PARALLEL WORLDS

When physicists are asked about "parallel worlds" or ideas along these lines, they have to be careful to distinguish among different interpretations of that idea. There is the "multiverse" of inflationary cosmology, the "many worlds" or "branches of the wave function" of quantum mechanics and "parallel branes" of string theory. Increasingly however, people are wondering whether the first two concepts might actually represent the same underlying idea with the branches being a truly distinct notion. When speaking of the multiverse we really just mean different regions of space-time. In inflationary cosmology however, these different regions can be relatively self-contained "pocket universes." When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be extremely different; they can have different particles, different forces, even different number of dimensions. So there is good reason to think about them as separate universes, even if they're all part of the same underlying space-time. The situation in quantum mechanics is superficially entirely different. Quantum mechanics describes reality in terms of wave functions, which assign numbers (amplitude) to the various possibilities of what we can see see when we make an observation. The many worlds interpretation and quantum mechanics explanation appear utterly different. In the cosmological multiverse, the other universes are simply far away, in quantum mechanics they're right here, but in different spaces.