Black holes, fast scrambling and the breakdown of the equivalence principle

Under reasonable assumptions, black holes have been argued to form firewalls, burning up anything crossing their horizons. This argument finds that a firewall would appear very late in a black hole's lifetime, when Hawking radiation has caused the horizon to shrink to one-half its original area. For stellar-mass black holes, this process surpasses the universe's current age and so no such black hole would currently possess a firewall. However, black holes have recently been conjectured to scramble their interior degrees-of-freedom, with a scrambling time scale comparable to the time it takes light to travel a Schwartzschild radius' distance. We prove that local observers will already experience a firewall from the scrambling time onwards after the black hole's formation. Here `local' means that the observer couples to fewer than one-half the black hole's total interior `qubits.' Indeed, for observers to fail to be local in this manner, it would mean that they couple to more `qubits' within such black holes than exist in all the stars of the observable universe. Therefore we find that if black holes are indeed fast scramblers, then every astrophysical black hole in the universe will already have a fully developed firewall for any local physical process.