What is the black hole information paradox?

The answer

The black hole information paradox, identified by Stephen Hawking in 1975, arises from the conflict between general relativity and quantum mechanics. Hawking showed that black holes emit thermal radiation (Hawking radiation) and eventually evaporate completely. Since this radiation is purely thermal and carries no information about what fell in, the detailed quantum state of the infalling matter appears to be permanently destroyed - violating quantum mechanics' fundamental principle of unitarity, which states information must be conserved. Proposed resolutions include black hole complementarity (information exists both inside and outside, but no observer sees both), the holographic principle (information is encoded on the event horizon as a 2D surface), and the firewall hypothesis (a wall of energy at the horizon). In 2019, calculations using the 'island formula' and quantum extremal surfaces by researchers including Netta Engelhardt and Ahmed Almheiri showed that information does eventually escape through subtle quantum gravitational effects, supporting unitarity - though the physical mechanism remains debated.