Light and Matter Group

Any proposed solution to the "screen problem" in quantum mechanics—the challenge of predicting the joint distribution of particle arrival times and impact positions—must align with the extensive data obtained from scattering experiments. In this paper, we conduct a direct consistency check of the Absorbing Boundary Condition (ABC) proposal, a prominent approach to address the screen problem, against the predictions derived from scattering theory (ST). Through a series of exactly solvable one- and two-dimensional examples, we demonstrate that the ABC proposal's predictions are in tension with the well-established results of ST. Specifically, it predicts sharp momentum- and screen-orientation-dependent detection probabilities, along with secondary reflections that contradict existing experimental data. We conclude that while it remains possible that physical detectors described by the ABC proposal could be found in the future, the proposal is empirically inadequate as a general solution to the screen problem, as it is inconsistent with the behavior of detectors in standard experimental settings. [To appear in Phys. Rev. A; arXiv:2509.07518.]

In recent years there has been quite some progress in understanding the effective descriptions of interacting many body systems. While finding analytical or numerical solutions for interacting systems of many particles is in many cases impossible with given techniques, physicists use effective, simplified descriptions to describe the main features of the systems. These effective descriptions significantly reduce the complexity of the system by considering only a selected limited number of the degrees of freedom of the system - the macro-variables of the system.

In the talk the most important steps in the derivation of some selected effective equations from microscopic principles will be given. A special emphasis will be the derivation of a time-irreversible macro-dynamics from time-reversible microscopic equations.

There is a lot of confusion about the entropy of gravitating systems. It is often said that the Boltzmann entropy of a classical gravitating system is infinite or not well-defined. In a different vein it is said, and this is presented as a puzzle, that, for a gas in a box, a state of high entropy should be a homogenous state while, for a gravitating system, it should the other way round, a homogenous state being a state of low entropy. We show that both problems can be resolved if one is ready to adapt the notion of the Boltzmann entropy to the context of gravity. To motivate this step, we study the similarities and differences between the Newtonian gravitational N-body system (NBS) and an ideal gas in a box (GB). We explain why a sensible definition of ‘gravitational entropy’ involves an adaption of the Boltzmannian macrovariables. This does not only lead to a well-defined, finite notion of entropy, but it also shows that entropy increases as the N-body system expands while clusters/galaxies form. This last result corroborates Penrose’s conjecture about the long-time behaviour of the entropy of gravitating systems.