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.