Gustav Kirchhoff is Born

March 12th, 1824

(Note: While the image is AI generated, the research and writing was carried out by a human, me).

Today 200 years ago, the German physicist and mathematician Gustav Kirchhoff was born in Königsberg, Prussia, which is today the Russian exclave Kaliningrad. Despite an unknown disability that confined him to crutches and wheelchairs for most his life, he made major contributions to the understanding of electrical circuits, radiation theory and spectroscopy. His contributions were rewarded with several prizes and memberships to learned societies. He died in 1887 in Berlin, after a bout of fever.

Students of the second semester intro physics course will likely encounter the “Kirchhoff’s laws” of electric circuits, but there is actually another “Kirchhoff’s law” which may be considered to be a key stepping stone to the discovery of our most fundamental theory of reality at the microscopic scale, quantum theory. He formulated it in 1859, the same year Darwin published his “On the Origin of Species”. Fortunately, the law is rather simple to describe, as it just equates two ratios.

The ratio of the radiation an object absorbs to the total amount incident on it is called its absorbtivity. The ratio of the radiation an object emits to the amount a perfect emitter of same shape-what Kirchhoff called a “blackbody”-would emit is called its emissivity. Kirchhoff’s law simply says that for any body receiving thermal radiation while in thermodynamic equilibrium, absorbtivity equals emissivity. There was no known single function of wavelength and temperature that described the emission behavior of a blackbody across the radiation spectrum, but Kirchhoff hypothesized that such a universal function exists. Forty years later, Max Planck found it, and in the process discovered that the equation requires a constant. This would turn out to be the central constant of quantum theory and was named in his honor Planck’s constant.