DA, the multiverse is merely a mathematical model that describes the apparent behavior of probalistic potentialities at the quantum level . No more, no less. This particular model is attractive because it dispenses with infinities caused by pertubation calculations. The side-effect is that you get these infinite "universes" instead of collapsing the Schrodinger Wave leaving only one universe and having all of these "left over" parts to have to explain away. Very messy.

Trying to explain a multiverse is like trying to explain what a complex number is as it pertains to everyday 4-dimensional reality. What exactly does it mean to have the square-root of negative two things in your hand? These numbers are used all of the time from light diffusion equations to electro-magnetism equations, from relativity to quantum mechanics, so... they must touch a reality that we are not ordinarily accustomed to in some fashion. They work, therefore, we can infer that there is something that we can't detect with present technology, but, in the vacuum of numerical purity, must be there.

There ARE philosophical problems with the Many Worlds theory not to mention that many physicists think that this approach is a cop-out - "just create another universe and that factors away your problems!". However, one must point out that, regardless of the far-fetchedness of the concept, quantum predictions work when equations are solved through using this model especially as they pertain to quantum computation. Allow me to quote an interview with Deutch:

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The key to Deutsch's model sounds peculiar. He treats the multiverse as if it were a quantum computer. Quantum computers exploit the strangeness of quantum systems-their ability to be in many states at once-to do certain kinds of calculation at ludicrously high speed. For example, they could quickly search huge databases that would take an ordinary computer the lifetime of the Universe. Although the hardware is still at a very basic stage, the theory of how quantum computers process information is well advanced.

In 1985, Deutsch proved that such a machine can simulate any conceivable quantum system, and that includes the Universe itself. So to work out the basic structure of the multiverse, all you need to do is analyze a general quantum calculation. "The set of all programs that can be run on a quantum computer includes programs that would simulate the multiverse," says Deutsch. "So we don't have to include any details of stars and galaxies in the real Universe, we can just analyze quantum computers and look at how information flows inside them."

If information could flow freely from one part of the multiverse to another, we'd live in a chaotic world where all possibilities would overlap. We really would see two tables at once, and worse, everything imaginable would be happening everywhere at the same time.

Deutsch found that, almost all the time, information flows only within small pieces of the quantum calculation, and not in between those pieces. These pieces, he says, are separate universes. They feel separate and autonomous because all the information we receive through our senses has come from within one universe. As Oxford philosopher Michael Lockwood put it, "We cannot look sideways, through the multiverse, any more than we can look into the future."

Sometimes universes in Deutsch's model peel apart only locally and fleetingly, and then slap back together again. This is the cause of quantum interference, which is at the root of everything from the two-slit experiment to the basic structure of atoms.

KurzweilAI.net

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Once again, we can infer that other universes exist through the numerical purity of the equations. And look! No left over parts!

I recommend the writings of David Deutch - Professor at Oxford University. He works at the Centre for Quantum Computation at The Clarendon Laboratory, University of Oxford.