CONF: Being Bayesian in a Quantum World (Konstanz)

Veiko Palge Veiko.Palge at
Tue Jul 26 00:13:32 CEST 2005

Conference Announcement


1-5 August 2005, Konstanz, Germany

Organizers: Carlton M. Caves (University of New Mexico)
              Christopher A. Fuchs (Bell Labs, Lucent Technologies)
              Stephan Hartmann (London School of Economics, University of
              Veiko Palge (University of Konstanz)
              Ruediger Schack (Royal Holloway, University of London)

To be a Bayesian about probability theory is to accept that
probabilities, whenever used, represent subjective degrees of belief and
nothing else.  This is in distinction to the idea that probabilities
represent long-term frequencies or intrinsic, chancy propensities "out
there" in nature itself.  But how well does a subjective account of
probabilities mesh with the existence of quantum mechanics?  To accept
quantum mechanics is to accept the calculational apparatus of quantum
states and the Born rule for determining probabilities in a quantum
measurement.  If there were ever a place for probabilities to be
objective, one would think it ought to be here!  This raises the
question of whether Bayesianism and quantum mechanics are compatible at
all.  For the Bayesian, it suggests that perhaps we should rethink what
quantum mechanics is actually about!

There is no doubt that we live in a quantum world.  From transistors to
lasers to nuclear warheads, the evidence is all around us.  One could
take from this that the individual elements in the quantum formalism
give a mirror image of nature:  That is, that the wave function is so
successful as a calculational tool precisely because it represents an
element of reality.  A more Bayesian (or, at least, Bayesian-like)
perspective is that if a wave function generates probabilities, then
they too must be Bayesian degrees of belief, with all that such a
radical idea entails.  In particular, quantum probabilities have no
firmer hold on reality than the word "belief" in "degrees of belief"
already indicates.  From this perspective, the only sense in which the
quantum formalism mirrors nature is through the normative constraints it
places on gambling agents trying to navigate through the world.  To the
extent that an agent should use quantum mechanics for his uncertainty
accounting rather than some foil theory tells us something about the
world itself --- i.e. the world independent of the agent and his
particular beliefs at any moment.  The task of the quantum Bayesian is
to make this argument explicit and rigorous and to reap any benefit such
a clarification can give to philosophy and physical practice.

Hogwash or deep idea?  The time seems ripe for a discussion, pro and
con.  At this meeting, in the pleasant surroundings of Lake Konstanz, we
envision a roughly fifty-fifty mix of philosophers (who have thought
long and hard about probability and quantum foundations) and
quantum-information physicists (who have developed an impressive box of
mathematical tools for prying apart the probabilistic structure of
quantum mechanics) to set the tone.  The goal is to make real progress
on these issues through a complementarity of talents and some
good-hearted debate.  Might we better understand the power of quantum
computation through Bayesian techniques?  Does a Bayesian conception of
the quantum state really make "the measurement problem" go away?  How
secure is quantum cryptography really if a quantum state is "just" a
state of belief?  Less secure?  Or maybe more (honestly) secure?  Does a
Bayesian conception of the quantum state lead us closer to or further
away from the Copenhagen interpretation?  What about many-worlds? ...
And so the list of topics goes on.  We hope the discussions will go long
into the night and fill the lakeside walks.

For more information about the conference, which is generously supported
by the VolkswagenStiftung, visit

There is a small number of seats left for guests. If you'd like to
attend the conference, please contact Veiko Palge by email:
bbqw2005 at

Mail group "philphys"
ESF Network for Philosophical and
Foundational Problems of Modern Physics
Help & Archive:

More information about the PhilPhys mailing list