of a BH, and they are there even if the gravitating object isn't a BH), and with a large enough BH you wouldn't even notice them at the horizon.

[ QUOTE ]
Also, at some point in such a compartment, the event horizon would be between the floor and ceiling, which could produce some interesting effects.

[/ QUOTE ]
No, not to a freely falling observer. If, however, you activated "sooper dooper rockets" located on the top and bottom of your box precisely at this moment, the top could in principle escape to infinity while the bottom could never break free. The horizon is not defined via local effects -- it is defined via the global properties of a spacetime as "the boundary of the past of future null infinity."

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
3. Is there anything special about the event horizon?

[/ QUOTE ]Not locally -- you wouldn't notice anything special if you were in a box falling across the horizon.

[/ QUOTE ]

Perhaps in the ideal case, but there would be a gigantic tidal force due to the gravitational gradients in a box with a non-negligible height.

[/ QUOTE ]
The tidal forces are there regardless of the horizon (both inside and outside the horizon of a BH, and they are there even if the gravitating object isn't a BH), and with a large enough BH you wouldn't even notice them at the horizon.

[/ QUOTE ]

Okay. The tidal forces at a given distance can be reduced by increasing the radius of the BH or whatever.

[ QUOTE ]
[ QUOTE ]
Also, at some point in such a compartment, the event horizon would be between the floor and ceiling, which could produce some interesting effects.

[/ QUOTE ]
No, not to a freely falling observer. If, however, you activated "sooper dooper rockets" located on the top and bottom of your box precisely at this moment, the top could in principle escape to infinity while the bottom could never break free. The horizon is not defined via local effects -- it is defined via the global properties of a spacetime as "the boundary of the past of future null infinity."

[/ QUOTE ]

I see where you're coming from that in principle a falling observer can't notice anything different in his frame of reference, but I think this is an ideal situation that wouldn't exist in reality. As any compartment with a finite height crosses the event horizon, a light beam directed laterally near the floor would take a closed path while a similar beam near the ceiling could still escape to an indefinite distance.

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
Okay. The tidal forces at a given distance can be reduced by increasing the radius of the BH or whatever.

[/ QUOTE ]

Wrong.

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
[ QUOTE ]
Gravitons exist?

[/ QUOTE ]

Only inside fuzzballs

[/ QUOTE ]
I'm so glad I don't sttudy physics.

Re: How do gravitons escape a black hole?
 

Thanks, Metric for detailed responses. Thanks to others too.

I have tons of questions but let me focus on one key question.

Let's assume the mass (and maybe charge and angular momentum) of a black hole is concentrated at the `singularity'?

Again, here `singularity' means whatever there is in quantum gravity in place of the classical singularity of GR.

Suppose it is a big black hole, so the event horizon is a long way from the `singularity'.

Then in the Hawking evaporation of the black hole (assume nothing is falling in, and don't worry about final stages for now) it seems to me that the mass (and maybe charge and angular momentum) must come not just from around the event horizon, but instead must makes its way out all the way from the `singularity', meaning that there is a flow of mass (and maybe charge and angular momentum) from the `singularity' all the way out to the event horizon and beyond.

QUESTION: Is something like this what is actually happening?

Moreover the strength of this flow should be calculable from standard formulas relating strength of Hawking/Unruh radiation to acceleration (gravitational or otherwise).
So the Hawking/Unruh radiation is stronger nearer the `singularity'. And it has mass. Maybe all of the mass is in this form leaving no singularity!

By the way, I assume that the principle of equivalence (relating acceleration and gravitational acceleration) means that Hawking and Unruh radiation are basically the same thing, and that freely falling bodies (anywhere, not just in a black hole) don't see it. But I don't think the observations of a freefaller in a black hole rule out what I said above.

Re: How do gravitons escape a black hole?
 

One way to think of the way gravity curves spacetime is to picture a bedsheet on your bed with different sized ball bearings on it representing stars. Now put a bowling ball representing a massive black hole in the middle. Well... the smaller bearings are going to roll towards it.

You see, there is no comminication between the bowling ball and the ball bearings at all, but they *are* affected by each other. The key concept here is that *gravity warps spacetime* just like that bowling ball warps your bedsheet.

Re: How do gravitons escape a black hole?
 

[ QUOTE ]
Then in the Hawking evaporation of the black hole (assume nothing is falling in, and don't worry about final stages for now) it seems to me that the mass (and maybe charge and angular momentum) must come not just from around the event horizon, but instead must makes its way out all the way from the `singularity', meaning that there is a flow of mass (and maybe charge and angular momentum) from the `singularity' all the way out to the event horizon and beyond.

QUESTION: Is something like this what is actually happening?

[/ QUOTE ]

The event horizon *is* the singularity / where the singularity occurs.