Max Planck

Max Planck (1858 to 1947) was the German physicist credited as the originator of quantum theory. In 1900, to explain the spectrum of black-body radiation, he proposed that electromagnetic energy is emitted and absorbed only in discrete amounts, called quanta, with energy proportional to frequency. This hypothesis introduced the quantum into physics and defined Planck's constant, opening the field now called quantum mechanics.

#The black-body problem

A black body is an idealized object that absorbs all radiation falling on it and re-emits a characteristic spectrum that depends only on its temperature. By the late 1890s the measured spectrum defied classical physics: existing formulas matched the data at either long or short wavelengths, but not both. Planck first found an interpolation formula that fit the full curve, then sought a physical derivation for it (Nobel biographical).

#Energy quanta and E = h nu

Planck's derivation, presented to the German Physical Society on 14 December 1900, required that the energy of the oscillators emitting radiation be restricted to whole-number multiples of a small unit. That unit is E = h times the frequency, written E = h nu, where h is a new constant of nature now called Planck's constant, approximately 6.626 x 10^-34 joule seconds. The Nobel committee cited his 1918 prize "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta" (Nobel Prize facts). Planck's constant later became one of the defining constants of the International System of Units.

#Reluctance about the implications

Planck regarded quantization at first as a mathematical device rather than a statement about physical reality, and he was uneasy about abandoning classical continuity. He described the introduction of the quantum as an act of desperation to match the data, and for years he tried to reconcile his result with classical electrodynamics (Nobel lecture). The bolder reading, that light itself is quantized, came from Albert Einstein in 1905, whose light quanta (later the photon) explained the photoelectric effect. Planck accepted the wave and particle character of radiation only gradually.

#Significance

Planck's 1900 hypothesis is the conventional starting point of the history of quantum mechanics. It supplied the constant h that appears throughout the theory, from Einstein's photons to the uncertainty principle and the wave function. Planck won the Nobel Prize in Physics for 1918, was a central figure in German science, and lent his name to the Kaiser Wilhelm Society, later renamed the Max Planck Society. His discovery marked the boundary between classical physics and the quantum era that Niels Bohr, Werner Heisenberg, and Erwin Schrodinger would go on to formalize.

#History

Planck studied in Munich and Berlin, where he was influenced by Hermann von Helmholtz and Gustav Kirchhoff, and he held the chair of theoretical physics in Berlin from 1889. His later life was shadowed by personal loss and by the upheavals of two world wars; his son Erwin was executed in 1945 for involvement in the plot against Hitler. Planck died in 1947. The Planck spacecraft and the Planck units, natural units built from h, the speed of light, and the gravitational constant, both carry his name.