Planck , Max Karl Ernst Ludwig

(1858–1947) German physicist

Planck was born at Kiel in Germany, where his father was a professor of civil law at the university. He was educated at the universities of Berlin and Munich where he obtained his doctorate in 1880. He began his teaching career at the University of Kiel moving to Berlin in 1889 and being appointed (1892) professor of theoretical physics, a post he held until his retirement in 1928.

Although Planck's early work was in thermodynamics, in 1900 he published a paper, Zur Theorie der Gesetzes der Energieverteilung im Normal-Spektrum (On The Theory of the Law of Energy Distribution in the Continuous Spectrum), which ranks him with Albert Einstein as one of the two founders of 20th-century physics. It is from this paper that quantum theory originated.

A major problem in physics at the end of the 19th century lay in explaining the radiation given off by a hot body. It was known that the intensity of such radiation increased with wavelength up to a maximum value and then fell off with increasing wavelength. It was also known that the radiation was produced by vibrations of the atoms in the body. For a perfect emitter (a so-called black body, which emits and absorbs at all wavelengths) it should have been possible to use thermodynamics to give a theoretical expression for black-body radiation. Various ‘radiation laws’ were derived. Thus Wilhelm Wien in 1896 derived a law that applied only at short wavelengths. Lord Rayleigh and James Jeans produced a law applying at long wavelengths, but predicting that the body should have a massive emission of short-wavelength energy – the so-called ‘ultraviolet catastrophe’.

Planck's problem was initially a technical one; he was simply searching for an equation that would allow the emission of radiation of all wavelengths by a hot body to be correctly described. He hit upon the idea of correlating the entropy of the oscillator with its energy. Following his intuition he found himself able to obtain a new radiation formula, which was in close agreement with actual measurements under all conditions.

There was, however, something unusual about the Planck formula. He had found that in seeking a relationship between the energy emitted or absorbed by a body and the frequency of radiation he had to introduce a constant of proportionality, which could only take integral multiples of a certain quantity. Expressed mathematically, E = nhν, where E is the energy, h is the constant of proportionality, ν is the frequency, and n = 0, 1, 2, 3, 4, etc. It follows from this that nature was being selective in the amounts of energy it would allow a body to accept and to emit, allowing only those amounts that were multiples of hν. The value of h is very small, so that radiation of energy at the macroscopic level wheren is very large is likely to seem to be emitted continuously.

Planck's introduction of what he called the ‘elementary quantum of action’ was a revolutionary idea – a radical break with classical physics. Soon other workers began to apply the concept that ‘jumps’ in energy could occur. Einstein's explanation of the photoelectric effect (1905), Niels Bohr's theory of the hydrogen atom (1913), and Arthur Compton's investigations of x-ray scattering (1923) were early successes of the quantum theory. In 1918 Planck was awarded the Nobel Prize for physics. The constant h (6.626196 × 10^–34 joule second) is known as thePlanck constant – the value

“h = 6.62 × 10^–27 erg.sec”

is engraved on his tombstone in Göttingen.

By the time of his retirement Planck had become the leading figure in German science and was therefore to play a crucial role in its relations with the Nazis. His attitude was that of prudent cooperation with the overriding aim of retaining the integrity of German science and preventing it from falling into international ridicule. Although he did not publicly protest against the harassment of Jewish scientists, considering such barbarisms a temporary madness, he did, in 1933, raise the issue with Hitler himself. He argued that the racial laws of 1933, barring Jews from government positions, would endanger the preeminence of German science. Hitler is reported to have expressed a willingness to do without science for a few years. Nor did Planck succeed in protecting the institutions of German science for in 1939 the presidency of the academy went to a party member, T. Vahlen, who lost no time in turning it virtually into an organ of the party.

Planck's later years, despite the honors that came his way, were indeed bitter ones. “My sorrow cannot be expressed in words,” he lamented at one point. During World War I his elder son Karl died from wounds suffered in action, and his twin daughters, Grete and Emma, died during childbirth in 1917 and 1919 respectively. In World War II he was forced to witness the destruction of his country and of German science and its institutions. His own home, with all his possessions, was totally destroyed by allied bombing in 1944. Worst of all, his one surviving child, Erwin, was executed in 1945 for complicity in the 1944 attempt to assassinate Hitler.