All together the development of quantum physics can be roughly divided into three phases: In the early phase, no complete theory was available. Instead, there was a mixture of models which combined new elements with classical results. This early phase began in 1900 with Planck’s quantum hypothesis and ended around 1925. Thereafter, a phase of breakthroughs and the establishment of a consistent new theory began. This was quantum mechanics in the modern sense, and a corresponding mathematical formalism was developed in parallel. This phase ended around 1935 with Bohr’s answer to the challenge presented by the EPR thought experiment. This marked also the tentative end of the philosophical interpretation debate around the newly developed theory, which had continued since the middle of the 1920s, after the Copenhagen interpretation, propagated by Bohr, Heisenberg, and other leading quantum physicists, was accepted by the majority of physicists—at least for the time being, but in fact continuing up to the present. Since the end of the 1930s, or at the latest in the course of the 1940s, the evolution of quantum mechanics began a third phase. It is characterized, on the one hand, by newer theoretical developments such as relativistic quantum mechanics, quantum field theory and quantum gravity, but on the other, also by important experimental verifications of the basic assumptions of quantum mechanics, by the development of innovative modern fields such as quantum information theory, and finally by the establishment of alternative approaches to the interpretation of quantum mechanics. (ibid, Chap 7)
