# Re: Decision theory

From: Jacques M Mallah <jqm1584.domain.name.hidden>
Date: Tue, 29 Dec 1998 14:16:41 -0500

On Thu, 24 Dec 1998, Wei Dai wrote:
> If you don't think it's a problem, then please explain how you would apply
> decision theory with the MWI (and include an example).

Since I explained why it should still work, how would an example
help? You can just take any non-MWI example and restate it in the MWI
language. OK, I'll be extra nice and do it myself.
Suppose that the spin of an electron is to be measured, and that
based on the way the system is prepared, the usual formulas give a 2 in 3
probability of spin up, and 1 in 3 of spin down.
Bob is in a game where he has to guess what the spin will be. If
he guesses correctly he gets \$10; if incorrectly, he loses \$10. He knows
what probabilities the QM formulas give.
He has to decide whether to guess spin up or down. What should he
do? Assume that he likes having money, and that the future versions of
himself are sufficiently similar to himself that he'd want the same for
them.
If were non-MWI, he'd choose spin up to maximize the probability
that the future version of himself has more money. So in the MWI, to
maximize the fraction that have more money, he should choose spin up
rather than spin down or not caring.

- - - - - - -
Jacques Mallah (jqm1584.domain.name.hidden)