2014/1/18 Hans von Baeyer <[email protected]>
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
_______________________________________________
fis mailing list
[email protected]
https://webmail.unizar.es/cgi-bin/mailman/listinfo/fis
http://en.wikipedia.org/wiki/GHZ_experiment
GHZ experiments are a class of physics experiments that may be used to generate starkly contrasting
predictions from local hidden variable theory and quantum mechanical theory, and permit immediate
comparison with actual experimental results. A GHZ experiment is similar to a test of Bell's inequality,
except using three or more entangled particles, rather than two. With specific settings of GHZ experiments,
it is possible to demonstrate absolute contradictions between the predictions of local hidden variable theory
and those of quantum mechanics, whereas tests of Bell's inequality only demonstrate contradictions of a
statistical nature. The results of actual GHZ experiments agree with the predictions of quantum mechanics.
The GHZ experiments are named for Daniel M. Greenberger, Michael A. Horne, and Anton Zeilinger
(GHZ) who first analyzed certain measurements involving four observers. [1] and who subsequently
(together with Abner Shimony, upon a suggestion by David Mermin) applied their arguments to certain
measurements involving three observers.[2]