The black hole information paradox is a puzzle resulting from the combination of quantum mechanics and general relativity.
Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.
This is controversial because it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.
A fundamental postulate of quantum mechanics is that complete information about a system is encoded in its wave function up to when the wave function collapses. The evolution of the wave function is determined by a unitary operator, and unitarity implies that information is conserved in the quantum sense.
Starting in the mid-1970s, Stephen Hawking and Jacob Bekenstein put forward theoretical arguments based on general relativity and quantum field theory that not only appeared to be inconsistent with information conservation but were not accounting for the information loss and state no reason for it. Specifically, Hawking's calculations indicated that black hole evaporation via Hawking radiation does not preserve information.
Today, many physicists believe that the holographic principle (specifically the AdS/CFT duality) demonstrates that Hawking's conclusion was incorrect, and that information is in fact preserved.
In 2004 Hawking himself conceded a bet he had made, agreeing that black hole evaporation does in fact preserve information.
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