How do gravitons escape a black hole?

[Metric]

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A couple of good responses so far. Let's expand the list of questions.

1. How do gravitons escape a black hole?

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A "graviton producing machine" inside the BH will not be able to send real gravitons out past the event horizon. (provided we remain in the limit where the BH solution can be thought of as a reasonable background)

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2. How does gravity escape a black hole?

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Gravity doesn't "escape" a black hole -- a black hole *is* a solution to the gravitational field equations.

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3. Is there anything special about the event horizon?

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Not locally -- you wouldn't notice anything special if you were in a box falling across the horizon. Outside the BH, though, it does have significance -- you'll never hear again from anything that crosses the horizon.

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4. If there is a graviton version of Hawking radiation, call it graviton-Hawking-radiation, then what is its strength?

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It should exist, and it should have a thermal spectrum just like any other field (provided again that the BH solution can be thought of as a good background -- i.e. the perturbative regime -- this should be an extremely good approximation for any reasonable BH, though).

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5. Is the strength of the graviton-Hawking-radiation basically the same strength as the gravity (gravitational acceleration) itself? (Set hbar, c, G to 1.)

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The temperature defining the thermal spectrum of gravitons should be proportional to the surface gravity of the BH (which will be inversely proportional to the mass of the BH).

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6. If so, or if not, what role does graviton-Hawking-radiation play in gravity?

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Due to the weakness of Hawking radiation, the gravitons can be thought of as tiny perturbations to the BH solution -- they shouldn't be significant in changing the causal structure of the BH solution.

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7. Since Hawking radiation comes from gravitional acceleration, does it not occur also inside the event horizon all the way down to the `singularity'?

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I seem to recall that a freely falling detector dropped into a BH isn't excited by Hawking radiation, but I could be mistaken.

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8. If not, then how does all the mass get from the `singularity' to the outside during the complete Hawking evaporation of a black hole via Hawking radiation?

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Hawking radiation is a property of the vacuum outside the BH -- there are some problems with thinking of Hawking radiation as something "escaping" from the BH. That said, there are some very big problems associated with "complete evaporation" of a BH that aren't properly understood by anyone.

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9. If so, how much of the mass of the black hole consists of Hawking radiation between the `singularity' and the event horizon?

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This is a sort of ill-defined question. The BH's mass is defined with respect to a "timelike Killing vector" that doesn't exist inside the BH. Energy and momentum inside the BH are only locally defined concepts.

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10. If you are accelerating `forwards' in space, does the Unruh radiation (similar concept to Hawking radiation, relating gravity to acceleration) hit you from the front or the back, or from all around, or what?

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It's isotropic.

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11. What are all the similarities, and differences, between gravitons and photons?

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This could lead to a huge, detailed discussion in and of itself. If you want to discuss it more, I wouldn't mind doing so, but I'll sidestep it for this particular post.

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12. (For genuine experts only) how much of the above is nonsense based on ignorance?

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They seem like reasonable questions, but as always a more detailed understanding reveals some "problems" to not be problems at all, and some questions not to be defined in the original sense that you imagined.

[Skidoo]

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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.

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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.

[Metric]

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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.

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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.

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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.

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

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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."

[Skidoo]

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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.

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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.

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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.

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

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

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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."

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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.

[Skidoo]

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

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Wrong.

[thylacine]

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.

[Upstairs]

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.

[BigBuffet]

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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.

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Wouldn't the bigger ball bearings roll toward and then hit the bowling ball first thus causing the smaller ball bearings to ricochet off them back in the direction they came from?

[Upstairs]

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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?

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

[Metric]

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

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No, it's not.