astro-ph/0302131 [abs, ps(600), other] :
Title: Parallel Universes
Authors: Max Tegmark (Penn)
Comments: 18 pages, 8 figs. A less technical adaptation is scheduled for the
May 2003 issue of Scientific American. Version with full-resolution figs at
this http URL
Journal-ref: In "Science and Ultimate Reality: From Quantum to Cosmos",
honoring John Wheeler's 90th birthday. J. D. Barrow, P.C.W. Davies, & C.L.
Harper eds. Cambridge University Press (2003)


I survey physics theories involving parallel universes, which form a natural
four-level hierarchy of multiverses allowing progressively greater
Level I: A generic prediction of inflation is an infinite ergodic universe,
which contains Hubble volumes realizing all initial conditions - including
an identical copy of you about 10^{10^29} meters away.
Level II: In chaotic inflation, other thermalized regions may have different
effective physical constants, dimensionality and particle content.
Level III: In unitary quantum mechanics, other branches of the wavefunction
add nothing qualitatively new, which is ironic given that this level has
historically been the most controversial.
Level IV: Other mathematical structures give different fundamental equations
of physics. The key question is not whether parallel universes exist (Level
I is the uncontroversial cosmological concordance model), but how many
levels there are. I discuss how multiverse models can be falsified and argue
that there is a severe "measure problem" that must be solved to make
testable predictions at levels II-IV.

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