Soviet physicist Lev Landau publishes "On the Theory of Phase Transitions", formulating a general theory of second-order phase transitions based on the concept of an order parameter: a quantity that is zero in the disordered phase and non-zero in the ordered phase, associated with spontaneous symmetry breaking. The theory does not depend on the system's microscopic details, allowing it to be applied equally to ferromagnetism, superconductivity (via the later 1950 Ginzburg-Landau theory), and, decades later, to completely different systems such as spin glasses or even particle physics. It is one of the most influential and enduring conceptual frameworks of 20th-century theoretical physics.