Anti-Semitism  -   · The hatred of Jews. Anti-Semitism was common in Germany even before the 1930s, but it increased as economic conditions grew worse and as the German people grew more discontented and looked for someone to blame. Adolf Hitler capitalized on this rising wave of anti-Semitism, riding it to power. Once Hitler was in power, anti-Semitism became a government policy, and Jews across the country were driven by their jobs and persecuted by their neighbors.

Appeal of the Ninety-Three Intellectuals -   · A manifesto signed by ninety-three German intellectuals in support of German military action in World War I. Many of these academics were so eager to announce their support of the fatherland that they signed without reading it. It was a move they came to regret. After the war, the German intellectual community was shunned by many of their European colleagues, in part due to their signatures on this type of document.

Blackbody Radiation  -   · A blackbody is a black box that absorbs all the radiation directed at it. The amount of energy it emits is independent of the size or shape of the box and depends only on temperature. The concept was introduced by Robert Kirchhoff in 1859. For decades afterward, physicists struggled with trying to figure out the spectrum of the energy emitted by the blackbody. But the problem remained unsolved until 1900, when Planck finally came up with a working formula.

Copenhagen Interpretation  -   · The interpretation of quantum physics that came to be accepted in the late 1920s. Planck disliked the theory, believing that it was a way of giving up on the important questions. If the Copenhagen Interpretation was correct, Planck reasoned, it would mean that scientists could never learn any objective facts about the universe.

E = hv -   · In this equation that made Planck famous, E stands for energy, v stands for frequency, and h is a constant, now known as Planck's constant. The equation defines the relationship between energy and frequency in a blackbody. The radical implication of this equation was that light came in finite packets, multiples of hv.

International Research Council -   · A scientific organization founded in 1919, which was designed to shut German scientists out of the international scientific community. Planck and his colleagues battled to get German scientists admitted to the organization's meetings, projects, and publications. They eventually succeeded.

Positivism  -   ·  A philosophy of science asserting that scientific truths can only come from and refer to scientists' direct experience with the world. According to the positivists, there is no objective reality, no deeper truth. Fact can only come from direct, subjective observation.

Quantum Physics  -   · The new system of physics created at the beginning of the twentieth century to explain the bizarre, counterintuitive behavior of light and subatomic particles. Physicists realized that activity on this small scale could not be described using physical models from the larger world, and they were forced to cobble together a new understanding of the way the basic particles of matter interact. The creation of quantum physics constituted an exciting revolution in the way people studied and applied physics.

Solvay Conference -   · A series of conferences about the problems of quantum physics that attracted top physicists from all over Europe. Planck was invited to the 1927 conference, even though his most productive years were behind him. The invitation was symbolic of his position as the father of quantum physics and of a new spirit of tolerance and acceptance toward the German scientists.

University of Göttingen -   ·  The German center of the quantum physics revolution.

Niels Bohr  -  Bohr's theory of complementarity was one of two components of the Copenhagen interpretation of quantum physics. It proposed that science will never be able to answer the question of whether light "is" a particle or a wave. Science is about the results of experiments, not about the way things really are, and the design of the experiment dictated whether the light would be measured as a particle or a wave.

Albert Einstein  -  The most famous scientist of the twentieth century. In 1905, Einstein introduced the theory of relativity, which overturned the classic Newtonian physics that had dominated the field for hundreds of years. This was the same year he published his paper on blackbody radiation, which reinterpreted Planck's famous equation as evidence that light was emitted in quanta, rather than waves. Einstein, a Jew, left Germany when the Nazis came to power and moved to the United States, where he lived out the rest of his life. For more information, see SparkNote on Albert Einstein.

Werner Heisenberg  -  Heisenberg's Uncertainty Principle was one of two components of the Copenhagen interpretation of quantum physics. It asserted that it was impossible for an observer to know both the exact position and the exact momentum of a subatomic particle. Heisenberg, like Planck, also stayed in Germany throughout World War II. After the war, some doubt remained as to whether Heisenberg had actively helped the Nazis develop an atomic bomb.

Adolf Hitler  -  The leader of Germany's national socialist workers' (Nazi) party. In 1933, Hitler was elected the Chancellor of the German Reich. He carried out a ruthless campaign against the German Jews, culminating in the Holocaust. His hunger for power and territorial conquest led the country into World War II.

Marga Von Hoesslin  -  Planck's second wife. Planck married Marga Von Hoesslin a year after his first wife died. Planck and Marga were married for twenty-three years, and they raised four children together.

Philipp Lenard  -  The author of Deutsche Physik, a work that proclaimed the superiority of German science. By German science, Lenard meant Aryan science–he claimed that relativity and quantum physics were perverse, non-Aryan creations, part of an international Jewish conspiracy led by men such as Einstein.

Erwin Planck  -  Planck's longest surviving son. Planck's other three children all died within a year after World War I. In 1945, Erwin Planck was executed for conspiring to assassinate Hitler.

Wilhelm Wien  -  A German physicist working on the blackbody radiation problem at the end of the nineteenth century. Wien developed a formula for the energy distribution that seemed to work but was eventually proven incorrect. Planck and Wien remained lifelong friends.