As early as 1930, Oppenheimer wrote a paper that essentially predicted the existence of the positron. This was after a paper by Dirac proposed that electrons could have both a positive charge and negative energy. Dirac's paper introduced an equation, later known as the Dirac equation, that unified quantum mechanics, special relativity and the then-new concept of electron spin, to explain the Zeeman effect. Drawing on the body of experimental evidence, Oppenheimer rejected the idea that the predicted positively charged electrons were protons. He argued that they would have to have the same mass as an electron, whereas experiments showed that protons were much heavier than electrons. Two years later, Carl David Anderson discovered the positron, for which he received the 1936 Nobel Prize in Physics.