The linearity of quantum mechanics leads, under the assumption that the wave function offers a complete description of reality, to grotesque situations famously known as Schrödinger's cat. Ways out are either adding elements of reality or replacing the linear evolution by a nonlinear one. Models of spontaneous wave function collapses took the latter path. The way such models are constructed leaves the question of whether such models are in some sense unique, i.e., whether the nonlinear equations replacing Schrödinger's equation are uniquely determined as collapse equations. Various people worked on identifying the class of nonlinear modifications of the Schrödinger equation, compatible with general physical requirements. Here we identify the most general class of continuous wave function evolutions under the assumption of no-faster-than-light signaling.

What is the right statistics for the measurements of arrival times of a quantum particle? Although this question is very old, it is still open. The usual experiments are performed in the far-field regime and this question becomes unimportant, as a semiclassical analysis suffices. Nevertheless, the development in the detector technology will soon allow for near-field investigations; thus, a better understanding of arrival time measurements is needed. Since outcomes of quantum measurements are always described by positive-operator-valued measures (POVMs), various arrival time POVMs have been proposed. On the other hand, many physicists would agree that the arrival time statistics is given by the quantum flux. This urges the question whether a POVM exists, which agrees approximately with the quantum flux values on a reasonable set of wave functions. We answer this question negatively for a very natural set of wave functions, but we remark that the answer is very sensitive to the choice of the set and provide evidence for the existence of a POVM that agrees with the quantum flux on a more restrictive set.

Abstract. Due to the finite speed of light, direct electrodynamic interaction between point charges can naturally be described by a system of ordinary differential equations involving delays. As electrodynamics is time-symmetric, these delays appear as time-like retarded as well as advanced arguments in the fundamental equations of motion -the so-called Fokker-Schwarzschild-Tetrode (FST) equations. However, for special initial conditions breaking the time-symmetry, effective equations can be derived which are purely retarded. Dropping radiation terms, which in many situations are very small, the latter equations are called Synge equations. In both cases, few mathematical results are available on existence of solutions, and even fewer on uniqueness. We investigate the situation of two like point-charges in 3 + 1 space-time dimensions restricted to motion on a straight line. We give a priori estimates on the asymptotic motion and, using a Leray-Schauder argument, prove: 1) Existence of solutions to the FST equations on the future or past half-line given finite trajectory strips; 2) Global existence of the Synge equations for Newtonian Cauchy data; 3) Global existence of a FST toy model that involves advanced and retarded terms. Furthermore, we give a sufficient criterion that uniquely distinguishes solutions by means of finite trajectory strips.

We consider the relation between so called continuous localization models -i.e. non-linear stochastic Schrödinger evolutions -and the discrete GRW-model of wave function collapse. The former can be understood as scaling limit of the GRW process. The proof relies on a stochastic Trotter formula , which is of interest in its own right. Our Trotter formula also allows to complement results on existence theory of stochastic Schrödinger evolutions by Holevo and Mora/Rebolledo.

Abstract. The direct interaction theory of electromagnetism, also known as WheelerFeynman electrodynamics, is often misinterpreted and found unappealing because of its reference to the absorber and, more importantly, to the so-called absorber condition. Here we remark that the absorber condition is indeed questionable and presumably not relevant for the explanation of irreversible radiation phenomena in our universe. What is relevant and deserves further scrutiny is the emergent effective description of a test particle in an environment. We therefore rephrase what we consider the relevant calculation by Wheeler and Feynman and comment on the status of the theory.