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The
idea that things exist in a particular, well-defined state at all times where
their properties can be determined so long as you can measure them well enough
was fundamental to how we conceived of the Universe.

When
quantum physics came along, that idea went right out the window, never to
return. The Universe, at a fundamental level, is indeterminate. One possible
interpretation — that of infinite parallel Universes — holds that every time a
quantum interaction occurs, all possible outcomes do actually occur somewhere,
with only one of them reflecting what happens in our observable Universe. But
if the right conditions exist, these parallel Universes will actually be real.

Quantum
indeterminism is a fundamental fact of the Universe, but how we interpret it is
up to us. If you fire a single electron through a double slit, you’d like for
it to go through either one slit or the other, but that’s not how the Universe
works.

Instead,
the electron acts as a wave, passing through both slits simultaneously and
interfering with itself. There’s a probability distribution describing where
each individual electron will wind up, but each one will only make a single
“hit” on a background screen. If you take thousands of these electrons in a row,
the interference pattern will emerge.

*The wave pattern for electrons passing through a double slit, one-at-a-time. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Note that more than one electron is required to reveal the interference pattern.*

There
are lots of processes that are inherently indeterminate in exactly this
fashion. Some are discrete: when you collide a particle and antiparticle to
create two photons, one of the photons will have spin +1 and the other will
have a spin of -1, but which is which has a 50:50 shot.

Other
indeterminate processes are continuous: colliding a particle and antiparticle
creates two photons, and those two photons will be created in opposite
directions (oriented 180 degrees) relative to one another in the
particle/antiparticle’s center-of-mass frame. But what direction will those
photons pick? North/South? East/West? Up/Down? Anything in between? It’s
entirely random.

*Particle-antiparticle annihilation will produce two photons of equal energy in opposite directions. But which direction that will be is completely random.*

Every
interaction between two particles in the Universe has this quantum
indeterminism, at some level, inherent to it. Every particle has an inherent
uncertainty to both its position and momentum, and when two of them interacts,
that uncertainty propagates into the final position and momentum, too. We have
a lot of different ways to try and understand this indeterminism, many of which
are equally valid.

*The idea of parallel Universes, as applied to Schrödinger’s cat.*

These
interpretations of quantum mechanics cannot be distinguished from one another,
and include ideas like wavefunction collapse (where an observation triggers the
collapse of the wavefunction), an ensemble approach to possible outcomes (where
all outcomes are possible, and the Universe selects one when an observation is
made), and the many-worlds approach, where all possible outcomes do occur in
some Universe, but we only have the one Universe to observe.

*The multiverse idea states that there are infinite numbers of Universes like our own, and infinite ones with differences.*

This
last one has a fantastic consequence, if true: there must exist a number of
parallel Universes that’s so great, it approaches infinity as time goes on.
There are some 10^90 particles in the observable Universe, which has been
around for 13.8 billion years since the Big Bang, and each particle has
undergone anywhere from millions of interactions to many quadrillions (or more)
over that time.

The
number of possible outcomes is ridiculously huge — a number greater than (10^90)! —
but that doesn’t mean the many-worlds approach is ridiculous. In fact, there
are a number of ways in which it could be exactly true.

*The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that.*

##
__1.
The Universe, of which our observable Universe is a small part, was born
infinite__

__1. The Universe, of which our observable Universe is a small part, was born infinite__
No
matter how many particles we have in our Universe, no matter how arbitrary
their initial configurations and no matter how many possible outcomes their
interactions could have given rise to, that number will still be finite. But
the Universe could have been born infinite!

Beyond
the stars, galaxies, matter and energy that we can see, we have every reason to
believe that there is more “Universe” just like our own, and that it’s simply
not observable to us due to the fact that the speed of light and the age of the
Universe (since the Big Bang) are both finite. If there’s an infinite amount of
Universe like this, then the exact configuration starting off our Universe
occurred an infinite amount of times, and everything that was ever possible
happened somewhere.

*Inflation set up the hot Big Bang and gave rise to the observable Universe we have access to, but we can only measure the last tiny fraction of a second of inflation’s impact on our Universe.*

##
__2.
Our Universe was born finite, but there were an infinite number of them born__

__2. Our Universe was born finite, but there were an infinite number of them born__
The
Big Bang was not the very beginning of everything, as we once thought, but was
merely the birth of our observable Universe. It was the first moment that our
Universe could be described as hot, dense, full of matter/antimatter/radiation,
and simultaneously expanding and cooling.

This
happened a finite amount of time ago — 13.8 billion years — and was preceded by
a period of cosmic inflation. Inflation creates an exponentially growing
spacetime, which means, if it occurred for an infinite amount of time to the
past, could have created an infinite number of finite Universes, one of which
contains ours.

*Even though inflation may end in more than 50% of any of the regions at any given time (denoted by red X’s), enough regions continue to expand forever that inflation continues for an eternity, with no two Universes ever colliding.*

##
__3.
Our Universe was born finite, there are a finite number of Universes, but there
are enough of them around that all possible outcomes still occur__

__3. Our Universe was born finite, there are a finite number of Universes, but there are enough of them around that all possible outcomes still occur__
This
is the trickiest case of all, because nothing — not even exponentially-growing,
inflating spacetime — grows as fast as the number of possible quantum outcomes
for the Universe. But a big enough, possibility-rich enough multiverse will
have created a Universe with identical initial conditions to our enough times
that all the possible outcomes to date have been realized somewhere. This will
change, given enough time; as interactions continue and quantum systems evolve,
we will eventually see the number of possibilities surpass the number of
Universes available to realize all of them.

*A representation of the different parallel “worlds” that might exist in other pockets of the multiverse.*

Somewhere,
the Nazis won World War II; somewhere, Hillary Clinton is president; somewhere,
humans have driven themselves to extinction; somewhere, we’ve achieved world
peace. We still have just the one Universe, though, and still have no prospects
for gathering information outside of what’s observable to us.

But
if the Universe was born infinite, if the state that gave rise to it existed
for an infinite amount of time, or we simply created enough pocket Universes
for these parallel Universes to exist today, then they’re real. And they could
be real if any of these three possibilities are true; there are three different
paths to success. But until we have some way of testing it, we have no way of
knowing what the ultimate truth of the matter is, and whether parallel
Universes truly are real.

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