The Franck–Hertz experiment, a pillar of quantum mechanics, is presented to the German Physical Society.

The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms. It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz. Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapour of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity of its kinetic energy before flying away. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million metres per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.

Quantum mechanics is the study of matter and matter's interactions with energy on the scale of atomic and subatomic particles. By contrast, classical physics explains matter and energy only on a scale familiar to human experience, including the behavior of astronomical bodies such as the Moon. Classical physics is still used in much of modern science and technology. However, towards the end of the 19th century, scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. The desire to resolve inconsistencies between observed phenomena and classical theory led to a revolution in physics, a shift in the original scientific paradigm: the development of quantum mechanics.

The German Physical Society is the oldest organisation of physicists. DPG's 2025 worldwide membership is cited as 50,668, with a peak of 63,012 in 2014. It holds an annual conference and multiple spring conferences, which are held at various locations and along topical subjects of given sections of the DPG. The DPG concerns the fields of pure and applied physics and aims to foster connections among German physicists, as well as the exchange of ideas between its members and foreign colleagues. The bylaws of the DPG commit the organization and its members to maintain scientific integrity and ethics, including freedom, tolerance, truthfulness, and dignity in scientific work, as well as the promotion of gender equality in the fields of physics and related sciences.