One of the biggest secrets of physics is that we still don't fundamentally understand what
happens when the smallest things interact with big things - that is, when quantum mechanics
meets our everyday world.
You've probably heard of the "Schrodinger's Cat" experiment where a particle (in this
case a cat) is in a superposition of two states (both dead and alive) at once, until it interacts
with the outside world - normally a photon of light that we send in to see what's going
on (but it could also just be a random particle that has nothing to do with us).
Upon interaction with the outside world, the cat is observed as being either dead, or alive,
but not both.
The problem is, physics can't explain how the cat, or particle, goes from being in a
combination of two states at once to being in "just one", or "just the other"; nor do
we know how the "decision" is made.
This isn't just a problem with cats, either - it plagues every single quantum mechanics
experiment, famous or otherwise, from the double slit experiment and quantum teleportation
to the Stern-Gerlach experiment and tests of the Bell inequalities.
In every case, we can predict how likely it is for a particle to be in one state or another,
but have no clue how it actually ends up that way.
That's where the many-worlds picture of quantum mechanics comes in - basically, "many-worlds"
proposes the idea that the quantum system doesn't actually decide - rather, at every
junction where large everyday stuff interacts with a quantum system, the timeline of history
splits and both possibilities happen on different, alternate branches.
You know, a choose-your-own-adventure where every possible story happens.
In this scenario, we'd think that only one possibility happened because we'd be stuck
on one of the branches, in a version of ourselves that only sees one possibility happen.
In some ways, this sounds pretty fishy - I mean, it's hard to test the reality of a universe
that hides its true nature from us by not allowing us to test or observe its true nature.
But anyway, in the many-worlds picture of quantum mechanics, "branch-points" are predicted
to happen all the time, everywhere.
Pretty much any time subatomic particles interact with each other, or with anything else, anywhere
in the universe, there are multiple possible outcomes.
Which means a LOT of branching of history.
There may even be infinitely many branches.
So is the Many Worlds hypothesis true?
We don't know - it hasn't yet been tested experimentally.
There's a bit of mathematics that supports it, and there are also a number of mathematical
models that don't require such an extravagantly big & complex picture of the universe.
But luckily, physics is science, not speculation, and eventually, someone, maybe even you, will
perform an experiment that helps us
discover the truth.