PeterDonis said:
I will try to recreate your argument.
In post #18 you say that:
(1) We never measure branch weightings only weights relative to our own branch.
(2) The ignorance interpretation relies of not having sufficient knowledge. But in MWI all outcomes occur. MWI is not about initial conditions.
Then you explain that (1) can be justified, but (2) cannot.
New question: why is so important 2? Is 2 what makes MWI probabilities ill-defined? Why do we need to invoke initial conditions?
PeterDonis said:
I already responded to your post #20 in my posts #21 and #22.
In post #21, you seem to be responding to my use of the terms but not clarifying.
In #22, you expand a bit more. You argue that:
(3) In classical mechanics, possibility is epistemic: the dynamics have a single outcome but we do cannot have full knowledge
(4) In MWI, possibility is ontic: "the deterministic dynamics is that
all of the "possible" outcomes, i.e., all the outcomes that have nonzero amplitudes in the wave function, actually occur. "
Then you argue that in MWI we end in all the branches but there is still a "we" in each branch that cannot detect the rest. You conclue by repeating that standard ignorance interpretation of probability doesn't even make sense.
You seem to affirm that we need a meaningful concept of probability to start with. My question is what in all these elements is what you find key in making probabilities ill-defined?
There are two questions here how to define the Born rule and how to define probabilities. I would argue that if you forget about (1) (sometimes justified as you say), we can come up with probabilities for MWI that would predict different branching that would correspond to our relative weight according to own branch. If this is enough to derive the Born rule is another question.