In 1995 the very successful interdisciplinary meeting Quantum Theory Without Observers was held at the ZiF (Zentrum für interdisziplinäre Forschung) in Bielefeld. The title refers to those quantum theories in which observers and measurement devices or, more to the point, the notions of observable and measurement, are neither part of nor crucial for the very formulation of the theory. The most famous and established examples of such theories are Bohmian mechanics and spontaneous localization models, as well as decoherent histories. In the earlier meeting these theories were thoroughly discussed, but mostly in the framework of Galilean space-time (the nonrelativistic case). While Bohmian mechanics and spontaneous localisation models work well and are more or less completely understood in the nonrelativistic case, the true challenge is to devise extensions of these models to relativistic space-time. The challenge of constructing a quantum field theory (the name usually given to relativistic quantum theory), which has taken place over the last 70 years, produced expressions thought to be valid as asymptotic formulas. The S-matrix formalism is the generic example. The true challenge, however, is to answer the question: Of which theoretical physical assertions are these expressions the asymptotics? What, in other words, is the underlying theory?
A common response of physicists has been to accept the asymptotic expressions as "all there is to physics," similar to their strange complacency with regard to the measurement problem of orthodox quantum theory, which they seem to regard simultaneously as an unsolvable puzzle of nature and as lying outside the domain of science. Bohmian mechanics and spontaneous localisation models clearly show that this view about the measurement problem is utterly without merit.
The problem of formulating a relativistic "quantum physics without observers" has been recently revived. One of the dominating difficulties is that of merging nonlocality with relativistic space time. This problem appears in various guises, arising either from Bell's inequalities or in more orthodox terms from the instantaneous collapse of the wavefunction. While these are by now well understood in nonrelativistic quantum theory, for relativistic quantum physics they present new and difficult challenges.
A further problem is of course the notorious plague of infinities, which is usually cured by renormalization. This will not be the main focus of the coming meeting. The complex problem of constructing relativistic quantum theories can be separated into various parts. The above mentioned problem of nonlocality, which is a necessary aspect of any relativistic quantum theory but which has so far been largely ignored, will be our primary concern.
The planned meeting Quantum Theory Without Observers II has two intentions: