Why the Many-Worlds Formulation of Quantum Mechanics is Probably Correct by Sean Carroll.
Our only assumption was that the apparatus obeys the rules of quantum mechanics just as much as the particle does, which seems to be an extremely mild assumption if we think quantum mechanics is the correct theory of reality. Given that, we know that the particle can be in “spin-up” or “spin-down” states, and we also know that the apparatus can be in “ready” or “measured spin-up” or “measured spin-down” states. And if that’s true, the quantum state has the built-in ability to describe superpositions of non-interacting worlds. Not only did we not need to add anything to make it possible, we had no choice in the matter. The potential for multiple worlds is always there in the quantum state, whether you like it or not.
The explanation is at a slightly lower level than the sequences, but it's a concise summary with a healthy dose of proselytization. I think it works nicely.
And the comments are predictably horrible. Sigh.
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