[kgwgk]

If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation?

Edit: the most significant problem with the MWI, apart from the "creepy" metaphysical aspects, is the meaning and quantification of probabilities. In particular being able to derive Born's rule, which works so well in practice.

[nessus42]

> If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation?

The "Many Worlds Interpretation" is actually something of a misnomer. This is why many people prefer to call it the Everett Interpretation.

It doesn't actually posit many worlds. It posits one very big complex world with very complex superpositions of state. But since your brain ends up in a superposition of states, different facets of this superposition of your brain state perceive this one big complex world, as smaller, simpler "worlds".

And the term for why different pieces of this superposition of states stop having an effect on each other is called "decoherence".

As for how the math works out in terms of probabilities, that is beyond me.

When I studied this, the discussion was usually simplified down to a quantum coin that when flipped would come up heads 1/3 of the time and tails 2/3 of the time.

This only results in two "worlds" though. A heads world and a tails world. So there was an issue that people debated at the time: Why do we perceive the .3333/.66667 probability for these two "worlds", rather than a .5/.5 probability?

I must admit that I am ignorant about the current state of this debate.

[kgwgk]

Is saying "measurement" really much more undefined and unscientific than saying "different facets of this superposition of your brain state perceive"?

[nessus42]

100% yes. You cannot give a scientific accounting of "measurement" since the term isn't even defined in the Copenhagen Interpretation.

There are various interpretations that attempt to define "measurement" in various ways, but those are not the Copenhagen Intepretation.

As for whether you can give a scientific account of how data is processed by a brain in a superposition of states, you most certainly can. (Ever heard of "quantum computing"?) It's just complicated.

[kgwgk]

“You most certainly can” is not a very satisfactory answer. I don’t say that the Copenhagen interpretation is very satisfactory but at least it predicts the probability of events. The term isn’t even defined in the Everett interpretation. In the best case, it’s incomplete and more metaphysics than physics.

Edit: I’ve never heard of any treatment of “quantum computing” which doesn’t include the concept of measurement, the Born rule and the projection postulate. Have you?

Edit2: it was maybe not fair to say that the probability of events is not defined in the Everett interpretation because many-world interpretations have addressed this issue since the original paper from Everett in 1957. But as far as I know they have not succeed. Measurement has also been addressed in countless papers and books for almost a century, for what it’s worth.