Onsagers

Onsagers is a surname most closely associated with Lars Onsager (1903–1976), a Danish‑born American physical chemist and theoretical physicist whose work laid foundational stones in non‑equilibrium thermodynamics and statistical mechanics. His name is linked to several pivotal concepts and results that continue to influence physics, chemistry, and mathematical physics.

Born in Copenhagen, Onsager studied chemistry at the University of Copenhagen and conducted research across Europe before emigrating to the United States in the 1930s. He joined Yale University as a professor of theoretical chemistry in 1940, where he remained for much of his career and became a naturalized American citizen. He died in Coral Gables, Florida, in 1976.

Onsager’s most celebrated achievement is the reciprocal relations for near‑equilibrium irreversible processes, published in 1944. The Onsager reciprocal relations show that the matrix relating thermodynamic fluxes to generalized forces is symmetric under microscopic reversibility, providing a general framework for transport phenomena such as diffusion, heat conduction, and chemical reactions. In the same era, he obtained the exact solution of the two‑dimensional Ising model in zero external field, a landmark result in statistical mechanics that influenced subsequent developments in critical phenomena.

In 1968, Onsager was awarded the Nobel Prize in Chemistry for his discovery of the reciprocal relations in irreversible processes. His work also inspired the so‑called Onsager algebra, a mathematical structure appearing in integrable systems and related areas of mathematical physics. Onsager’s contributions continue to underpin modern approaches to transport theory, phase transitions, and exactly solvable models.