  We present a framework in which well-defined predictions are obtained in an
eternally inflating multiverse, based on the principles of quantum mechanics.
We show that the entire multiverse is described purely from the viewpoint of a
single "observer," who describes the world as a quantum state defined on
his/her past light cones bounded by the (stretched) apparent horizons. We find
that quantum mechanics plays an essential role in regulating infinities. The
framework is "gauge invariant," i.e. predictions do not depend on how spacetime
is parametrized, as it should be in a theory of quantum gravity. Our framework
provides a fully unified treatment of quantum measurement processes and the
multiverse. We conclude that the eternally inflating multiverse and many worlds
in quantum mechanics are the same. Other important implications include: global
spacetime can be viewed as a derived concept; the multiverse is a transient
phenomenon during the world relaxing into a supersymmetric Minkowski state. We
also present a theory of "initial conditions" for the multiverse. By
extrapolating our framework to the extreme, we arrive at a picture that the
entire multiverse is a fluctuation in the stationary, fractal
"mega-multiverse," in which an infinite sequence of multiverse productions
occurs. The framework discussed here does not suffer from problems/paradoxes
plaguing other measures proposed earlier, such as the youngness paradox, the
Boltzmann brain problem, and a peculiar "end" of time.
