It is a staple of science fiction: the mirror world or
mirror universe, or parallel universe. Star Trek has used it any number
of times, as have some of the Marvel movies. You know, the characters you are
following somehow transition to an alternate universe where there are copies of
themselves, but the alternate universe selves are altered. Perhaps if the
originals are good, the alternates are bad. Or, perhaps the captain in one
universe is a bricklayer in the other.
It is a fun concept to play around with in a fictional
setting, but like the concept of travel, it doesn’t really work if you think
about it too hard. There could certainly be other universes that were belched
out of the Big Bang at the same time ours was. They might even start out with
the same materials in the same quantities. But a parallel universe? One that
has another version of you and me in it? That doesn’t work. The two universes
would have to be absolutely identical and our doppelgangers doing the same
things we are, in other words, no different from our universe:
indistinguishable. Or it would be completely different. Because even if they
started out identical, any change that happened three seconds after the Big
Bang would propagate an infinite number of further changes.
In the Star Trek episode “Mirror, Mirror,” we learn
that Kirk’s doppelganger became captain by assassinating Captain Pike. A big change like that would change everything
that happened after it. A change like that, if it happened a thousand years
earlier, would make the parallel universe unrecognizable. There would be no “parallels.”
The only way for this to work would be that everything in both universes was
identical for billions of years and only started to diverge from each other very
recently. What would make them identical for billions of years and then suddenly
diverge? It doesn’t make any logical sense (other than as a plot device).
Some scientists have theorized the possible existence of
other universes. But there is no reason to believe those universes would be
similar to ours; indeed, those universes would not necessarily even be governed
by the same laws of physics that our universe is.
But in a fictional world parallel universes and time travel
can be great fun. Endless stories could be generated by them. Just don’t think
about them too hard.
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