The next contradiction that physicists faced was between quantum mechanics (which had been developed over the thirty years following Planck's seminal insight) and the special theory of relativity. Most of the work in quantum mechanics was in the Galilean (or non-relativistic) approximation.

To be sure, Dirac had developed a relativistic wave equation for the electron, which was an important advance, but there was still a basic contradiction that needed to be resolved. The new feature that is required in a successful union of quantum mechanics and special relativity is the possibility of the creation and annihilation of quanta (or `particles'). The non-relativistic theory does not have this feature. .