There are some times when a science word is downright confusing.
One such word is multiverse.
After all, we already have the word “universe.”
Universe means everything- all of creation as far as we can see from the moment it was
created to the very end of time.
Everything.
So that is the beginning of the confusion and even apparent absurdity.
Multiverse means multiple universes.
Multiple everythings.
I’ll wait a minute while that sinks in so you can think about it and go “Huh… but
wait a minute…”
So let’s put aside the obvious language and logic issues and see exactly what scientists
mean.
To do that, we need to slightly limit what the word “universe” means to a practical
scientist.
It means “everything that could affect us in any way.”
That means everything in a bubble around us that is 14 billion years old.
Taking into account the expansion of the universe, that’s a bubble 92 billion lightyears across.
So that’s a huge volume, but it isn’t infinite.
In addition, we mean anything that exists in our familiar dimensions of forward and
backward, up and down, and left and right.
You can imagine that other dimensions exist that we can't communicate with.
That’s kind of a mind blowing idea, but it’s at least possible.
So if this is what we mean by the universe, we can then see what scientists mean when
they talk about the multiverse.
Now it’s important to realize that we don’t know that multiverses exist.
They very well may not.
But here are some ideas for different kind of possible multiverses.
The first kind is the simplest and least exotic.
We’ll call it version 1 and this one hinges on the idea that the universe is infinitely
large.
That means that our visible universe is just one of literally an infinite number of volumes
the size of our universe.
These other universes could be similar to ours or quite different.
If the laws of physics change with position, that also means that some of these volumes
include ones in which life isn’t even possible.
Perhaps even more mind blowing, with an infinite number of universes, there is somewhere another
universe that is utterly identical to ours and the identical you is currently listening
to the same video.
And there another universe in which I can only speak if it is permitted by our cat overlords.
Infinite means infinite, and that means all things are possible.
So that’s a pretty amazing kind of multiverse, but the others are even more exotic.
Multiverse version 2 is an offshoot of an idea called cosmic inflation.
I’ve talked about inflation in another video, but the basic idea is that when our universe
was very young, briefly it expanded at speeds faster than the speed of light.
You can expand that idea even further and imagine that that during inflation many small
bubbles expanded super fast.
These independent bubbles aren’t part of our three dimensional universe- they are completely
separate from ours.
Further, and here’s a weird twist, throughout the universe inflation continues to occur,
with bubbles popping up all the time, each creating its own infinite universe.
In fact, it’s kind of hard to create a mathematical model of inflation that doesn’t lead to
the multiverse.
Okay- so I’m not done blowing your mind.
Version 3 of the multiverse idea originated in thoughts about quantum mechanics, specifically
the Many Worlds Hypothesis.
You may know that in quantum mechanics that everything is handled probabilistically and
that objects can be in two different states at the same time.
Perhaps the most famous example of this is Schrödinger’s cat, in which a cat is put
in a box with a vial of cyanide and a radioactive material.
When the radioactive material decays, it breaks the vial and the cat dies.
But, according to one interpretation of quantum mechanics, until we open the box to see if
the radioactive material has decayed or not, the cat is both alive and dead.
In the many worlds interpretation of quantum mechanics, when you open the box, the universe
splits in two.
In one universe the cat is alive and in the other, it is dead.
This creation of universes continues all the time, with every decision that’s made.
And none of these universes can communicate with one another.
With a proliferation of universes, we now have a multiverse.
There are a few other proposals about what a multiverse means, but hopefully, this gives
you the basic idea.
Given that these ideas seem outlandish, you may be wondering why scientists believe that
multiverses are even worth thinking about.
That’s a long story and one that I talk about in another video, but basically it’s
one possible explanation for why the universe we live in is so perfect for life to form.
Watch the other video to understand that better.
So now, let me put on my skeptic hat and talk about multiverses in a practical way.
A very real criticism of multiverse theory is that it can’t be tested, even in principle.
By definition, the other universes can’t communicate with our own.
Karl Popper suggested that in order for an idea to be a scientific theory, it must be
able to be proven wrong.
Thus, according to Popper, if we can never talk to other universes, even in principle,
then the multiverse theory isn’t a scientific theory at all.
Personally, I think that’s a bit of a harsh criticism, as it’s at least possible that
a multiverse theory will be devised that leads to predictions about something we can observe
in our own universe.
But, until that prediction is made and tested, we certainly should be a bit skeptical about
the whole multiverse idea.
Another criticism of multiverse theory is that it fails to satisfy Ockham’s razor,
which says that, everything being equal, the simplest explanation must be true.
And multiverse theory is anything if simple.
Again, I think this isn’t a great criticism.
After all, a scientific theory must also explain data, provide a clarifying principle and-
this is important- be right.
Multiverse theory does at least a couple of those things and we need to remember that
the universe (or multiverse) is under no obligation to agree with our idea of what constitutes
simple.
So what’s the bottom line?
Do I believe in multiverses?
The answer is no.
But neither do I think they are silly, nor do I think the researchers who ponder them
are bad scientists.
I do think that it will take a lot of evidence for me to be able to say that multiverses
are proven.
But I’m willing to be convinced.
So how about you and I just wait and watch and see what kinds of new ideas are proposed.
Maybe, just maybe, the multiverse is real.