Max Planck
Max
Planck,
(born April 23, 1858, Kiel,
Schleswig [Germany]—died October 4, 1947, Göttingen,
Germany), German theoretical physicist who originated quantum
theory,
which won him the Nobel
Prize
for
Physics in 1918.
His
father received an appointment at the University
of Munich,
and Planck entered the city’s renowned Maximilian Gymnasium,
where a teacher, Hermann Müller, stimulated his interest in physics
and
mathematics.
But Planck excelled in all subjects, and after graduation at age 17
he faced a difficult career decision. He ultimately chose physics
over classical philology
or
music because he had dispassionately reached the conclusion that it
was in physics that his greatest originality lay.
His
intellectual
capacities
were, however, brought to a focus as the result of his independent
study, especially of Rudolf
Clausius’s
writings on thermodynamics.
Returning to Munich, he received his doctoral degree in July 1879
(the year of Einstein’s
birth) at the unusually young age of 21. The following year he
completed his Habilitationsschrift
(qualifying
dissertation) at Munich and became a Privatdozent
(lecturer).
In 1885, with the help of his father’s professional connections, he
was appointed ausserordentlicher
Professor
(associate
professor) at the University of Kiel. In 1889, after the death of
Kirchhoff, Planck received an appointment to the University of
Berlin, where he came to venerate Helmholtz as a mentor and
colleague. In 1892 he was promoted to ordentlicher
Professor(full
professor). He had only nine doctoral students altogether, but his
Berlin lectures on all branches of theoretical physics went through
many editions and exerted great influence. He remained in Berlin for
the rest of his active life.
Planck
recalled that his “original decision to devote myself to science
was
a direct result of the discovery…that the laws of human reasoning
coincide with the laws governing the sequences of the impressions we
receive from the world about us; that, therefore, pure reasoning can
enable man to gain an insight into the mechanism of the [world]….”
He deliberately decided, in other words, to become a theoretical
physicist at a time when theoretical physics was not yet recognized
as a discipline
in
its own right. But he went further: he concluded that the existence
of physical laws presupposes that the “outside world is something
independent from man, something absolute, and the quest for the laws
which apply to this absolute appeared…as the most sublime
scientific
pursuit in life.
The first instance of an absolute in nature that impressed Planck deeply, even as a Gymnasium student, was the law of the conservation of energy, the first law of thermodynamics. Later, during his university years, he became equally convinced that the entropy law, the second law of thermodynamics, was also an absolute law of nature. The second law became the subject of his doctoral dissertation at Munich, and it lay at the core of the researches that led him to discover the quantum of action, now known as Planck’s constant h, in 1900.
In 1859–60 Kirchhoff had defined a blackbody as an object that reemits all of the radiant energy incident upon it; i.e., it is a perfect emitter and absorber of radiation. There was, therefore, something absolute about blackbody radiation, and by the 1890s various experimental and theoretical attempts had been made to determine its spectral energy distribution—the curve displaying how much radiant energyis emitted at different frequencies for a given temperature of the blackbody. Planck was particularly attracted to the formula found in 1896 by his colleague Wilhelm Wien at the Physikalisch-Technische Reichsanstalt (PTR) in Berlin-Charlottenburg, and he subsequently made a series of attempts to derive “Wien’s law” on the basis of the second law of thermodynamics. By October 1900, however, other colleagues at the PTR, the experimentalists Otto Richard Lummer, Ernst Pringsheim, Heinrich Rubens, and Ferdinand Kurlbaum, had found definite indications that Wien’s law, while valid at high frequencies, broke down completely at low frequencies.
Planck
learned of these results just before a meeting of the German Physical
Society on October 19. He knew how the entropy
of
the radiation had to depend mathematically upon its energy in the
high-frequency region if Wien’s law held there. He also saw what
this dependence had to be in the low-frequency region in order to
reproduce the experimental results there. Planck guessed, therefore,
that he should try to combine these two expressions in the simplest
way possible, and to transform the result into a formula relating the
energy of the radiation to its frequency.
The result, which is known as Planck’s radiation law, was hailed as indisputably correct. To Planck, however, it was simply a guess, a “lucky intuition.” If it was to be taken seriously, it had to be derived somehow from first principles. That was the task to which Planck immediately directed his energies, and by December 14, 1900, he had succeeded—but at great cost. To achieve his goal, Planck found that he had to relinquish one of his own most cherished beliefs, that the second law of thermodynamics was an absolute law of nature. Instead he had to embrace Ludwig Boltzmann’s interpretation, that the second law was a statistical law. In addition, Planck had to assume that the oscillators comprising the blackbody and re-emitting the radiant energy incident upon them could not absorb this energy continuously but only in discrete amounts, in quanta of energy; only by statistically distributing these quanta, each containing an amount of energy hν proportional to its frequency, over all of the oscillators present in the blackbody could Planck derive the formula he had hit upon two months earlier. He adduced additional evidence for the importance of his formula by using it to evaluate the constant h (his value was 6.55 × 10−27 erg-second, close to the modern value of 6.626 × 10−27 erg-second), as well as the so-called Boltzmann constant (the fundamental constant in kinetic theory and statistical mechanics), Avogadro’s number, and the charge of the electron. As time went on physicists recognized ever more clearly that—because Planck’s constant was not zero but had a small but finite value—the microphysical world, the world of atomic dimensions, could not in principle be described by ordinary classical mechanics. A profound revolution in physical theory was in the making.
Planck’s concept of energy quanta, in other words, conflicted fundamentally with all past physical theory. He was driven to introduce it strictly by the force of his logic; he was, as one historian put it, a reluctant revolutionary. Indeed, it was years before the far-reaching consequences of Planck’s achievement were generally recognized, and in this Einstein played a central role. In 1905, independently of Planck’s work, Einstein argued that under certain circumstances radiant energy itself seemed to consist of quanta (light quanta, later called photons), and in 1907 he showed the generality of the quantum hypothesis by using it to interpret the temperature dependence of the specific heats of solids. In 1909 Einstein introduced the wave-particle duality into physics. In October 1911 Planck and Einstein were among the group of prominent physicists who attended the first Solvay conference in Brussels. The discussions there stimulated Henri Poincaré to provide a mathematical proof that Planck’s radiation law necessarily required the introduction of quanta—a proof that converted James Jeans and others into supporters of the quantum theory. In 1913 Niels Bohr also contributed greatly to its establishment through his quantum theory of the hydrogen atom. Ironically, Planck himself was one of the last to struggle for a return to classical theory, a stance he later regarded not with regret but as a means by which he had thoroughly convinced himself of the necessity of the quantum theory. Opposition to Einstein’s radical light quantum hypothesis of 1905 persisted until after the discovery of the Compton effect in 1922.
Planck was 42 years old in 1900 when he made the famous discovery that in 1918 won him the Nobel Prize for Physics and that brought him many other honours.
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