Jesse Mazer wrote:
> Hal Finney wrote:
> >Measure is important. It is what guides our life every day.
> >We constantly make decisions so as to maximize the measure of good
> >outcomes, as nearly as we can judge. I don't think we can neglect it
> >in these thought experiments.
> What type of "measure" are you talking about? I had gotten the impression
> reading this list that the measure on "everything", however it's
> defined--all possible computations, for example--was an open question, and
> that different TOEs might disagree.
That's true, but the important point is to consider why we are searching
for a measure, or why we even think there might be a measure that is
relevant to our experience.
The reason is because our own existence is not chaotic, but ordered.
Presumably there are observers who see universes that are much more
chaotic than ours, universes where there are no natural laws but the
observers just manage to hang together somehow. Why do we see a lawful
And in our own universe, why do more probable things happen more often
than less probable ones? It's not tautological! Remember our discussion
of the magical universe where dice always come up "6" but everything
else works OK. Why don't we live in one of those universes?
The same thing happens in the MWI. If you send almost-vertically-
polarized photons through a vertical polarizer then 99 times out of 100
they go through. Each time, the universe splits into two branches.
After 100 photons, only one universe in 2^100 of them will see the
right statistics. By sheer numbers of universes, almost all of them
will see about 50% go through. Why aren't we in one of those universes?
The answer to all of these puzzles must be that fundamentally, some
universes are more likely to be experienced than others. This is the
concept which we refer to as measure. It is a weighting factor that
somehow must make some universes more important in the grand scheme
You are right that there are many different ideas about how measure works
and how it could apply, in both the MWI and in the larger multiverse.
But this uncertainty doesn't mean that we can reject or ignore the concept
of measure. Its reality is forced upon us by every observation we make.
> Are you talking about a type of measure
> specific to the MWI of quantum mechanics? I thought there was supposed to be
> a problem with this due to the "no preferred basis" problem.
The proper manner for incorporating measure into the MWI is indeed an
open question at this point. The simplest is to just introduce it ad
hoc and define the measure of a branch as the square of its amplitude.
Others claim that they can derive this from more elementary and/or
obvious assumptions. But it's got to be there.
> In any case, if there is some sort of theory that would give objective
> truths about first-person probabilities in splitting experiments (and I'm
> not sure if you believe in continuity of consciousness or that such a theory
> is out there waiting to be found),
Well, I do believe in continuity of consciousness, modulo the issues
of measure. That is, I think some continuations would be more likely to
be experienced than others. For example, if you started up 9 computers
each running one copy of me (all running the same program so they stay
in sync), and one computer running a different copy of me, my current
theory is that I would expect to experience the first version with 90%
However I don't see any way at this point to test this model.
> then if first-person probabilities
> disagree with "measure", however it's defined, I think most people would
> care more about maximizing the first-person probabilities of good outcomes
> as opposed to measure.
Our experiences every day prove that first person probabilities do
correspond to measure, but that is because we define measure to correspond
to what we experience. That is where the amplitude-squared formula for
probability came from in QM; it is there to make theory match experience.
> The main reason to care about measure would be for
> altruistic reasons, that you don't want friends and families to have a high
> probability of suffering because they see you die, but even this could be
> stated in terms of maximizing the subjective probability of happy outcomes
> for other people.
It seems that for QS to be an attractive option, you have to believe
that measure applies all the time, except when you die. What is the
justification for making an exception, when all the rest of the time
you act as if you believe in measure? You would take a good bet rather
than a bad bet, but if your death is involved you'll stop caring?