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So as I understand it, Hawking radiation occurs when virtual antiparticle-particle pair are created near the event horizon of a black hole due to vacuum fluctuations because of Heisenberg uncertainty and one particle flies away from the black hole and the other falls in. The black hole loses mass because the one that falls in has negative energy measured by a far away observer. My question is how can a particle have negative energy in a certain reference frame? Be as techincal as possible in your answer, thank you.