From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork, cobbled together by incorporating some quantum ideas into essentially classical physics. The work of Planck, Einstein, Bohr, and Sommerfeld can be bracketed as old quantum theory. This classification is not meant to undermine the tremendous achievement that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which emerged from the work of Heisenberg, Schrodinger, Born, and Dirac. The development of the photon concept is a key example of a radically new idea gradually gaining acceptance. The process works via a growing realization that more and more things can be explained in the context of the photon model.
Now that we have reached the end of our brief tour through the early history of quantum theory, we need to stop and consider some important themes. From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork cobbled together from classical physics with a few quantum ideas thrown in. e works of Planck, Einstein, Bohr, and Sommerfeld can be classed as old quantum theory. is is not meant to undermine the tremendous achievements that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which slowly emerged from the works of Heisenberg, Schrödinger, Born, and Dirac.