As you interact with the things in your environment, they get entangled with you, and then other things get entangled with them, and so on. Eventually, we have a single complicated and messy wave function for the entire universe—the universal wave function. And as more events happen, that universal wave function splits into more and more noninteracting parts, each merrily marching along to the deterministic beat of the Schrödinger equation. These are the many worlds of Everett’s interpretation. They may seem absurd on the face of it: there is, after all, only one world that we experience. But
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