Planck, Max (Karl Ernst Ludwig) (1858-1947), a German theoretical physicist. He was awarded the 1918 Nobel Prize in physics for originating the quantum theory in 1900. In his work in quantum theory, Planck introduced a fundamental physical constant—Planck's constant—which scientists use in relating the energy of radiation to its wavelength or frequency. Planck's concepts, extended and applied by Albert Einstein, Niels Bohr, and others, revolutionized the science of physics.

Planck, the son of a professor of constitutional law, was born in Kiel, Germany. He studied mathematics and physics in the universities of Munich and Berlin. At Berlin, one of his teachers, Gustav Kirchhoff, encouraged his interest in thermodynamics, the science of heat. In 1885, Planck became professor of physics at the University of Kiel. Four years later, he became a physics professor at the University of Berlin.

Planck was appointed permanent secretary to the Prussian Academy of Science in 1912, and in 1930 became president of the Kaiser Wilhelm Society for the Advancement of Science, the highest academic post in Germany. During his last years, Planck turned to philosophical writing. He was an outspoken opponent of Nazi policies. He died in Göttingen, Germany, where after World War II the Kaiser Wilhelm Society was reorganized as the Max Planck Society for the Advancement of Science.

Planck's writings, translated into English, include: Introduction to Theoretical Physics (5 volumes, 1932-33); Where Is Science Going? (1932); Philosophy of Physics (1936); Treatise on Thermodynamics (3rd English revised edition, 1945); Scientific Autobiography and Other Papers (1949).