Dear Friends: In keeping with the message of my lecture, that knowledge of
the world is based on the ensemble of individual experiences, more than on
assumed objective, actual properties of an external reality, I will tell
you about my experiences of writing and discussing the New Year Lecture. I
enjoyed the entire process enormously, and wish once more to applaud Pedro
for inventing this new tradition!
Even as I started this email I learned something that piqued my interest.
Gregory Bateson was quoted: "Kant argued long ago that this piece of chalk
contains a million potential facts (Tatsachen) but that only a very few of
these become truly facts by affecting the behavior of entities capable of
responding to facts." Google.de informed me that Tatsache is probably an
18th century translation of the English "matter of fact". "Tat" is a deed,
a "factum", something done or performed, while "Sache" means a thing or a
matter. This tenuous etymology connects factuality with action rather than
with some intrinsic essence. Kant's words "affecting", "behavior" and
"responding" are QBist to the core. More and more I realize that philosophy
matters. Chris Fuchs, the chief spokesman for QBism, is among the rare
physicists who give credit to philosophers for the contributions they make
to natural science. In return he hopes that they will listen to physicists
who bring news from the furthest reaches of nature.
My most intense experience in connection with the New Year Lecture was the
writing of it. The first challenge was brevity: "The letter I have written
today is longer than usual because I lacked the time to make it shorter"
quipped Blaise Pascal. In order to introduce QBism to you, I had to explain
the Q and the B. How to do that within the allotted length? The
distinction between Bayesian and frequentist probability is an old subject
among mathematicians, so I was able to steal from them. ("Schreiben ist
Borgen", writing is borrowing, according to the aphorist G.C. Lichtenberg.)
But in order to talk about the Q, I had to show succinctly what's so
special about quantum mechanics. At this point I was considerably aided by
the GHZ prediction and its fairly recent corroboration, because, unlike all
previous experiments, GHZ is a one-shot deal, rather than a subtle
statistical effect. Like finding a single white raven to falsify the claim
that "all ravens are black." But even so, although I could easily
demonstrate the WRONG classical prediction, I was not able to show those of
you who are not trained in theoretical physics how the correct quantum
mechanical prediction for GHZ comes about. Unfortunately I would need a
semester for that! In any case, by keeping to the prescribed format of the
lecture, I was able to clarify my own thinking and to streamline my
presentation of the unfamiliar topic.
My timing was very fortunate in that two unusually accessible articles
about QBism appeared in November and December 2013 -- both available for
free at <arxiv.org>. (ID numbers 1311.5253v1 and 1312.7825.) What a
welcome coincidence! It reassured me that the topic I had chosen for my
lecture is emerging from its niche in quantum foundations research and
slowly seeping out into the broader community.
>From the subsequent discussion I discovered several important things that
are new to me. I learned that there is the possibility, by means on
non-Kolmogorovian probabilities, to avoid the troublesome certainty of
probability 0 and 1 -- in particular via Logic in Reality. I learned about
the interesting concept of "feed-forward", in contrast to feedback, which
corrects for disruptions of a system BEFORE the disrupting influence kicks
in. (In order to do that, the mechanism has to make use of an accurate
model of the system's performance, so that it can PREDICT how the system
will react. I think it's an exaggeration to call this maneuver "inverting
the cause-and-effect sequence", but it comes close.) I learned about
instrumentalism, and will try to understand how it relates to pragmatism.
I was surprised when the conversation on the list veered from probability
and epistemology to communication and information. But I shouldn't have
been. The QBist point of view divides science into two realms. On the one
hand each individual agent assembles the totality of her experiences
(experimenting, reading, talking, calculating...) into a web of probability
assignments that is as coherent and comprehensive as possible. That's the
easy part, and, as usual, physicists have picked it as their domain. But
the hard part is the effort of agents to correlate their private
experiences -- i.e. to communicate with each other in order to develop a
common scientific worldview. Agent A's description of an experience serves
as input for updating B's personal probability assignments via Bayes' law.
And this is done through language as well as math. Niels Bohr more clearly
than any of the other pioneers of quantum mechanics realized the importance
of language -- he was "steeped in language" in the apt phrase of one
biographer. He thought that language is necessary to relate the abstract,
quantum mechanical description of matter to everyday experiences of the
world. QBists would add that it also enables agents to relate to each
other.
So, my fellow agents, I hope that my lecture has given you a few tidbits of
new information to serve as input for updating some of the probability
estimates you use to make decisions on your own future action. By future
action I mean thinking, talking, reading, writing... Your emails have
certainly caused me to re-think! And, being fundamentally an optimist, I
hope that in infinitesimal ways our worldviews will converge, and improve,
and lead to a better world. That's my New Year's wish for FIS!
Sincerely,
Hans Christian von Baeyer
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