SAHA, MEGHNAD

(b. Scoratali, Dacca district India [now Bangladesh], 6 October 1894; d. New Delhi India, 16 February 1956)

theoretical physics.

Saha was the fifth child of Jagannath Saha, a small shopkeeper, and Bhubaneswari Debi. The family lived in modest circumstances, and Saha was able to go to primary school through the kindness of a local partron. He entered the Government Collegiate School on a scholarship when he was eleven, but was soon expelled for participating in a political demonstration; he then attended a private school and, in 1911, enrolled in the Calcutta Presidency College. He received the M.A. in applied mathematics in 1915 and, in the following year, was appointed lecturer in mathematics on the all Indian faculty of the University College of Science, which had just been founded by Sir Astosh Mukherjee.

A personality conflict with the chairman of the mathematics department soon led Saha to transfer to the department of physics, where he taught and did research. He received the D.Sc. in 1918. In the meantime, he had become interested in astrophysics, and had begun a systematic study of twenty-five years of the Monthly Notices of the Royal Astronomical Society. Since he was also teaching thermodynamics on the graduate level (although he had not previously studied the subject in depth), he often considered the relationship between thermodynamics and astrophysics. Thus, when he read J. Eggert’s paper “Über den Dissoziationzustand der Fixsterngase”, published in Physilalische Zeitschrift in 1919, Saha was prepared to begin the work on thermal ionization that won him a permanent place in the history of science.

By boldly applying thermodynamics and quantum theory to stellar matter and by drawing an analogy between chemical dissociation and atomic ionization, Saha derived a formula by which the degree of ionization in a very hot gas could be expressed in terms of its temperature and electron pressure. He set out his results in a paper entitled “On Ionization in the Solar Chromosphere,” published in Philosophical Magazine in 1920. There is some dispute concerning where this paper was written, since Saha left India in September 1919 on a scholarship that permitted him to visit both A. Fowler’s laboratory at Imperial College, London, and later Nernst’s laboratory in Berlin. At any rate, in London Saha worked not only with Fowler but with E. A. Milne, who developed important extensions of Saha’s theory; while in Berlin he met Planck, Einstein, Laue, and Sommerfeld and set up experiments to confirm his theory, of which he gave an account in an article, “Versuch einer Theorie der physikalischen Erscheinungen bei höhen Temperaturen ...,” published in Zeitschrift für Physik in 1921.

Saha’s theory may be considered to be the starting point of modern astrophysics. The long-noted absence of rubidium and cesium in the solar spectrum could be understood for the first time, since it follows from Saha’s formula that the degree of ionization increases with temperature and decreases with pressure, and is therefore smaller for elements with lower ionization potentials. It is thus possible to predict whether or not the spectrum of an element may be expected in a region with a given temperature and pressure if its ionization potential is known. It was also soon confirmed that other lines existed in the cooler regions of the solar atmosphere, as the theory predicted, and H.N.Russell was thereby able to identify rubidium in the spectra of sunspots. (It must be noted, however, that in the years immediately following the publication of Saha’s theory, enthusiasm led some astrophysicists to apply the equation rather rashly, and some significant errors were made.) But in general, important knowledge of stellar atmospheres could be obtained by an analysis of their spectra through Saha’s theory.

Saha returned to India in 1921 to accept a professorship at the University of Calcutta, but lack of funds and encouragement prompted him to look for a position elsewhere. In 1923 he went to the University of Allahabad, where he taught for the next fifteen years. As confirmations and extensions of his theory brought him fame he received a number of honors. He was elected president of the physics section of the Indian Science Congress Association in 1925, and two years later he was named a member of the Royal Society. In 1938 he left the University of Allahabad to become professor of physics at the University of Calcutta, where he worked to create an institute of nuclear physics (which was officially opened by Irène Joliot-Curie, and is now named in Saha’s honor). He also initiated researches on cosmic rays, the ionosphere, and geophysics, and traveled widely in connection with his increasing organizational responsibilities. He was directly or indirectly responsible for the establishment of a number of scientific bodies and institutions, including the United Provinces Academy of Sciences (later the National Academy of Sciences), the Indian Science News Association, and the National Institute of Sciences.

Saha was further active in planning scientific and industrial projects for India. In particular he was involved in river valley projects (the result of having in his youth witnessed devastating floods), calendar reform, and geophysical explorations. In 1935 he founded the influential socioscientific journal Science and Culture: but although he was interested in the social implications of science, and although he was sympathetic to the Indian struggle for independence, he remained aloof from politics in general. He was opposed to the khadi movement of Ghandi and his followers because he thought that it would impede Indian industrialization. Following India’s political independence, Saha devoted more time to social, political, and economic problems, and in 1951 he was elected, as an independent, to Parliament. He died during a trip to New Delhi to discuss some matters pertaining to the administration of his nuclear physics institute.

BIBLIOGRAPHY

A complete bibliography of Saha’s works may be found in S. N. Sen, ed. (see below).

For further details on the life and works of Saha, consult S. N. Sen, ed., Professor Meghnad Saha, His Life, Work and Philosophy (1954); Science and Culture, 19 (1954), 442; Science and Culture, 22 (1956), a complete issue devoted to Saha: D.S.Kothari, “Meghnad Saha,” in Biographcial Memoirs of Fellows of the Royal Society, 4 217–236; and an obituary by F.J.M.Stratton in Nature, 177 (1956), 917.

For discussions of Saha’s theory of thermal ionization, any work on astrophysics may be consulted. In particular, a nonmathematical treatment of the ideas as they stood in the early years of the theory may be found in H. Dingle, Modern Astrophysics (New York, 1924), 197–217. See also Giorgio Abetti, The Sun, J. B. Sidgwick, trans. (London, 1955), ch. 5. For a more technical discussion, see, A. Unsöld, Physik der Sternatmosphären, 2nd ed. (Berlin, 1955), 40–48; and S.Rosseland, Theoretical Astrophysics (Oxford, 1936), 170–183.

V. V. Raman