  "Boltzmann brains" are human brains that arise as thermal or quantum
fluctuations and last at least long enough to think a few thoughts. In many
scenarios involving universes of infinite size or duration, Boltzmann brains
are infinitely more common than human beings who arise in the ordinary way.
Thus we should expect to be Boltzmann brains, in contradiction to observation.
We discuss here the question of whether Boltzmann brains can arise as quantum
fluctuations in the vacuum. Such Boltzmann brains pose an even worse problem
than those arising as fluctuations in the thermal state of an exponentially
expanding universe. We give several arguments for and against inclusion of
vacuum Boltzmann brains in the anthropic reference class, but find neither
choice entirely satisfactory.
