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.