Paul Kwiat: "the physical reality of an experiment is affected by the
knowledge of the experimenter - or more precisely, by what can in principle
be known. This inextricable link between reality and information..."The inextricable link exists between facts and probabilities of possible
facts. The knowledge of experimenters is based on facts (specifically,
actual events or states of affairs indicating the outcomes of
measurements). The outcome of a measurement allows experimenters to assign
probabilities to the possible outcomes of other possible measurements,
regardless of temporal order and of whether measurements are performed in
timelike or spacelike separation. It is facts, rather than anyone's
knowledge, that affects, and it is probability assignments, rather
than "the physical reality of an experiment" that is affected. I wish the
frequent but wholly gratuitous and seriously misleading reference
to "knowledge" or "information" would be avoided wherever it can be
avoided. It also doesn't help to blur the difference between probability
assignments and physical reality.
PK: "Quantum information, in the end, describes not only what can be known,
but the subtle effect that knowing has on nature."
"What can be known" is ambiguous. It could mean (1) what can be known by
merely looking at a macroscopic pointer or hearing a click, (2) what can be
known by supplementing the experimental apparatus that is already in place,
or (3) by using the appropriate apparatus. In the experiment of Englert,
Scully, and Walther (ESW, see my posting of 28 Feb) the slit taken by an
atom can be known in sense 2 as long as the shutters separating the two
cavities remain closed, by inserting a photosensor in each cavity. When the
shutters are opened (exemplifying sense 3), the phase relation with which
the atom went through both slits can be known in sense 2, by inserting a
photosensor in the appropriate place. This illustrates that what has a
subtle effect on nature is not knowing per se but the facts on which
quantum-mechanical probability assignments are based. (I do not deny that
these probability assignments reflect or imply subtle effects on nature.)
PK: "in-principle knowledge of a quantum system can alter the observed
behavior of that system."
Again, "in-principle knowledge" is a seriously misleading terminology. What
can alter the behavior of the ESW atom is such a modification of the setup
as opening the shutters between the cavities and replacing the photosensors
in each cavity (indicating the slit taken by an atom) by a photosensor in
the center of the joint cavities (indicating the phase relation). And what
is altered is the behavior that is indicated by detectors, whether or not
this is observed by an experimenter.
PK: "by throwing away information in a particular way, we are then able to
alter the quantum mechanical outcomes."
We don't throw away information that we have. We only "throw away" the
possibility of obtaining a certain kind of information, by substituting one
setup for another. What matters is not information but the setup that is in
place.
Ulrich Mohrhoff
Sri Aurobindo International Centre of Education
Pondicherry 605002 Indiahttp://members.tripod.com/ujmvjm/ujm.htm
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