When the escape velocity from EH is c, then how can Hawking radiation escapes it? Does it travel faster than c? Or, it escape from outside the EH? If so, how a BH can evaporate beyond EH?

  • $\begingroup$ Both parts get created outside the horizon, but one escapes and one falls towards the horizon. $\endgroup$ – Yukterez Mar 4 '16 at 6:36
  • $\begingroup$ Hi kpv. see the question I've linked - the Hawking radiation doesn't come from inside the event horizon so it doesn't have to escape. For more on what Hawking radiation is see Black holes and positive/negative-energy particles. $\endgroup$ – John Rennie Mar 4 '16 at 6:37
  • $\begingroup$ @JohnRennie: What should I do with this question given that it is answered, and likely also a duplicate? Should I delete it? $\endgroup$ – kpv Mar 4 '16 at 8:32
  • $\begingroup$ I wouldn't worry about it. While we try to eliminate duplicates I don't think it does any harm to have some duplication. Arguably it makes searching easier - the Physics SE site search isn't the best search facility in the world. Anyone finding this question will see the link to the "master". $\endgroup$ – John Rennie Mar 4 '16 at 8:56
  • $\begingroup$ @John Rennie : please don't close a question as a duplicate when the answers on the "master" question are incorrect. $\endgroup$ – John Duffield Mar 4 '16 at 13:59

In quantum field theory, particle antiparticle pairs spontaneously pop out of the vacuum all the time, essentially borrowing energy from the vacuum and then subsequently annihilate each other.

However, when an event horizon is present, you can get situations where the pair is generated very close to the horizon. If the pair is generated such that they both are highly energetic, but one particle happens to be just inside the horizon whereas the other is just outside the one that is outside will fly away since even if you are just outside the horizon it is still possible to escape. No particle here goes faster than light or crosses the event horizon in the wrong direction.

Please note that this is a very commonly asked question, such as here From where (in space-time) does Hawking radiation originate? (you should check out some of the answers there, some are of much higher quality and greater depth than my own).

Please try to do research before you ask!

Edit answering why this causes the BH to lose mass

Contrary to your intuition, the black hole has lost a particle's worth of energy since the energy to create the pair came from the spacetime on or around the event horizon, so really energy was borrowed from the event horizon to create the pair but it only got one particle back so the black hole was 'short changed'.

  • $\begingroup$ If the particles are popping out by borrowing energy from vacuum, then how the BH evaporate? Is BH is loosing anything here, on the other hand, doesn't it gain one of the particles? $\endgroup$ – kpv Mar 4 '16 at 6:08
  • $\begingroup$ I have edited my answer to explain evaporation. $\endgroup$ – Mason Mar 4 '16 at 6:15
  • $\begingroup$ even out of BH context, how energy "borrowed" is given back (thus instantiating the mass loss) ? $\endgroup$ – Fabrice NEYRET Mar 4 '16 at 6:36
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    $\begingroup$ The virtual particle pairs explanation is just an analogy, and actually rather a poor one since it causes much confusion. The reason for the radiation is that observers near and far from a black hole disagree about what constitutes the vacuum state. $\endgroup$ – John Rennie Mar 4 '16 at 6:39

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