Royal Norwegian Society of Sciences and Letters · CC BY-SA 4.0

Klein's name is attached to several load-bearing pieces of twentieth-century physics: the Klein-Gordon equation, part of the mathematical bedrock of relativistic quantum mechanics; the Kaluza-Klein theory, an early attempt to unify gravity and electromagnetism by proposing extra, curled-up spatial dimensions; and the Klein-Nishina formula describing how photons scatter off electrons, still used in quantum electrodynamics.