Hawkings realized that the perfectly thermal nature of black-hole radiation and also further in the ability to have a consistent quantum description of black holes. His observation was based on the fact that in quantum mechanics a state of the thermal density of radiation is a mixed (or density matrix) as opposed to a pure state. However, the unitary time evolution of quantum mechanics (which is central to ensuring that the sum of all quantum probabilities adds up to one) prevents pure states from evolving into density matrices. There arises then a paradox of how a black hole, which can be formed from the collapse of matter prepared in a pure state, can evolve into a thermally radiating object. In fact, eventually, a black hole can completely evaporate leaving only the radiation behind. This puzzle is called the 'information paradox' and continues to be actively debated to the present day. What does this mean in simpler terms?
Hawkings realized that the perfectly thermal nature of black-hole
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