  We argue that the many-worlds of quantum mechanics and the many worlds of the
multiverse are the same thing, and that the multiverse is necessary to give
exact operational meaning to probabilistic predictions from quantum mechanics.
  Decoherence - the modern version of wave-function collapse - is subjective in
that it depends on the choice of a set of unmonitored degrees of freedom, the
"environment". In fact decoherence is absent in the complete description of any
region larger than the future light-cone of a measurement event. However, if
one restricts to the causal diamond - the largest region that can be causally
probed - then the boundary of the diamond acts as a one-way membrane and thus
provides a preferred choice of environment. We argue that the global multiverse
is a representation of the many-worlds (all possible decoherent causal diamond
histories) in a single geometry.
  We propose that it must be possible in principle to verify quantum-mechanical
predictions exactly. This requires not only the existence of exact observables
but two additional postulates: a single observer within the universe can access
infinitely many identical experiments; and the outcome of each experiment must
be completely definite. In causal diamonds with finite surface area,
holographic entropy bounds imply that no exact observables exist, and both
postulates fail: experiments cannot be repeated infinitely many times; and
decoherence is not completely irreversible, so outcomes are not definite. We
argue that our postulates can be satisfied in "hats" (supersymmetric multiverse
regions with vanishing cosmological constant). We propose a complementarity
principle that relates the approximate observables associated with finite
causal diamonds to exact observables in the hat.
