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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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marked as duplicate by John Rennie black-holes Jan 20 '16 at 11:33

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    $\begingroup$ 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. $\endgroup$ – ACuriousMind Jan 19 '16 at 18:11
  • $\begingroup$ 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? $\endgroup$ – user Jan 19 '16 at 18:13
  • $\begingroup$ @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. $\endgroup$ – Jimmy360 Jan 19 '16 at 18:50
  • $\begingroup$ 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. $\endgroup$ – CuriousOne Jan 19 '16 at 18:50
  • $\begingroup$ @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. $\endgroup$ – Jimmy360 Jan 19 '16 at 18:55
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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.

Your question:

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?

In the popular quantum model of virtual pair loops within the Heisenberg uncertainty, close to the horizon, one has to keep in mind that virtual particles have to interact with something to become real particles which could be absorbed by the black hole. To become real there should be an interaction, and that interaction can only take energy from the black hole. From momentum conservation, if one real created particle has enough energy to leave against the gravitational field, the other will be absorbed because of its direction. The emitted particle in this frame takes away energy from the black hole and adds it to the rest of the universe.

In this link there is the paper itself where the connection with black body radiation is given. And here is another link with theoretical calculations.

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  • $\begingroup$ You say "To become real there should be an interaction, and that interaction can only take energy from the black hole". Why that interaction can only take energy from the black hole? Couldn't the interaction happen inside the black hole? $\endgroup$ – user Jan 21 '16 at 20:16
  • $\begingroup$ @user When it happens inside the black hole, and it is possible this model, both products will be swallowed if not near the horizon. By definition of the horizon every particle inside the horizon cannot get out. $\endgroup$ – anna v Jan 22 '16 at 4:30
  • $\begingroup$ Yes, but I am still not clear on why when there is an interaction the energy is taken from the black hole, and why the energy cannot be taken from the universe. $\endgroup$ – user Jan 22 '16 at 9:51
  • $\begingroup$ The universe away from any matter has zero energy to give , certainly not enough to give enough momentum for one of the pair to be able to leave. The temperature of the cosmic microwave background says this which is close to zero. It is similar to evaporation, the high energy molecules in the liquid have enough momentum to leave the surface, but the energy in the liquid diminishes. $\endgroup$ – anna v Jan 22 '16 at 10:31

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