Max Planck

The year 1900 is the watershed between what are known as classical Physics and modern Physics – not because it makes the transition from the 19^th century to the 20^th century, but because it saw the birth of quantum theory which changed the character Physics. The author of that path breaking theory was Max Planck who was born exactly a century and a half ago, on April 23, 1858 in the city of Kiel in northern Germany. Planck studied in the University of Berlin under Helmholtz, Clausius and Kirchoff and got his doctorate in 1879. After teaching in Munich and Kiel universities or some time, he replaced his teacher, Kirchoff, as professor of Physics in Berlin University when the latter died in 1889.

The quantum theory concerns the nature of light, a topic hotly debated in the 17^th century. Newton held that light is a stream of corpuscles (tiny particles) whereas his contemporary, Christian Huygens said that it is a sort of wave, As years rolled by, overwhelming evidence in favour of the latter view got accumulated – notably the phenomena of diffraction and interference, characteristics of wave motion – and, as a result, the wave theory was more or less accepted.

It came to be understood that light consists of electromagnetic waves, waves made up of oscillating electric and magnetic waves and travelling with a speed of about 300,000 kilometers per second. Coloured lights, found in the spectrum of white light, are electromagnetic radiations having different wave lengths varying from 3800 angstroms (an angstrom is ten billionth of a meter) at the violet end of the spectrum to 7200 angstroms at the red end. Consequently they have different frequencies (number of oscillations per second) which can be got by dividing the velocity of light by the wave length. The visible part of the spectrum is skirted on either side by the ultraviolet and infrared radiations.

Planck got interested in the problem of the black body, one that absorbs light of all frequencies and so, when heated, emits all frequencies of light. But, it does not emit them equally. No one, at that time, could explain why the black body radiates different frequencies in the way it does. Planck, however, managed to arrive at an equation which accurately described the distribution of frequencies in the blackbody radiation. But his equation was based on a seemingly untenable assumption that radiant energy does not flow continuously, but does so in packets o energy which he called quanta (plural of quantum). It looked as though he was bringing Newton’s corpuscular theory from the back door. But it was not so. For, he had not rejected the wave theory, but had established a link with it. He had assumed that the amount of energy in the quantum is proportional to the frequency of the radiation.

Energy of the quantum = h X frequency

The constant h is now known as Planck’s constant. Since some phenomena like diffraction are explained by the wave theory and others, like black body radiation, are explained by the quantum theory, the two theories are not mutually exclusive, but are complementary.

The theory was so revolutionary that it was not accepted by physicists immediately. Planck himself half-suspected that it might be a mathematical trick and may not represent anything real. But, in 1905, Einstein used the theory to explain the photoelectric effect, emission of electrons by some metals when light falls on them. It cannot be explained by the wave theory. Niels Bohr, in 1913, used it to describe the atomic structure and explain a great deal which cannot be explained by classical physics. By 1918 the quantum theory had become so important that it fetched the Nobel Prize for its author. Einstein and Bohr were awarded that prize in 1921 and 1922 for applying the theory fruitfully.

In 1930, Planck was made the president of the Kaiser Wilhelm Society which was later renamed Max Planck Society. Although Planck was already in his seventies during the rise of Hitler in Germany, he resisted it. He sought to intercede on behalf of the persecuted Jewish scientists and, as a result, had to step down from the presidentship of the Max Planck Society. But, in the world of science, the veneration accorded to him was next only to that accorded to Einstein.

During the second world war, his house was destroyed by allied bombings; his older son was killed in action and the younger one was accused of plotting against Hitler and was executed. Planck lived into his ninetieth year to see Nazism destroyed. He was renamed president of the Max Planck Society and spent the last two years loved and respected. He died in October 3,1947.

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My father, J R Lakshmana Rao, wrote this for publication on Max Plank's 150th birthday on April 23. For various reasons it did not see the light of day. I decided to post it here.

Picture: Courtesy Wikipedia