Abstract Firewalls in black holes are easiest to understand by imposing time reversal
invariance, together with a unitary evolution law. The best approach seems to
be to split up the time span of a black hole into short periods, during which no
firewalls can be detected by any observer. Then, gluing together subsequent
time periods, firewalls seem to appear, but they can always be transformed
away. At all times we need a Hilbert space of a finite dimension, as long as
particles far separated from the black hole are ignored. Our conclusion con tradicts other findings, as these assume that information that entered into a
black hole, cannot re-emerge. But re-emergence of that information is exactly
what our version of firewalls is supposed to ensure. Indeed, the firewall trans formation removes the entanglement between very early and very late in- and out-particles, in a far-from-trivial way.