Sep
06
The True Science of Parallel Universes


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 you 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? I mean, if “the universe” is everything that
there is, you can’t have two versions of it, right? Otherwise the pair would really be
everything and what you started 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. And 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,
but that’s a well-understood question and not what physicists normally talk about when
they discuss multiple observable universes, or “multi-verses.” So let’s cut to the chase: in physics, the
word “Multiverse” normally refers to one of three distinct and largely unrelated proposed
physical models for the universe – none of which has been tested or confirmed by experiment,
by the way. The three “multiverse” models are: Type 1) Bubble universes or baby black hole
universes. This is the most straightforward kind of multiverse: 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 so good at
making stars and galaxies and 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. If you’re not convinced by this logic, don’t worry too much: there’s
not yet any experimental evidence for this kind of multiverse.
Multiverse 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. Multiverse type 3) The many-worlds picture
of quantum mechanics. Surprisingly, physicists still don’t fully understand how the collapse
of the wavefunction 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
“maybe anything that can happen, does. And we just happen to exist in the series of happenings
that were 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. 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 hypotheses. 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 some time 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 – systems that approach the line between the quantum realm and our
everyday experience. So as always, we must remember that physics
is science, not philosophy; and in our attempts to explain the universe that we observe, we
have to make claims that can in principle be tested – and then test them!