[ExI] Probability is in the Mind

scerir scerir at libero.it
Thu Mar 13 16:51:00 UTC 2008


Yakir Aharonov writes:
"It is a quite trivial fact that if we acquire
new information we can affect the probabilities of events
that happened in the past. This happens not only in
quantum mechanics but in ordinary classical probabilities
as well. For example suppose we have a bag with an equal
number of white and black balls and extract one ball at
random and put it, without looking, into a bag containing
only black balls. The probability that the ball is white
is 1/2. But suppose we than extract a ball from the
second bag and see that the ball is white. In the light
of this new information we can now infer that in this
situation the probability that a white ball was extracted
from the first bag is actually 1 and not 1/2. The future
information affects our knowledge about the past, but
there is nothing surprising about this."

It is perhaps interesting to point out that Aharonov (et al.)
developed a new quantum-mechanical formalism (ABL formalism). 
According to this two-state time-symmetrical formalism a |ket> 
vector originates at t1, and it is determined by a time 
boundary condition in the past and evolves toward the future,
while a <bra| vector originates at t2, and it is determined by 
a time boundary condition in the future and evolves toward 
the past.

Now, there is no problem if these vectors only represent
probabilities (or probability amplitudes). There is
a deep epistemological issue if we interpret these 
vectors as having *objective* *physical* meaning, although
in the quantum domain. Because, i.e., if the <bra| has 
an *objective* meaning, than we have to admit that it really 
propagates backwards in time. 

For more, see the last two (readable) pages of this paper
http://arxiv.org/PS_cache/arxiv/pdf/0712/0712.0320v1.pdf






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