[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 poi
Re: [Fis] New Year Lecture
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 th
