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> Some of them are now in the "red" universe, and some of them are now in the "blue" universe and give matching answers. The percentage of them giving each answ

> Some of them are now in the "red" universe, and some of them are now in the "blue" universe and give matching answers. The percentage of them giving each answer is the same as in the situation above The objection is: what fraction are in red universes, and why? Why is it 2/3rds and not, say, 3/5ths or zero? In QM we can compute these fractions using the Born rule, which says that the probability of each outcome is proportional to its measure in the wavefunction. This is natural to postulate in Copenhagen, where we interpret the wavefunction as a probability density. If the measure of spin-up is twice that of spin-down, it means that spin-up is twice as likely. But in MWI, the wavefunction is definitely NOT a probability density. Both spin-up and spin-down are physically realized. So why should it matter that spin-up has a higher measure? Where does the Born rule come from in MWI? Can it be derived from the Schrödinger equation? If so, how? If not, what additional axioms are needed?

jiggawatts 6y ago
The Born rule is basically saying that the wave function and the measurements are related by a squaring relationship one way, or a square root the other way. This squaring in QM is basically saying that there are two probability distributions, not one. There isn't just the particle, there is the particle and the experimenter. Similarly, there's the particle and itself in interference experiments. Or the particle and the detector. There's always (at least) two required for an interaction. Hence a squaring. It's saying that the observed and the observer are part of the same system. We're not Gods viewing squiggly cartoons on a page, like we like to draw. We are the drawing.