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I understand that one of the simplified ways of looking at Hawking radiation is a pair of virtual particles close to the event horizon (but outside of it). The particle with negative energy falls into the black hole, thus decreasing its mass/energy, while the one with positive energy escapes into the Universe outside of the black hole. My question is: Why is it that the negative energy virtual particle always falls into the black hole (with respect to the observer)? That is, why is there not a random distribution of either a negative particle or a positive particle falling into the black hole?