At the same time, investigations of black-body radiation carried out over four decades (1860–1900) by various researchers culminated in Max Planck 's hypothesis that the energy of any system that absorbs or emits electromagnetic radiation of frequency ν is an integer multiple of an energy quantum E = hν . As shown by Albert Einstein, some form of energy quantization must be assumed to account for the thermal equilibrium observed between matter and electromagnetic radiation ; for this explanation of the photoelectric effect, Einstein received the 1921 Nobel Prize in physics.