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Comment by sillysaurus3 | Hacker News Reader

A question for the physicists on HN: I've heard that black holes emit radiation. But if nothing can go faster than the speed of light, and a black hole's gravity is so strong that not even light can escape, then how can a black hole emit anything?

Is there a kind of black hole which is so massive that not even that radiation can escape, or do all black holes emit some kind of radiation? (In fact, do they emit radiation proportional to their size?)

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Armchair Physicist here, not a real one.

There are two reasons why you'd say black holes emit radiation. An interesting one and a very interesting one (warning, other peoples scales may be calibrated differently to mine).

1) Black holes accelerate things massively, so they're travelling at an astonishing speed before they "enter". This can result in huge amounts of x-rays being emitted due to heating things to millions of degrees (well beyond white hot!). It's not the black hole itself, but the black hole is certainly to blame. http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkbin.html

2) The weirder one. Hawking radiation: http://en.wikipedia.org/wiki/Hawking_radiation

Hawking radiation happens when a pair of virtual particles "pop" into existence near the event horizon. Normally these pairs annihilate quickly, but if it happens near the event horizon it's possible for one of the particles to fall in and the other to escape. This results in a loss of mass of the black hole (told you it was weird) so could be considered to be the black hole emitting radiation.

Also some particles might travel faster than speed of light by tunneling or similar quantum mechanisms.

> one of the particles to fall in and the other to escape. This results in a loss of mass of the black hole (told you it was weird)

How does the energy to create the virtual particles come from the black hole (which it has to in order for the accounting to work: -2+1=-1)? Is it a "Quantum Field Theory doesn't care about the event horizon" type thing?

It doesn't take any energy to create virtual particles; virtual particle pairs are constantly being created and destroyed everywhere, according to Quantum Field Theory, but when they're created, on average, they have zero net energy: one has positive energy and one has negative energy. (Note that this is a heuristic description and not every quantum field theorist would agree with it. The only really unambiguous way to describe the process is using math; but translating math into everyday language is often difficult because our intuitions don't really match up with what the math is telling us. I'm doing the best I can.)

However, if a virtual particle pair happens to be created just outside a black hole's horizon, the hole's tidal gravity can pull the negative energy particle inside the horizon before it can be annihilated by the positive energy particle. The positive energy particle can then escape. Effectively, this means the positive energy particle's energy is taken from the hole's mass, so the hole's mass decreases slightly.

IanCal and blaze33 both gave good answers, but just to clarify one thing: the radiation we see coming from regions where there are black holes is of the first type: radiation emitted by objects like gas clouds that are falling into the holes, before those objects cross the event horizon. If we leave out quantum effects like Hawking radiation (see below), it's impossible for light, or any kind of radiation, or indeed anything at all, to escape from inside the event horizon of a black hole.

Hawking radiation is a quantum effect, which nobody has ever observed; the reasons for thinking that it exists are purely theoretical.

Isn't the nonexistence of large black holes from cosmic ray collisions proof of black hole evaporation? Or have we not been able to measure that / have reason to doubt micro-black-holes are created by cosmic ray collisions in the first place?

I would say we don't have accurate enough measurements or an accurate enough theoretical understanding to know how many micro-black-holes we should expect to see from cosmic ray collisions, on the assumption that none of them ever evaporate, or to be able to measure how many there actually are, so as to be able to compare the two numbers to see if there's a significant difference. In principle this would certainly be a good experimental test for the existence of black hole evaporation; I just don't think it's a test we can make with any confidence now or in the near future.

You may be referencing two different kind of radiations:

- the one emitted by the heated matter falling in / spinning around the black hole (the quasar referenced in the article [1])

- the Hawking radiation. Basically a pair of particle/antiparticle randomly appears near the black hole event horizon, one of the particle falls into the black hole while the other one escapes. The black hole would eventually evaporate if you waited long enough. [2]

[1] http://en.wikipedia.org/wiki/Quasar [2] http://en.wikipedia.org/wiki/Hawking_radiation

Also armchair physicist here - my understanding is that since information can never be destroyed, Blackholes must emit something.

http://en.wikipedia.org/wiki/Black_hole_information_paradox

I think there's pretty general agreement among physicists that we will eventually confirm that black holes emit Hawking radiation. However, that by itself doesn't help us to choose between all the various proposed resolutions of the information paradox.

The simplest answer: it's the region immediately above the black hole that these reports refer to, not the black hole itself (its event horizon and below there).

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