Happy New Year and Goodwill to all FIS'ers and distinguished guests!
I found the concept of Quantum Bayesianism as presented by Professor von Baeyer
most interesting. From the point of view of bringing the subject-object balance
back into physics it is very congenial to Logic in Reality (LIR). I have
several criticisms of this approach, however, which I will try to make clear in
the absence of any real skills in quantum mechanics:
1. QBism seems not to consider the option of using non-standard,
non-Kolmogorivian probabilities to describe quantum and non-quantum nature,
that is, with values >0 but <1.
2. It excludes the case, impossible by classical logic, but basic to physics
and LIR, of a dynamic interaction between the subject and the object which
allows both views ("belief" and "facts") to be partly true or better operative
at the same time or at different times.
3. Since the QBism interpretation does not deal with points 1. and 2. above
(also in the Fuchs, Mermin, Shack paper), it leaves the door open to an
anti-realist interpretation not only of quantum mechanical reality, but of
reality /tout court/ which must be based on and reflect the quantum
'situation'.
I would welcome responses to the above that might help me and others understand
the scope of QBism and whether, as I hope, my LIR approach, which is based on
values like non-standard probabilities might actually supplement rather than
contradict it.
Best wishes,
Joseph
----- Original Message -----
From: Hans von Baeyer
To: fis@listas.unizar.es
Sent: Thursday, January 02, 2014 9:25 PM
Subject: [Fis] New Year Lecture
Quantum Bayesianism (QBism): An interpretation of quantum mechanics based on
quantum information theory
Hans Christian von Baeyer, Professor of Physics, emeritus
College of William and Mary, Williamsburg, Virginia
January 2014
I am honored and proud to be asked by Pedro to inaugurate the
tradition of “New Year Lecture” to the FIS community, in the spirit of the
Royal Institution’s “Christmas Lectures”, which have been presented in London
almost every year since 1825. Those shows were originally intended for a
“juvenile audience”, but have always captivated young and old alike. My
electronic lecture is not for children, but like many of its famous
predecessors it features a mind-boggling experiment. In spite of the scholarly
nature my topic – the interpretation of quantum mechanics – my principal
message is simple, and I hope relevant to our quest for the meaning of
information. I look forward to a lively discussion after my virtual lecture!
QBism (with a capital B) is a radical new interpretation of
quantum mechanics that resolves many of the paradoxes that have bedeviled the
theory since its invention. The technical successes of quantum theory are
unchanged and undisputed -- only the meaning of the formalism is re-appraised.
The revision has far-reaching implications for the scientific worldview in
general.
The crucial move for QBism, inspired by quantum information
theory, is very simple. It consists of revising the predominant interpretation
of probability. Most physicists accept the frequentist interpretation of
probability as “favorable outcomes/all possible outcomes”. Even though this
definition becomes rigorous only in the unrealistic limit of an infinite number
of trials, it is claimed to be objective. QBism is based instead on the older
Bayesian interpretation, which defines probability as “degree of belief.”
Specifically, the probability that an event will occur is an agent’s personal
assignment of betting odds for the occurrence of the event. It is based on all
the information available to the agent, and is explicitly subjective. Bayesian
probability, unlike frequentist probability, is meaningful for a single,
unrepeatable event.
Bayesianism is more general than frequentism. In many cases,
such as normal laboratory practice, Bayesian probability can be measured by
conventional frequentist procedures, but the meaning of the result remains
Bayesian. (Similarly, temperature is measured by a thermometer, but its meaning
runs much deeper.) Bayesianism thus absorbs the successes of frequentism.
By combining Bayesian probability with conventional quantum
mechanics, QBism locates the result of a calculation in the mind of the agent
who makes it. The Schrödinger wavefunction, which is a compendium of
information about a quantum system, and in turn yields probabilities for the
outcomes of future experiments, becomes subjective as well. Input for
assigning betting odds comes from the experiments the agent performs herself,
added to information she gathers from the written and oral records of science,
i.e. from the totality of her personal experiences. Since wavefunctions are
not real in this scheme, the problems associated with such phenomena as the
“collapse of the wavefunction” (when probability snaps into certainty as a
result of a measurement), Schrödinger’s cat, nonlocality, and Bell
inequalities, issues that were interminably debated during the twentieth
century, all dissolve.
The notorious problem of wavefunction collapse, for example,
which defies both mathematical description and the relativistic speed limit, is
interpreted as the modification of a probability assignment by a measurement.
It is a straightforward application of Bayes’ Law (also known as Bayes’ Theorem
or Rule) for updating a probability upon the acquisition of new information.
In this way QBism provides a natural and convincing explanation of the
mysterious collapse.
Apparent nonlocality is displayed most dramatically in an
experiment suggested in 1989 by Daniel Greenberger, Michael Horne, and Anton
Zeilinger (GHZ). The spin of a “spin 1/2 particle” (such as an electron) can
be measured along one axis at a time -- say pointing up or down (U/D) along the
z axis, or, alternatively, right or left (R/L) along the x axis. Three
identical particles are brought into close contact, and prepared in the special
GHZ configuration, in which they are said to be “entangled.” They are then
separated by large distances and it is found that whenever two of them point in
the same horizontal direction, the third one points UP. (DOWN, if the first two
point in opposite directions.) Thus LLU, RRU, RLD and LRD are found among the
measurement results, but LLD, RRD, RLU and LRU never occur. A mnemonic: If
your two index fingers point in the same horizontal direction, one thumb
(representing the third particle,) points up. If they point in opposite
horizontal directions the thumb points down. In short: thumbs UP for
agreement, thumbs DOWN for disagreement.
This configuration displays classical correlations, reminiscent
of two bar magnets which, when in contact, align north pole to south pole. If
the magnets are then separated without rotation, their orientations remain
correlated throughout their subsequent histories. Observing one instantly
reveals the direction of the other, regardless of their distance of separation.
The horizontal and vertical spin directions of particles in a GHZ
state could conceivably assume their values during preparation, and retain them
as they are separated. GHZ states have, in fact, been assembled, and their
properties have been confirmed experimentally. They are robust. Once two
horizontal spins have been measured, the third, vertical spin can be predicted
with certainty. Einstein would have considered it to be a “real” property of
the third particle, pre-existing any observation.
Quantum mechanics throws a spanner into the works. Suppose that
the experimenter assembles a GHZ state, but measures the first two spins along
the z axis, obtaining UU. What is the prediction for the third vertical spin?
In any triangular relationship, if two pairs agree, the third pair must agree
also (transitivity). Since all three horizontal spins (if they were measured)
would point in the same direction, the third spin should point UP, so the
classical, logical prediction for the allowed state is UUU. Quantum mechanics,
however, decrees that UUU is forbidden, and that the result is UUD instead.
Experiments confirm this seemingly bizarre prediction.
GHZ presents a choice between realism and locality, defined as
the absence of spooky action-at-a-distance. The paradox can be resolved in one
of two ways. Many physicists insist on realism – believing that theory should
describe nature as it really is. This implies that properties predictable with
certainty pre-exist the measurement – that they are carried along by the
particle. Some realistic interpretations of quantum mechanics require that the
Schrödinger wavefunction is nonlocal. In the case of GHZ this means that the
first two measurements yielding UU, even though they take place far from the
third particle, instantaneously influence its spin and constrain it to point
down.
The second option for GHZ is to give up realism in favor of
locality – the QBist approach. The wavefunction becomes a purely abstract
mathematical object designed to predict measurement outcomes. The unmeasured
horizontal spins cannot be invoked to make logical inferences: they have no
values at all unless they enter an agent’s experience. Once the first two
vertical spins are measured, they co-exist at one location – the agent’s mind –
where they become input information for the (correct) quantum mechanical
calculation.
Quoting from the second reference below: “[B]ecause everything
any of us knows about the world is constructed out of his or her individual
private experience, it can be unwise to rely on a picture of the physical world
from which personal experience is explicitly excluded, as it has been from
physical science. Schrödinger traces this exclusion back more than two
thousand years to the ancient Greeks. It worked for over two millennia and
played an important role in the construction of classical science.
But when we attempted to understand phenomena not directly
accessible to our senses, our ingrained practice of divorcing the objects of
our investigations from the subjective experiences they induce in us got us
into trouble. While our efforts at dealing with phenomena at these new scales
were spectacularly successful, we have just as spectacularly failed for almost
a century to reach agreement about the nature or meaning of that success.”
QBism, by placing human experience at the center of a new
worldview, suggests a way of “restoring the subject-object balance” to science.
My popular exposition of QBism was published in the June 2013
issue of SCIENTIFIC AMERICAN and some of its translations. A more
sophisticated but non-mathematical primer dated 20 November 2013, by
Christopher Fuchs, David Mermin, and Rüdiger Schack, is available gratis at
www.arxiv.org with the ID number 1311.5253v1.
HAPPY NEW YEAR 2014!
------------------------------------------------------------------------------
_______________________________________________
fis mailing list
fis@listas.unizar.es
https://webmail.unizar.es/cgi-bin/mailman/listinfo/fis
_______________________________________________
fis mailing list
fis@listas.unizar.es
https://webmail.unizar.es/cgi-bin/mailman/listinfo/fis
