Can there be a parallel existence?
Many scientists have speculated that there may be universes
other than ours. Proving it is another matter. See the book by
Martin Rees, Before the Beginning for a discussion of how the
universe may consist of many different interacting universes.
Another interesting idea is that a "shadow" or "mirror" universe
exists that could explain the dark matter.
Another is the many-worlds interpretation of quantum mechanics.
Quoting from an interview with David Deutsch, one of the
foremost expositors of this theory today: "It is rather like the
discovery of other planets or other galaxies. Having previously
known only the Milky Way, we did not just find that there are
vast numbers of stars out there, far more than in the Milky Way.
There are more galaxies out there than there are stars in the
Milky Way. We also found that most of the stars outside the
Milky Way are actually arranged in other little Milky Ways
themselves. And that is exactly what happens with parallel
universes. It is of course only an analogy but quite a good one;
just like the stars and galaxies, the unseen parts of reality
are arranged in groups that resemble the seen part. Within one
of these groups, which we call a parallel universe, the
particles all can interact with each other, even though they
barely interact with particles in other universes. They interact
in much the same way as the ones in our seen universe interact
with each other. That is the justification for calling them
universes. The justification for calling them parallel is that
they hardly interact with each other, like parallel lines that
do not cross. That is an approximation, because interference
phenomena do make them interact slightly. "
Also see the article by M. Tegmark and J. Wheeler in the Feb.
2001 issue of Scientific American: 100 Years of Quantum