# Hawking radiation and black hole evaporation [duplicate]

As I understand it, Hawking radiation predicts black hole evaporation because of quantum effects. After the creation of a particle/anti-particle near the horizon of the black hole, if the one gets trapped inside the black hole and the other escapes, the black hole will have lost part of its mass.

Why can't we interpret this as the universe instead losing part of its mass inside the black hole? Why is the particle emitted viewed as the one with positive energy? What if, instead, the one with positive energy is the one trapped causing the black hole to grow and the universe to shrink?

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• What do you mean "the one with positive energy"? Hawking radiation can be derived and predicted (from the Unruh effect + equivalence principle) without the need for the popular image of a "pair" forming at the horizon and one falling in. – ACuriousMind Jan 19 '16 at 18:11
• Is the popular image of a pair forming at the horizon and one falling in an incorrect explanation? Why is it the one always being used then? – user Jan 19 '16 at 18:13
• @user79303 It is not that it is outlandishly incorrect. It is just not as good as other explanations. Since it is simple, the image of a pair of particles provides a good explanation to people of little background in physics. – Jimmy360 Jan 19 '16 at 18:50
• Yes, the popular explanation is completely wrong and I don't know why it's being used or who popularized it. The kind of radiation escaping a black hole follows trivially from thermodynamics once we know the black body temperature. @Jimmy360: It is never a good idea to popularize physics with bad explanations. – CuriousOne Jan 19 '16 at 18:50
• @ACuriousMind Although I hate seeing incorrect explanations, I believe that they can induce a love of science in those who would usually be indifferent. So the analogy is not as horrible as flat misconceptions that do not contribution to a growing love of science. One example of those is the rumor that our modern understanding of physics provides no explanation for how a honeybee can fly. – Jimmy360 Jan 19 '16 at 18:55

Here is the abstract of the original paper by Hawking:

In the classical theory black holes can only absorb and not emit particles. However it is shown that quantum mechanical effects cause black holes to create and emit particles as if they were hot bodies with temperature hκ2/πk≈10^−6(M⊙/M)∘K where κ is the surface gravity of the black hole. This thermal emission leads to a slow decrease in the mass of the black hole and to its eventual disappearance: any primordial black hole of mass less than about 10^15 g would have evaporated by now. Although these quantum effects violate the classical law that the area of the event horizon of a black hole cannot decrease, there remains a Generalized Second Law:S+1/4A never decreases where S is the entropy of matter outside black holes andA is the sum of the surface areas of the event horizons. This shows that gravitational collapse converts the baryons and leptons in the collapsing body into entropy. It is tempting to speculate that this might be the reason why the Universe contains so much entropy per baryon.

Bold mine. It stresses that in classical General Relativity, nothing can escape the black hole horizon.