Einstein's Revolutionary Light--Quantum Hypothesis

David C. Cassidy

David C. Cassidy

Norbert Straumann

Einstein's revolutionary light quantum hypothesis of 1905 and his further contributions to quantum theory are reviewed.

Harald Weinfurter

Einstein is rightly famed for his revolutionary work on relativity. But he was also one of the founders of quantum physics and in 1905 became the first physicist to apply Max Planck's quantum hypothesis to light. Einstein realized that the quantum picture can be used to describe the photoelectric effect – that only light above a certain frequency can eject electrons from the surface of a metal.Indeed, it was mainly for deriving the law of the photoelectric effect that he was awarded the 1921 Nobel Prize for Physics.

John Lekner

In 1905 Einstein wrote five history-making papers (on the particle nature of light, Brownian motion, special relativity, energy and inertia, and on molecular dimensions). He was 26, married to Mileva Mari, with one-year-old child Hans Albert, and employed as Technical Expert (third class) by the Bern Patent Office. His relativity theory captured the public imagination, but his quantum ideas were even more revolutionary. His light quantum (now the ‘photon’) was rejected by Planck, who is often thought of as the originator of the concept, and by Millikan, whose 1916 photoelectric data agreed beautifully with Einstein’s predictions (‘a bold, not to say reckless, hypothesis of an electromagnetic light corpuscle ... which flies in the face of thoroughly established facts of interference’). Compton’s 1923 paper on the scattering of X-rays and gamma rays uses the light quantum idea, but makes no mention of Einstein. There followed more than 20 papers on various aspects of quantum theory, substantial enough to make Einstein one of the founders of quantum mechanics, possibly the founder. This paper traces the beginnings of quantum mechanics, giving an outline of Einstein’s contributions. It begins with a sketch of his early life.