Hans van Leeuwen (physicist)

Hans van Leeuwen (1915-2001) was a Dutch theoretical physicist known for his contributions to the field of plasma physics and statistical mechanics. He is particularly recognized for the van Leeuwen theorem, a crucial result in classical statistical mechanics.

Van Leeuwen Theorem: The van Leeuwen theorem states that classical statistical mechanics cannot explain diamagnetism or paramagnetism. In other words, the magnetic susceptibility of a classical system in thermal equilibrium is always zero. This theorem arises from the fact that in a classical system, the magnetic field only enters the Hamiltonian through the Lorentz force, which does not affect the canonical distribution due to cancellations in the integration over velocities. This theorem highlights the necessity of quantum mechanics to explain magnetic phenomena in materials.

Career: Van Leeuwen held a position at the Delft University of Technology. His research focused on applying statistical mechanics to understand the behavior of plasmas and other many-body systems. His work was instrumental in shaping the understanding of classical limitations in describing magnetic properties and paved the way for understanding the quantum mechanical origins of magnetism.

Legacy: While the van Leeuwen theorem demonstrates a limitation of classical physics, it is fundamental to understanding the importance of quantum mechanics in explaining magnetism. It is widely taught and cited in textbooks on statistical mechanics and condensed matter physics.