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But since then, evidence has accumulated that unitarity must be upheld.įor some time, it was believed that an idea called "black hole complementarity" would resolve the conflict. Hawking went with the equivalence principle and famously predicted that information is lost. Either information is lost, or the horizon of a black hole is a special place called a "firewall".

It turns out that these two demands are mutually exclusive. Meanwhile, in general relativity, the horizon of a black hole is empty space, so it should look the same as empty space everywhere else. Quantum mechanics requires this cloud to contain information: in principle, we could measure the cloud and use a powerful computer to reconstruct what formed the black hole. General relativity is based on the equivalence principle: empty space should look the same everywhere and to everyone.īlack holes evaporate, converting their energy into a cloud of radiation. Quantum mechanics rests on the principle of unitarity: information cannot be fundamentally lost. The central principles of quantum mechanics and of general relativity (our classical theory of gravity) come into sharp conflict at the horizon of a black hole. My interests are in theoretical cosmology and quantum gravity.