Is Hawking Radiation unique? (And one more question)

[Tio Barnabe]

Are there kinds of black hole radiation other than that proposed by Hawking? Note that I'm talking about truly black hole radiation, not radiation from matter that orbits the black hole, etc.

How can we conciliate such phenomenon with General Relativity? I mean, this seems to completely contradict what General Relativity predicts, i.e. light coming out of a black hole.

So what led physicists to even talk about black hole radiation? That a black body should absorb as well as emit radiation? How can they claim that by completely violating General Relativity?

 

[PeterDonis]

Are there kinds of black hole radiation other than that proposed by Hawking?

I'm not sure what you mean. Hawking radiation, in principle, includes every possible kind of particle species.

How can we conciliate such phenomenon with General Relativity?

By understanding that GR is a classical theory, while the prediction of Hawking radiation from black holes requires a quantum theory. Hawking's original prediction used quantum field theory in curved spacetime, specifically QFT done using Schwarzschild spacetime (the spacetime that describes a static black hole) as the background spacetime. More recent efforts have tried various other theoretical frameworks. We probably won't fully understand how all this works until we have a good theory of quantum gravity.

light coming out of a black hole

The problem with Hawking radiation from a classical GR standpoint is not the radiation itself; that can be viewed as being emitted from just above the hole's horizon, which is perfectly possible classically. The problem is that the radiation carries away energy from the hole, i.e., it decreases the hole's mass. This violates classical theorems which say, in effect, that the mass of a static black hole cannot decrease. However, quantum field theory effects can violate crucial assumptions, called "energy conditions", on which those classical theorems are based. So when QFT is taken into account, Hawking radiation is perfectly consistent with GR at the classical level--you just have to take proper account of the QFT effects involved in the classical limit.

So what led physicists to even talk about black hole radiation?

Because they were trying to understand how thermodynamics works in the presence of a black hole. A good summary of the relevant history is here:

https://arxiv.org/pdf/hep-th/0409024.pdf

 

[Tio Barnabe]

Thanks for the response.
After the opening post, I realized that it doesn't make sense to talk about a thing ruled out by a theory while studying it from another theory!

That is, we have theories. They are chain of arguments, rules, etc which we use to predict phenomena. A particular theory is like a particular representation of a given phenomenon. In our case, Einstein's theory predicts black holes. It's a matter of experiment to check whether they actually exist or not. We do know they exist from independent experiments. So, now, we are looking at them and using Quantum theory, and that theory predics radiation. It's again a matter of experiment to see whether they actually radiate. So we should forget about what is forbidden in Einstein's theory, because it's another theory! Its like asking why a blue sock is not red! By definition a blue sock is blue and that's it.

would any of you like to make a observation about my above reasoning?

 

[PeterDonis]

it doesn't make sense to talk about a thing ruled out by a theory while studying it from another theory!

I agree.