Here's what it takes to make parallel universes real

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.

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.