Session D5: Pais Prize Talk; Sam Goudsmit: Physics, Editor, and More

Abraham Pais Prize for History of Physics Talk: Henry Cavendish, John Michell, Weighing the Stars

This talk is about an interaction between two 18th-century natural philosophers (physical scientists), Henry Cavendish and John Michell, and its most important outcome, the experiment of weighing the world (their name for it) using a torsion balance (our name for it). Michell was the most inventive of the 18th century English natural philosophers, and Cavendish was the first of his countrymen to possess abilities at all comparable with Newton's. By their interests and skills, they were drawn to one another. Both were universal natural philosophers, equally adept at building scientific instruments, performing experiments, constructing theory, and using mathematics; both had a penchant for exacting, quantitative work. Both also had fitful habits of publication, which did not begin to reveal the range of their work, to the mystification of later scientists and historians. Late in life, Cavendish and Michell turned their attention to the force that Newton had examined most completely, a singular triumph of his natural philosophy, the force of universal gravitation. Over the course of the 18th century, abundant evidence of attraction had been gathered from the motions of the earth, moon, planets, and comets, phenomena which span the intermediate range of masses, sizes, and distances. But in three domains of experience, involving the extreme upper and lower limits of masses and dimensions, the universality of gravitation remained an article of faith. These were the gravity of the ``fixed'' stars, the mutual attraction of terrestrial bodies, and the gravitation of light and other special substances. Michell took on himself the task of deducing observable consequences from each of these prospective instances of universal gravitation. Cavendish encouraged Michell, and he followed up the resulting observational and experimental questions. The experiment of weighing the world was the last experiment Mitchell planned and the last experiment Cavendish published. The capstone of two distinguished careers, the experiment outlived the world in which it was conceived and carried out. Today gravitation is at the center of the physics of the very small and the very large, and experiments that followed in Michell and Cavendish's footsteps find a place in it. The ``most important advance in experiments on gravitation,'' to quote an authority, ``was the introduction of the torsion balance'' by Michell and Cavendish and independently by Coulomb; ``it has been the basis of all the most significant experiments on gravitation ever since.'' Another authority traces the ``noble tradition of precision measurement to which we are heirs'' to Cavendish's experiment, which he calls the ``first modern physics experiment.''   

Samuel Goudsmit - Early Influences

Samuel Goudsmit, born in 1902 in The Hague, Netherlands, earned his Ph.D. at the University of Leiden in 1926 with Paul Ehrenfest. The present talk will describe some aspects of his background and early formative years in order to provide context for the broad range of his professional life. Sam belonged to a large tribe of paternal and maternal uncles, aunts and first cousins; including his parents, grandparents and sister Ro, they numbered forty. Sam was the first of the tribe to be educated beyond high school. Early interests as a child and later as a university student in the Netherlands prefigured his significant and diverse contributions in several realms including not only physics but also teaching, Egyptology and scientific Intelligence. Bibliographic sources will include: The American Institute of Physics' Oral History Transcripts and photographs from the Emilio Segre visual archives, memoirs and conversations of those who knew Sam and also letters to his daughter, Esther.   

A Keen Eye for Clues

Samuel Goudsmit, a pioneering atomic theorist who specialized in the exacting, quantitative art of interpreting line spectra and who, with George Uhlenbeck, discovered electron spin, also contributed key studies of nuclear moments, neutron scattering, and the statistics of experimental measurement. Beyond the traditional ambit of laboratory, desk, and blackboard, Goudsmit was drawn to a wider world of inquiry -- to museums and archaeological sites in Cairo as a respected amateur Egyptologist; to the MIT Radiation Lab early in WWII and to the briefing rooms of British pilots, analyzing the effectiveness of radar; and across wartime Europe by jeep, as head of an Allied mission in pursuit of clear information on Germany's secret fission program. After the war he took up chairmanship of a major physics department and editorship of the \textit{Physical Review}, where he created the ambitious new journal, \textit{Physical Review Letters}. The present author, Goudsmit's assistant at the journal forty years ago, looks for a common element that might explain this extraordinary diversity of interests and contributions, and finds one in Goudsmit's abiding delight in solving puzzles of every kind, coupled with a detective's keen eye for clues.   

Sam Goudsmit--His Physics and His Statesmanship

Sam Goudsmit was already a famous theoretical physicist in his thirties, mainly because of his co-discovery of electron spin with George Uhlenbeck while both were students of Paul Ehrenfest in Holland in 1925. He and Uhlenbeck continued their thriving careers at the University of Michigan. Goudsmit's style as a physicist was always to make as close a connection between theory and experiment as possible. Thus, for example, his development with his student Robert Bacher of the technique called ``fractional parentage'' used fruitfully in both atomic and nuclear physics to compute energy levels of unknown states in terms of know ones. He also delved deeply into problems related to determinations of nuclear spins and moments. Partly because of his service as scientific leader of the Alsos project at the end of WWII he became a leading statesman of science. I will describe some of his achievements both as a physicist and as a statesman, prior to his becoming Editor in Chief of the American Physical Society.   

Electron spin from Goudsmit and Uhlenbeck to Spintronics

While the electron's spin was postulated by Goudsmit and Uhlenbeck to explain atomic spectra of gases, it was adopted in a very different setting a decade later to explain the unusual physical and electrical transport properties of ferromagnetic metals. A discovery in 1988 lead to control currents through the spin of the electron, i.e., spintronics. The initial idea of spin dependent transport dates back to Neville Mott's work in the mid- thirties in which he developed the s-d or two current model of conduction in the 3d transition-metal ferromagnetic metals. The methodology used for semiconductor heterostructures lead one to grow high quality metallic multilayers by the 1980's, and it was apparent to Albert Fert and Peter Gr\"unberg [Nobel Laureates in Physics 2007] that one could alter the magnetic configuration in ferromagnetic metals with moderate magnetic fields, and thereby change their resistivities. I will review the principle ideas and developments that lead to this new field, called Spintronics, and focus on developments in three distinct time periods. The first from 1988-1995 which was dominated by metallic multilayers which displayed giant magnetoresistance (GMR), the second from 1995-2000 when reproducible magnetic tunnel junctions (MTJ's) were studied for their tunneling magnetoresistance (TMR), and the third period from 2000-2005 in which the ideas of Berger and Slonczewski were realized on the back action of currents on the magnetic background of the materials doing the conducting, i.e., current induced magnetization switching (CIMS). Current interest has focused on spin dependent transport in oxides and carbon based materials. These developments illustrate the broad range of activities in Spintronics; a field which is barely twenty years old.