# RE: PhD-thesis on Observational Selection Effects

From: Bostrom,N <N.Bostrom.domain.name.hidden>
Date: Mon, 7 Aug 2000 20:02:34 +0100

Nick Bostrom
Dept. Philosophy, Logic, and Scientific method
London School of Economics
Homepage: [I tried sending parts of this message before, but don't think it
worked, so I re-send. Nick.] <http://www.hedweb.com/nickb>

Jacques wrote:

So I take it you don't believe in "quantum immortality" or the "RSSA".

Given MWI, you know what happens if you commit suicide. You may affect an
increase in your average well-being at the cost of a decrease of your total
measure. (By "you" I here mean the totality of observer-moments that are
subjectively similar in certain ways to your present observer-moment). It is
then depends on your preferences whether suicide is rational. I believe most
people actually do care about their total measure. If qm suicide became
common, creatures would evolve to care about their total measure. Such
creatures would get a greater measure.

Suppose, without the MWI, he
would know that on a usual day a deer is 1% likely to turn
up. But since
this is not a usual day, but the day that will determine
whether he gives
rise to a race, he would think a deer probably will turn up
which then means
he won't reproduce. (And not reproducing would give his own
case more
effective probability, which is why he thinks it's likely.)

So far I'm with you...

If he believes the MWI however, then he knows there is a
branch where a
deer turns up and one where it doesn't. He knows that for a
usual day the
effective probability of a deer turning up is about 1%.
Since he has no
reason to believe the laws of nature are specially
configured to correlate
the amplitude of the wavefunction's branches with his
actions (especially
since such correlation would be hard to reconcile with QM),
he will still
believe the effective probability of a deer turning up is
just 1%.

You may be confusing subjective and objective probability. Let's say that
the subjective probability is 1%. This is compatible with the objective
probability being, for instance, 99.9%. Adam may be ignorant as to whether
the objective probability is 99.9% or 0.0001%, but his degrees of beliefs in
these two possibilities may be such that his subjective probability for deer
turning up is 1%. (Subjective probability can be set equal to the weighted
sum over the potential objective probabilities that the system may have (for
all you know) of producing a given outcome; the weights being given by your
subjective probability of each such objective chance being the actual
chance.)

Applying this to the present case: he may think that there is a 99%
probability that the objective probability of deer is 0.000001% ("no deer in
the neighbourhood) and a 1% probability of the objective probability being,
say, 99.9% (a deer is about to appear). After forming his reproductive
intention, the objective probability would remain unchanged, but Adam's
subjective probability that the objective chance is high (99.9%) would
increase dramatically (perhaps to 80%, just to pick a number), provided the
no outsider requirement is satisfied. This would lead to his subjective
probability of deer appearing also increasing.

In addition, in QM there will be many branches even
before that day. So
it's also true that branches will exist, with about equal
amplitude to his
own where e.g. maybe the weather was different and he chose
a different day
to go "deer hunting", where the Earth has two moons, or
indeed where the
first man's name was Jack, etc.
These sort or branches do act exactly like regular old
alien worlds for
this purpose. Thus even if someone hands him a spin 1/2
particle, state
unknown, he should not consider it just to have some
wavefunction which he
must also
consider the already parallel branches where the equivalent
particle has
some other wavefunction, and there are other versions of him
there to see
it.

If there are all these other fat branches in the world, then yes, I agree
with that. However, Adam and Eve were there from the beginning, before there
deer paths had begun to spread out much as a probability cloud over the
terrain. Or at least we can suppose they were - that's the nice thing about
thought experiments!

And I do find the premise that he can be certain that no
type of MWI can
be true hard to swallow.

He wouldn't have to be certain about that.

>Of course, everything I say in the thesis is compatible
with accepting R=Ru
>(i.e. setting the reference class equal to the class of all
observers)
>provided one is also willing to accept the Adam&Eve

You still haven't presented any way to determine the
proper "reference
class". I suggest you use the logical approach: include all
observers.

I don't see what's so particularly "logical" about that. In either case, we
have to deal with borderline cases and such, and make some seemingly rather
arbitrary postulations.

Nick Bostrom
Dept. Philosophy, Logic, and Scientific method
London School of Economics
Homepage: http://www.hedweb.com/nickb <http://www.hedweb.com/nickb>
Received on Mon Aug 07 2000 - 12:09:49 PDT

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