SCIENCE OF PARALLEL UNIVERSE
Everyone
loves the idea of parallel universes. Maybe it's the appeal of an ideal
world where you have second chances and things turn out differently -
an alternate reality where do get into Hogwarts and the Star Wars
prequels aren't made and you finally plug in your asymmetric computer
cord correctly on the first try. But is there really a place in science
for such wistful speculation?
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| The idea of parallel universe |
I
mean, if the universe is everything that there is, then you can’t have
two versions of it, right? Otherwise the pair would really be everything
and what you would start off calling the universe wasn't. The problem
here is terminology. Physicists speaking informally often say universe
when they really mean observable universe, that is, the part of the
whole universe that we've so far been able to see.
It's
perfectly fine to talk about multiple different observable universes.
For example, an alien near the edge of our observable universe will see
parts of the whole universe that we cannot yet see because the light
hasn't had time to reach us yet. But that's a well understood question
and not what physicists normally talk about when they discuss multiple
observable universes or multiverses.
So
let's cut to the chase, in physics the word 'Multiverse'
normally refers to one of the three distinct and largely unrelated
proposed physical models for the universe, none of which has been tested
or confirmed by the way.
The three multiverse models are:
Type 1 - Bubble universes or baby black hole universes:
This is the most straightforward kind of universe. The basic idea is
that perhaps there are other parts of the universe which are so far away
that we will never see them or are inside black holes so similarly we
will never see them. This kind of model was created as an attempt to
explain why our universe is good at making stars, galaxies, black holes
and life. As the argument goes, if each of these separate mutually
un-seeable 'bubbles' in the universe had slightly different laws of
physics, then by definition we could only exist in one that had the
right physical laws to allow us to exist. Like you have to live in a
universe where the earth could form because if the earth couldn't form
then you wouldn't be here. If you're not convinced by this logic don't
worry too much, there's not yet any experimental evidence for this kind
of universe.
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| Bubble universe |
Type 2 - Membranes and extra dimensions:
Inspired in part by the inability of the mathematics of string theory
to predict the right number of dimensions for the universe in which we
live, string theorists proposed the idea that perhaps what we think of
as our universe is actually just a three dimensional surface embedded
within a larger super universe with 9 spatial dimensions. Kind of like
how each page of a newspaper is its own two dimensional surface embedded
within our three dimensional world. And of course, if space had 9
dimensions rather than three, there'd be plenty of space for other three
dimensional surfaces that appeared, like ours, to be universes in their
own right, but like the pages of a newspaper were actually part of a
bigger whole. These kinds of surfaces are called 'membranes' or 'branes'
for short. And as a reminder, there is not yet any experimental
evidence for this kind of multiverse.
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Type 3 - The many-worlds picture of quantum mechanics:
Surprisingly, physicists still don't fully understand how the collapse
of the wave function in quantum mechanics happens and the many-worlds
hypothesis makes an attempt at explanation by proposing that every
possible alternate timeline for the universe is real and they all happen
in an ever-larger, ever-branching way. Like a universal choose your own
adventure - where every possible story happens. If this were the case,
we might not realize it because we'd be stuck living out just one of the
infinitely many possible lives available to us. In some ways,
many-worlds is similar to the bubble multiverse model by proposing 'may
be anything can happen, does and we just happen to exist in the series
of happenings necessary for us to exist. If you're still not convinced
by this logic, don't worry. There is not yet any experimental evidence
for this kind of multiverse.
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| Many worlds |
Of
course if you want to get imaginative, you could also combine several
of these models together into a multi-multiverse - a new super
speculative model based itself on speculative and experimentally
unconfirmed models. But that's not to say we couldn't test these
multiverse hypothesis. For example, if our observable universe were
really just one of many disconnected bubbles or membranes and if it
happened to collide with another bubble or membrane sometime in the
past, then that collision would certainly have had some sort of effect
on what we see when we look up at the night sky. On the other hand, the
many-worlds interpretation might be tested fairly soon since
experimentalists are becoming increasingly able to manipulate and
control ever larger quantum mechanical systems in their labs. We must
remember physics is science not philosophy and in our attempts to
explain the universe that we observe, we have to make claims that in
principle be tested and test them
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