[ExI] Probability is "subjectively objective".

Lee Corbin lcorbin at rawbw.com
Tue Jul 15 03:13:08 UTC 2008


Bryan, on January 9, evidently posted the following (although
it never showed up in *my* inbox, and obody replied to Bryan's
email either, if you sort by thread):

> http://lists.extropy.org/pipermail/extropy-chat/2008-January/039920.html

Anyway, Bryan there quotes some guy who says

     The particles might then separate to even cosmological distances, but as soon as the spin of one
     particle is observed, the other particle must have the opposite spin, which means that the wave
     function has collapsed across those cosmological distances and caused the other particle to
     assume a predictable spin. If this occurs instantaneously, it would violate the limitation of
     the velocity of light in Special Relativity.

     This has now been shown to actually occur on the basis of Bell's Theorem (from John Bell,
     1928-1990), meaning that Quantum Mechanics does violate Special Relativity by allowing
     instantaneous interactions across even cosmological distances.

Evidently the author is not at all worried about the way the concept of "instantaneously"
clashes with special relativity. So far as I can see, you have to choose between the
mysterious and weird "collapse" theory (which does affect things "instantaneously"
as described) or many-worlds.  MWI has *no* problem reconciling these things.
(One copy of you ends up in a universe with a distant friend who reports
the same result you got, and the other copy of you ends up in the same 
universe with a copy of your friend, and once again *they* agree upon the
result. The basic mathematical operation of QM exhibits just these two universes,
not four.)

> John K Clark wrote:
>> knows. All I know is that if Copenhagen is right then a event
>> is frequent because it is probable and if Many Worlds is right
>> then a event is probable because it is frequent.

Not a bad way to put it.  What it sacrifices in accuracy is more
than made up for its brevity. 

Jaynes did manage to make a Bayesian out of me (albeit an
"objective Bayesian" as its called), and we have only the result
that probability is indeed subjective, which is the point of 
Eliezer's post (also being discussed in this thread).

However, Quantum Theory is still for me the one and only Great
Sticking Point to the whole thing.  For in any version of QM that
I know of, probability is quite objective. In MWI, there exists
a definite fraction of universes in which one carefully defined result
obtains, and that fraction is identical to the probability we calculate.
Thus that fraction is as objective as, say, the proportion of human beings
alive at this instant who have blood type O.  Even in the Copenhagen
interpretation, the probability is taken as objective as I understand it,
though since the Copenhagen interpretation is not coherent (given
its clash with special relativity), I can't really defend that claim.

Jaynes does have quite a bit to say about QM, but I've never
discovered exactly what he'd say about probability in QM,
and whether the fraction of worlds in which a certain outcome
obtains is somehow subjective.

Lee




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