A historical interlude Max Planck 18581947

Max Planck was born on 23 April 1858 in Kiel, then the Danish city of Holstein, and died on 4 October 1947 in Göttingen. His father,

Julius Wilhelm, was Professor of Constitutional Law at the University of Kiel. His mother was Emma Patzig. He entered the University of Munich at the very young age of 16, to study physics. Ironically, his professor Philipp von Jolly (1809-1884) told him at the time that Physics was a closed science, in the sense that all of the subject, with the exception of a few details, had been mastered. Accordingly, he warned, there was little prospect of future research or discovery. Little did he know how wrong he would be proved in the future by the young person he was addressing!

Planck remained in Munich until 1880, apart from one year (1877-1878) in Berlin, where he studied under Gustav Kirchhoff (1824-1887) and Herman von Helmholtz (1821-1894). He did not, it appears, enjoy his time there, describing Kirchhoff's lectures as 'dry and monotonous', while those of Helmholtz were 'badly prepared and careless, 'giving us the unmistakable impression that the class bored him as much as it did us'.

On his return to Munich, Planck received his doctorate at the age of 21, with a thesis on thermodynamics, and one year later he was appointed to a teaching post at Munich, at what was again an exceptionally young age. In 1885 he obtained a professorship at the University of Kiel, and soon afterwards married his childhood sweetheart, Marie Merck. Marie died in 1909 and some time later he was to remarry her cousin Marga von Hosslin.

In 1889, Planck moved to Berlin, where he had been offered the Chair of Theoretical Physics, previously held by Kirchhoff. Planck

stayed in this post for a period of 38 years until 1927, during which time he produced his most brilliant work, including the discovery of his radiation formula in 1900.

In his autobiography Planck describes the lonely path he plotted, which was clouded by an anxious and uncertain mood, unlike the dramatic successes of some of his contemporaries, such as the Curies or Lord Rutherford. He called his postulate of energy quanta 'an act of desperation', because 'a theoretical explanation of black-body radiation had to be found at all cost, no matter what the price'. Even after the publication of his famous paper he tried to fit the introduction of his constant h into the framework of classical physics, but without success.

During the initial period after 1900, practically nobody seems to have realised the significance of Planck's discovery. The quantum postulate was reported by Arthur L. Day at the 1902 Washington meeting of the American Philosophical Society, but was deemed to deserve little or no reference in texts and papers published over the next few years. In short it seems to have been regarded as an expedient methodology without deeper physical significance.

The general skepticism may be illustrated by the statement made by Hendrik A. Lorentz during a series of lectures entitled 'New and Old Problems in Physics' which he gave in Gottingen as late as 1910: 'We cannot say that the mechanism of the phenomena has been unveiled by Planck's theory...it is difficult to see the reason for this partition of energy into finite portions, which are not even equal to each other, but vary from one resonator to another'.

Whatever about the opinion of German physicists, quantum theory took a long time to cross the English Channel to Britain. Rutherford, in a letter to his friend William Bragg, about 1908, wrote: 'Continental people do not seem to be the least interested to form a physical idea of Planck's theory I think the English point of view is much more to be preferred.' In his lecture at the presentation of the Nobel Prize in 1919, Planck gave the story of

(Continued)

the origin of quantum theory from his own unique perspective. Quoting Goethe's 'Man errs as long as he strives', he described how he encountered many pitfalls and difficulties, punctuated with steps which brought him 'conclusively nearer the truth'.

The quantum hypothesis obtained its greatest support when Niels Bohr introduced it in 1913 into his theory of the atom. Planck further recalled that 'it fell upon this theory to discover, in the quantum of action, the long sought key into the wonderland of spec-troscopy...there was no other decision left to the critic, who does not intend to resist the facts, but to award to the constant h full citizenship in the system of universal physical constants'.

The consequences of the new philosophy of science introduced by Planck continued to reach further and further as quantum mechanics was being developed. The ideas were so strange that even the originators became bewildered. Planck himself was often confused: 'If anybody says he can think about quantum problems without getting dizzy, that only shows he has not understood the first thing about them'. In a lecture given in New York in 1949, he remarked: 'An important scientific innovation rarely makes its way by winning over and converting its opponents. What does happen is that its opponents gradually die out, and that the growing generation is familiarized with the ideas from the beginning.'

Planck was once described by Kurt Mendelssohn as 'a spare figure in a dark suit, starched white shirt, and black tie, with the look of a typical Prussian official, but for the penetrating eyes under the huge dome of his bald head'. He had the great strength of character to blaze a new and original path despite the lack of interest and encouragement on the part of many of his peers.

Planck's life was punctuated with drama and tragedy from the time when, as a child, he watched Prussian troops take over the city of Kiel. His eldest son was killed in 1916 at the battle of Verdun. During the period of Nazi government, he was openly opposed to some of the policies, particularly the persecution of the Jews. He suffered another great personal tragedy when his other son, Erwin, was executed in 1945 for plotting to assassinate Hitler. A short time later an American army unit was sent to rescue him from the ruins of his bombed-out house in Berlin.

Appendix 11.1 Deriving the Stefan-Boltzmann law from Planck's radiation formula and calculating the value of Stefan's constant

0 X5 ehcXkt -1

x dx

Let x _

hc/XkT

fi X _

hc/xkT

fi dX

_ -(hc/x 2kT) dx

c 3h3

p 15

8p5k4 T4

15c3h3

But, it can be shown that the radiant exitance M _ —

E _ 5.66 x 10-8 T4 Jm-2 s-1 K4 (The Stefan-Boltzmann law)

Showing good agreement with experiment. a = 5.70 x 10-8 Wm-2 K-4

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