Or, as it’s often called, the Many Worlds Interpretation. It’s one of these things that’s often misunderstood. You’ve probably heard of the idea that every time a choice is made, the universe branches in two, and in one branch the choice went one way, and in the other, the choice went a different way. It’s a compelling idea, but it is, in fact, wrong. Or, to be more accurate, it’s been misinterpreted and oversimplified.

Alas, it’s hard to properly describe it without using words like "wavefunction" and "quantum state", but I’m going to try anyway.

Your basic elementary particles like electrons have a property called spin, which is kind of like the particle spinning on its axis like a spinning globe, but in fact is actually very much not like that at all. Anyways, a particle can either be spinning one way, or spinning the other, and these are known as spin up and spin down states.

Which again, isn’t entirely true, because particles don’t have to be in either of the states, they can exist in what is known as a superposition, where the particle is a mixture of spin up and spin down, and only when you try and measure the spin state of a particle does it become up or down, a process known as collapse. This is the choice alluded to in the first paragraph - in one universe, the particle was seen to be spin up, and in the second, the particle was seen to be spin down. The superposition has collapsed into two definite outcomes.

Where the idea of the branching universes isn’t right is in this idea of collapse. What the originators of the orthodox Quantum Mechanics forgot to include was the vital element that essentially the person doing the observing and measuring is himself (or herself, with a lower probability…)  a quantum system, and the state of that quantum system is affected by the result of the measurement. Something different will get written down in the results.

In essence, the superposition of the spin states of the particle doesn’t collapse into certainty - the physicist instead enters a superposition! This is the crux of the idea - that large systems can enter superpositions, then each state in that superposition evolves independently of each other - forming individual universes, unreachable and undetectable.

This approach extends naturally to the entire universe, evolving in time as a single quantum system, with every single possible event playing out simultaneously.

It also has a rather grisly underside - the idea of a quantum suicide. As long as there exists a possible state in which you remain consciously aware, then a version of yourself will be consciously aware. Subjectively, this means that you can continue to survive an indefinite number of suicide attempts, murders, deaths by natural causes, etc. What worries me is that there are likely to be many more states in which you survive indefinitely in horrible pain than there are where everything goes really well.

This realisation really freaked me out the first time I sat and thought about it. Still unsettles me now.

Actually kinda makes me hope that Everett was wrong, and that wavefunction collapse does actually occur.