Though Planck spent much of his time after the war on administrative matters, he still turned a great deal of attention to the new field in physics: quantum mechanics. And there was certainly a lot to think about, as the best and brightest of European physics worked to figure out the quantum puzzle. And, with German scientists at the forefront, this exciting new field seemed the best way for Germany to regain its former position as the leader of physics communities.
In the years just after World War I the central issue confronting physicists was one that stretched back to the startling results of Planck's work on blackbody radiation: was light a particle or a wave? Scientists in Copenhagen and at the University of Göttingen worked diligently at the problem, and, by combining their efforts, they were eventually able to reach a workable solution.
In 1926, physicist Erwin Schrödinger came up with a theory that seemed to reconcile the bizarre data of quantum theory with the classical understanding of wave mechanics. He discovered an equation for a wave, which he thought of as a wave function showing the distribution of electricity in an atom. This direction of thinking greatly pleased old guard physicists such as Albert Einstein, who were reluctant to throw out a century's worth of beliefs about light and energy. But the other quantum physicists were less impressed.
They put forward a new theory about Schrödinger's wave function, describing it as a probability wave – a wave measuring the probability that an electron will occupy a certain place and certain state at any given time. Planck despised the idea, and he refused to accept a worldview that involved never knowing anything for certain about the makeup of atoms. For Planck, the entire goal of physics was to completely dissect the workings of the universe, and the idea that some things were just unknowable was completely repugnant to him, and reminiscent of the positivism that he had attacked so passionately in years past.
To Planck's dismay, the quantum physicists continued their inexorable march toward the concept that the universe was, at its core, unknowable. Things came to a head when the quantum physicists working in Copenhagen put forward what would come to be known as the Copenhagen interpretation of quantum physics; by 1927, it had become the definitive response to the wave/particle question.
As formulated by Werner Heisenberg and Niels Bohr, the Cophenhagen interpretation had two parts. The first was Heisenberg's uncertainty principle, which dictated that it was impossible for an observer to know both the exact position and the exact momentum of a subatomic particle. The more exact your measurement was of one, the less exact it would be of the other. Bohr's contribution of the principle of complementarity, which sidestepped the question of whether light was a particle or a wave. According to Bohr, it was both: light was a particle or a wave, depending on how the observer measured it.
Basically, Bohr argued that the experimenter was an inextricable part of the experiment and that the method he chose for measuring a phenomenon would determine the results. Asking whether light "is" a particle or a wave is a useless question–physics, Bohr claimed, is about the results of experiments, not about the way things really "are." This philosophy represented everything Planck had always stood against in physics; if Bohr was right, there was no such thing as an objective reality independent of the observer. Planck felt that the Copenhagen interpretation was nothing more than a way of giving up on the real questions and, essentially, was a scientific expression of the pessimism rampant in German life.
Planck was not the only one to dislike the new trend in physics. Einstein detested the Copenhagen interpretation, calling it "the Heisenberg-Bohr tranquilizing philosophy, or religion." Indeed, scientists like Einstein, Schrödinger, and Planck hoped that the Copenhagen interpretation was just a detour on the way to a true understanding of quantum mechanics. But because they had hope for the future, none of these scientists gave up on quantum mechanics. And though he disagreed with their results, Planck held fast to his belief that supporting the quantum physicists would bring prestige to German science and to Germany itself.
For a time, he was right. No one could deny the remarkable contributions that German scientists had made to the new field. German science seemed to be on the right track. But once again, politics was about to overtake science. Once again, Planck would have to fight for his field's survival, though it would no longer be so clear, to him or to anyone else, whom he was fighting and whom he was protecting. In 1933, the Nazi party came to power, and Germany turned against itself.