Bohr–Van Leeuwen theorem

The Bohr–Van Leeuwen theorem is a result in statistical mechanics that states that classical mechanics cannot provide a satisfactory explanation of certain magnetic phenomena, particularly the presence of diamagnetism in equilibrium systems. Specifically, the theorem asserts that in a classical system at thermal equilibrium, the average magnetic moment of an ensemble of particles, such as electrons, will be zero when the system is in a uniform magnetic field.