In the 1950s, a student at Princeton University named Hugh Everett III was studying quantum mechanics. He learned about the Copenhagen interpretation, which says that at the very, very smallest level—what we mean when we say quantum—matter exists not just as a particle and not just as a wave, but in all possible states at once (all of those states together is called its wave function; the phenomenon of existing in all of those states at once is called superposition). It also says that when you observe a quantum object, you break that superposition and it essentially "chooses" one state to be in. He also learned about the Heisenberg Uncertainty Principle, which says that because we affect a quantum object's behavior through observation, we can never be completely certain where it is or what it's doing at any given time.

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Everett understood these principles, but he took issue with one part: what if, instead of a quantum object "choosing" a state when you observe it—say, it becomes a particle instead of a wave—there was an actual split in the universe that created separate timelines? According to Everett's theory, in this timeline, the object is a particle, but there's another timeline where it's a wave. Even more baffling, this implies that quantum phenomena aren't the only things that split the universe into separate timelines. For everything that happens, every action you take or decide not to take, there are infinite other timelines—worlds, if we may—where something else took place. That's the many-worlds interpretation of quantum physics. It may not seem like it, but it's actually simpler than the Copenhagen interpretation—it doesn't strike an arbitrary line between the quantum world and everything else, because everything behaves in the same way. It also removes randomness from the picture, which helps the math work out nicely.