Meghnad Saha- His Science and Persona through Selected Letters and Writings

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Societies, Nations and their future are sculpted by the dreams of a few wise men, who contribute to it by providing a vision and knowledge based wherewithal for their development. These people provide a road map, on which the Nation treads for its well-being. Meghnad Saha was one such persona who thought of India and its prosperity determined by science.

The Indian Academia and the society owe a great deal to Meghnad Saha, who besides his fundamental and path breaking contributions to astrophysics, envisioned the need for an Academy of Science in India to serve as an interface between science, society and government as well as the link with other international science bodies like ICSU. His article, A Plea for an Academy of Sciences, in the Allahabad University Magazine in 1929 was sufficiently impactful that it led to the formation of UP Academy of Sciences. Later the National Institute of Sciences of India (NISI) was in place by 1935. NISI was renamed as Indian National Science Academy (INSA), in 1970. Saha was the Second President of INSA. Saha believed in Gothe's Maxim of, to strive, to seek, to find and not to yield and his Presidential Address to the UP Academy of Sciences should be a must read for all serious students of Science. This address presents his amazing vision almost a century ago and INSA is striving diligently to live up to this vision of Saha.

This year we celebrate the 125th birth anniversary of Meghnad Saha. We celebrate a brilliant intellect, a true patriot and a man with earthy humanness. We celebrate his 125th year, with great humility and a sense of reverence for one of the few, who provided solid foundation for the development of Science in India. This book is a mark of our deep gratitude to this goliath of Indian Science.

was given the responsibility of producing this collection of letters and writings several years ago by Prof. Chitra Roy and late Dr. Prasenjit Saha (daughter and son of Meghnad Saha), and our beloved teacher, late Prof. Supriya Roy, husband of Chitra Roy. This volume is not a comprehensive collection of Meghnad Saha's letters but a selection from what was available in the family archive. My criteria for inclusion of a letter in this volume are: (a) it should have some scientific and/or professional content; (b) it should at least convey a sense of time (social, political, cultural), especially if it is devoid of the first criterion; and (c) it should be compatible with the spirit of a celebratory volume. The science historians may take this selection and integrate it with other letters and writings of Meghnad Saha to develop a more complete picture of him as a person and a scientist. However, this collection of letters will bring out, I hope, the impact of Meghnad Saha's scientific contributions; his enormous enthusiasm and passion for science and scientific research, despite the struggle that he faced in carrying out his research in an economically underdeveloped subject nation under the British rule; his undaunted spirit; and show how highly respected he was among the stalwarts of his time that was inarguably the most revolutionary period of physics since Newton.

Meghnad Saha was one of the greatest scientists to come out of India in the modern era. His scientific contributions are versatile, ranging from Astrophysics to Nuclear Physics, as well as some areas of applied sciences that were motivated by his social commitments and Nationalistic ideals. His seminal contribution on the thermal excitation and ionization of elements, which led to the formulation of the famous "Saha Equation", gained him international acclaim immediately after its publication in 1920, when Saha was only 27 years of age (but in the prime age for making a major contribution as a physicist at that time), and secured a permanent place for him in the field of Astronomy and Astrophysics. To explain some of the puzzling aspects of the spectroscopic data of the stars, notably the Sun, Saha came up with the bold suggestion that these data reflect different stages of excitation and extent of ionization of elements "under the stimulus prevailing in the star", rather than major compositional differences among the stars. He then went on to derive a formal relationship (1920: Philosophical Magazine) among temperature, pressure, ionization potential and relative abundances of different ionization states of an element. It was a remarkably imaginative step that combined the knowledge of the atomic structure in the new subject of quantum mechanics (the Bohr model) and statistical thermodynamics (it is said that Bohr himself was also considering the problem). The immediate success of Saha's theory and also some of his predictions led to intense synergistic activities within the Astronomy- Astrophysics community during the 1920's and 30's to further refine the theory and to determine ionization potentials of elements and their spectral characteristics, as one would appreciate from many of the letters in this collection.