Here is a truly delightful explanation of the Quantum Multiverse. It was introduced this way: The story you're about to hear by Tom Wiser was recorded live at the Moth Mainstage.
"There are some scientists who say that the universe that we inhabit is not the only universe there is. There are alternate universes. They say that these universes arise from the subatomic realm of quantum physics. Here's how they say it works: Every time a quantum event happens, it produces all of its possible results. This is like saying if you flip a coin, it comes up heads... but it also comes up tails. Since those two results are mutually exclusive, the flipping of the coin splits the universe into two alternate universes: the 'heads' universe and the 'tails' universe. And since quantum events are happening all the time, it means that alternate universes are bubbling up continually. They're always being created, splitting off from ours. And these alternate universes are real. They're not just figments of the imagination. They're complete, whole, real things..."
The only thing I might add to this explanation is that you, the observer, will become part of both the 'heads' universe and the 'tails' universe, and your two selves will be identical until you make an observation. When your two selves observe the coin, one of them will see 'heads' and the other will see 'tails', and at that instant your two selves will become distinct. This raises the tricky question, which you is the real you? The answer: both are equally legitimate yous which have lost touch with one another, and will never see each other again. The you who is reading this webpage is one of the two, made distinct by the act of observing the coin.
This thought experiment raises two key issues: One, it's not the superposition of states of the coin which is fixed as 'heads' or 'tails' by observation, but rather the superposition of observers, identical in their distinct universes, which become distinct by the act of observation. Two, it calls into question the nature of identity, since the observer becomes a distinct part of distinct universes through the act of observing. By identity, I mean the youness of you.
The identity of the observer is called into question because the act of observing causes the single entity, you, to take on ever-so-slightly different characteristics in each of the two daughter universes - the 'heads' universe and the 'tails' universe. Are both of these people really the same person, just with different memories? Or are they different people? The flipping of the coin is not the only quantum event, of course; they're happening all the time. Cosmic rays cause cell damage in one universe, but not so much in another. One you gets more wrinkles in his face, while the other keeps his baby skin. The wrinkly guy doesn't get that plum sales job, can't afford the car, doesn't marry the supermodel. The sales guy marries the supermodel, gets caught up with drug addiction, you get the idea. (No need to write my own Sliding Doors.) At what point did these two yous become different people, each in their own universe? Which quantum event was the straw that broke the camel's back? If these two quite different yous do not have the same identity, then is it the case that you don't share an identity with your two-year-old self?
It's hard to answer these questions consistently, and I think I know why: the concept of identity is not well formed, or well defined. In fact, this concept may defy such an effort to pin it down.
Multiverse Quantum State Collapse