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This is a very common misunderstanding based on the sloppy way people tend to explain Many Worlds. In the Many Worlds picture, branching of wave functions isn’t

This is a very common misunderstanding based on the sloppy way people tend to explain Many Worlds. In the Many Worlds picture, branching of wave functions isn’t built into the basic postulates of QM. Not every quantum event results in branching. Branching an emergent property of large systems. Consider the following: * I have a qbit in a superposition for 0 and 1 * I as a large, hot, wet macroscopic system interact with that qbit in such a way that my future behaviour depends on the state of that qbit (say if I measure it to be 1 I shoot myself and if I get 0 I go for a walk) If I try to measure the qbit, I become entangled with it. According to unitary evolution, my wave function would be an entangled superposition of the two events. However, for macroscopic systems entanglement and superpositions are basically meaningless. I couldn’t do an interferometry experiment on my body that would determine if I’m in a entangled state so a many worlds person would say that the wave function of be unvierse has effectively branched into two parts that never very little overlap or influence on each other. —————————————————— Maybe this analogy will help: If I have a cup of water and we look at it on a macroscopic scale, there’s a very clear difference between the liquid and the air above it. There’s a division. However, if you zoom in, that division becomes very fuzzy. There’s a region near the waters surface where air molecules and water molecules freely mix and interact and there’s actually a somewhat smooth crossover between the air and water. So the phase separation between a cup of water and the air above it isn’t a fundamental part of the rules of physics. It’s just an emergent behaviour like the branching of the wave function.

millstone 7y ago
Right. The problem of interpretation arises when the qbit has value 0 with probability 75%, and value 1 with probability 25%. In what sense can the universe be said to have branched into two, yet the probabilities are not evenly divided? What can probability even mean if both branches are in some sense realized - is one branch 3x as "real" as the other?
- eigenspace 7y ago
  Again, I’d say you’re misunderstanding the point. The branches aren’t a real thing in quantum mechanics. They’re just a particular way of looking at the wave function of the universe. In the evolution of the wave function, it sometimes will ‘split’ into two parts that are essentially independent of each other and may have different amplitudes associated with them. These amplitudes may be shown to correspond to the (square root of the) probability of finding yourself in one branch or another. However, if you knew about the entire wavefunction as an outside godly observer to the universe might, then there’d be no point in speaking of ‘branches’.

millstone 7y ago

Right. The problem of interpretation arises when the qbit has value 0 with probability 75%, and value 1 with probability 25%. In what sense can the universe be said to have branched into two, yet the probabilities are not evenly divided? What can probability even mean if both branches are in some sense realized - is one branch 3x as "real" as the other?

eigenspace 7y ago
Again, I’d say you’re misunderstanding the point. The branches aren’t a real thing in quantum mechanics. They’re just a particular way of looking at the wave function of the universe. In the evolution of the wave function, it sometimes will ‘split’ into two parts that are essentially independent of each other and may have different amplitudes associated with them. These amplitudes may be shown to correspond to the (square root of the) probability of finding yourself in one branch or another. However, if you knew about the entire wavefunction as an outside godly observer to the universe might, then there’d be no point in speaking of ‘branches’.