This is a tentative theory of quantum measurement performed on particles with unspecified mass. For such a particle, the center of the wave packet undergoes a classical motion which is a precious guide to our approach. The framework is manifestly covariant and a priori nonlocal. It allows for describing an irreversible process which lasts during a nonvanishing lapse of time. The possibility to measure a dynamical variable in an arbitrary slate is discussed. Our picture is most satisfactory if we focus (...) on free particles and constants of the motion. Two-particle systems and measurement of individual observables in a correlated slate are considered. (shrink)

Directly interacting particles are considered in the multitime formalism of predictive relativistic mechanics. When the equations of motion leave a phase-space volume invariant, it turns out that the phase average of any first integral, covariantly defined as a flux across a 7n-dimensional surface, is conserved. The Hamiltonian case is discussed, a class of simple models is exhibited, and a tentative definition of equilibrium is proposed.

The open Nambu string is revisited in the spirit of an early approach by Rohrlich. Strictly timelike motions only are considered. The proper-time of the center-of-mass is taken as preferred parameter. We propose a canonical formalism in terms of a countable infinity of variables, among them the modes. But the barycentric coordinates have noncommuting components, which makes possible a consistent quantization (in any dimension, four in particular) within the framework of a transverse space of states. If a maximal number of (...) modes is further fixed, a transverse Hilbert space emerges, where spacetime displacements preserve the (positive-definite) scalar product. (shrink)