With his idea of light particles Einstein is able to explain certain experimental findings, such as the photoelectric effect. Nevertheless, his quantum hypothesis of light meets with widespread rejection, since it contradicts the mostly well-functioning wave theory of light. In 1913, Planck even finds it necessary to excuse Einstein for overshooting the mark with his quantum hypothesis.

Only in 1923 Compton’s explanation of the scattering of X-ray quanta from electrons brings the breakthrough. The idea is established that light has to be considered both as a wave and as a particle – the wave-particle dualism. In the same year, de Broglie demonstrates that, vice versa, one can also attribute wave properties to particles. One year later, Bose makes it clear that light quanta are not independent, classical particles. Einstein applies this insight to a material gas to describe the thermal properties of bodies near absolute zero temperature. However, a theory that fully harmonizes wave and particle properties remains elusive even then.