Ice melts but does not spontaneously re-form on a warm day. Gases spread out instead of gathering themselves together again. Black holes grow as they absorb matter and light but never shrink. Blackboards are erased but our formulae never emerge from a sponge. Visser: ‘Yet at the microscopic level, the fundamental laws of physics generally allow processes to evolve both forwards and backwards in time. Explaining how this reversible microscopic world gives rise to the irreversible world we experience has been an unsolved puzzle since the 19th century.’
Molecular chaos
The aim of the project is to develop a unified understanding of irreversibility, entropy, randomness, and chaos. The researchers will approach the problem from three closely connected directions. Visser: ‘First, we will investigate how irreversible behaviour can emerge from complex microscopic dynamics. Using modern mathematics, we aim to make ideas such as “molecular chaos” more precise and to determine how microscopic randomness can lead to predictable large-scale behaviour.’
Second, the team will study irreversibility in the most extreme physical systems in the universe: black holes. Black holes behave in strikingly thermodynamic ways: they can be assigned entropy and temperature, while the area of their horizons can only increase. The project will investigate how this thermodynamic behaviour of black holes works both mathematically and physically, the latter especially when black holes are changing and out of equilibrium. The third strand examines the historical and philosophical foundations of concepts such as chaos, randomness, entropy and coarse-graining. A central question is why simplified, large-scale descriptions such as thermodynamics can nevertheless provide reliable, opbjective accounts of the physical world.
The new research programme, Origins of Irreversibility, brings together mathematics, theoretical physics, and history and philosophy of science at the Radboud Center for Natural Philosophy. This center, which started in 2023 and was officially opened in January 2025, is a joint initiative involving three of Radboud University’s research institutes: IIMAPP, ISiS en CHPS creating an interdisciplinary research environment in which mathematicians, physicists, historians and philosophers work closely together.