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In the many worlds interpretation of QM why does the universe splits to multiple universes after a measurement and not all the time?

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    Decoherence gives an account of how the wavefunction branches emerge out of entanglement of a system with the environment. This video gives a fairly simple explanation. Of course, the validity of decoherence is independent of any interpretation. – Maximal Ideal Jan 15 '22 at 00:13
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    It's not clear whether you're aware, but "measurement" doesn't necessarily mean a scientist taking a reading. It could potentially be just about any event, indeed splitting the worldlines "all the time". I believe there is some controversy over what exactly does and doesn't qualify as a measurement, but hopefully you get the idea. – electronpusher Jan 15 '22 at 00:55
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    @MaximalIdeal any account of branching based exclusively on decoherence is necessarily inconsistent, as decoherence just transforms pure density matrix states into diagonal thermalized states representing classical probability distributions, which cannot magically by itself turn those probability distributions into individual eigenstates. You still need a mechanism for observing said individual states, and that only can be understood as entanglement between observer and observee states – lurscher Jan 15 '22 at 01:26
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    If you have an isolated particle in state $|\psi(t)\rangle$, it's just a particle in one state. If it evolves and changes state to something else like $|\psi(t')\rangle$, it's still one particle in one state. There is no reason for any splitting to occur. It's not even something you can shoehorn into the situation. However, when a particle begins entangling with MANY other particles that's when things get interesting. That's what I wanted to get across with my link. Hopefully that clarifies some things for @CartoonRyan. – Maximal Ideal Jan 15 '22 at 01:47
  • @lurscher True. I admit, the video I linked does seem to give the wrong impression in that regard. I think I didn't get across what I wanted to say. Thanks for clarifying. – Maximal Ideal Jan 15 '22 at 01:50
  • This may help - Does the collapse of the wave function happen immediately everywhere?. My answer address it toward the end. – mmesser314 Jan 15 '22 at 02:01
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    Here are a couple more relevant videos. This one is from Veritasium - Parallel Worlds Probably Exist. Here’s Why. This is from Sean Carroll - Sean Carroll explains: what is the many-worlds interpretation? – mmesser314 Jan 15 '22 at 03:08
  • According to many worlds, the number of universes that have existed or ever will exist is constant. The so called branching is just an evolution of how "populated" they all are by the wavefunction. – Connor Behan Jan 15 '22 at 07:00
  • Check out this answer: https://physics.stackexchange.com/questions/208149/where-are-the-worlds-in-many-worlds-interpretation/208161#208161 – Victor Novak Jan 16 '22 at 10:42

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