- Brian Pippard
Fröhlich, Herbert (1905–1991), theoretical physicist, was born on 9 December 1905 in Rexingen, in the Black Forest of south-west Germany, the second of three children of Julius Fröhlich (1879–1952), a cattle dealer, and his wife, Frida, née Schwarz (d. 1959). The family moved to Munich while Herbert was still young. Both parents were Jewish. Fröhlich took no interest in his mother's devout faith and soon showed his independent spirit. He chose his own school and later transferred to a technical college where he discovered the attractions of radio and mathematics. At the University of Munich he studied theoretical physics, first with Wilhelm Wien and then, after Wien's death in 1928, with Arnold Sommerfeld, gaining his DPhil in 1930. During this period he was an enthusiastic hiker, mountaineer, and skier, and until late in life would lead colleagues and students on long, fast, cross-country walks, illuminated by lively discussion of physical problems. He was always generous of time and ideas, and inspired great devotion. An appointment in 1933 as a Privatdozent at Freiburg University was cut short when Hitler assumed power. His father suffered a severe beating, and the whole family, except Fröhlich himself, took the first opportunity to escape to France and thence to Palestine. Fröhlich (as he was known to almost everyone, even his wife) hazarded his life for a year in Germany to rescue what he could of the family property. The local Nazi leader being absent for a day to confer with Hitler, Fröhlich slipped away to Russia and the Ioffe Institute in Leningrad. But in 1935, along with other foreigners, he was expelled, and eventually reached haven in Bristol by way of Rome and Holland, with support from the Academic Assistance Council. Apart from a few months of internment in 1940 as an enemy alien he spent the war in Bristol, and then went to Liverpool in 1948 at Sir James Chadwick's invitation, the last of his many migrations. There, on 3 July 1950, he married the much younger Fanchon Aungst, an American and a talented artist who had studied philosophy at Oxford. To his disappointment, for he loved children, the marriage was childless.
Fröhlich, who was elected a fellow of the Royal Society in 1951, remained professor of theoretical physics at Liverpool until 1973; he was then professor of solid state electronics at Salford University from 1973 until 1976. He had wide-ranging interests in physics and a ready skill in the mathematical analysis of whatever problem took his fancy. His approach was strongly influenced by the German models of his youth, with a delight in systematic development from basic ideas until he was satisfied that his result was compatible with the known facts. For the facts themselves he was content to rely on the experimenters, having no personal interest in the practical affairs of a laboratory. One merit, to him, of Liverpool was the university's willingness to establish a separate theoretical physics department for his research students and collaborators, with only a small number of undergraduate students to teach.
Fröhlich's university career in Munich had started little more than a year after the discovery of quantum mechanics, and his professor, Sommerfeld, a master of the older quantum theory, was one of the pioneers of applying the new theory to the conduction of electricity by electrons in metals. Fröhlich's book, Elektronentheorie der Metalle (1936), was perhaps the first of its kind (after the ground-breaking article of Sommerfeld and Bethe in the Handbuch der Physik) and the first to include any discussion of semiconductors, obscure and little considered as they were then, but later dominating every aspect of electronics and computers. Between then and 1950 he set aside his interest in metals, and concentrated on insulators, their polarization by weak electric fields, and their catastrophic breakdown in stronger fields. At the same time he became fascinated by new ideas in nuclear physics, especially those of Hideki Yukawa on the as yet undiscovered mesons that bind protons and neutrons into a stable atomic nucleus. His work in this area showed his ability to master and develop the latest mathematical methods involved in the quantum theory of fields. From time to time he returned to these problems with great fertility of imagination but, as befell most other workers in particle physics, his conceptions suffered severe competition, and perhaps little of his achievement survived explicitly in the fundamental theories that later commanded assent.
By contrast Fröhlich's work on dielectrics, on which he published a small but still widely quoted text in 1949, led him to a succession of discoveries which guaranteed his place in the history of solid-state physics. In Bristol he began, with Mott, to study the motion of electrons under the influence of strong electric fields, and the collisions with vibrations of the crystal lattice which deflect their motion and hinder the growth of an avalanche of fast electrons—an important mechanism of dielectric breakdown. In examining the process further he brought to bear his expertise in quantum field theory, possibly its first application to solids, to analyse the deformation of an ionic lattice by the field of an electron. Not only did he describe the 'large polaron', moving as a free particle with the electronic mass somewhat enhanced by the locally deformed lattice, but he realized that the deformation itself could attract another electron to form a loosely bound pair. This led him to propose in 1950 the first theory of superconductivity that had the merit of a sound physical basis.
It was recognized after 1933 that the ability of certain metals to conduct electricity without resistance, at very low temperatures, depended on their expulsion of magnetic flux, and after 1936 this property of perfect diamagnetism became linked with the presence of an energy gap—unlike ordinary metals at zero temperature, an electron in a superconductor cannot be excited to a higher state without the supply of a certain minimum energy. Fröhlich showed that the attractive force resulting from lattice distortion could produce the desired gap without (as in insulators) inhibiting electron motion. While preparing his paper for publication he learned that different isotopes of mercury make the transition to superconductivity at slightly different temperatures, as his theory predicted. This was enough to convince him and others that electron lattice interaction lay at the heart of superconductivity. Fröhlich's detailed theory was not readily accepted, and it was seven years before the idea of electron pairs was incorporated into the later renowned theory of John Bardeen, Leon Cooper, and Robert Schrieffer. But Fröhlich's achievement inspired this breakthrough, and was entirely novel in that until then the ionic lattice had been treated as a rigid structure, playing at most a minor role in superconductivity.
Fröhlich's earlier work on dielectrics also seems to have inspired the ideas which from 1966 dominated his thought—a complicated system, such as a living cell, might be stimulated into organized oscillatory motion by irradiation with relatively weak electromagnetic waves. Despite the handicap of an analysis hard enough for many physicists and opaque to most biologists, he attracted an enthusiastic following who were encouraged by observations that the rate of cell division could be altered by irradiation, and that the effects were sharply dependent on the frequency used. There is no direct evidence to support Fröhlich's precise mechanism, but the occurrence of any such effect was enough to stimulate further experiments, which he supported to the last, especially at the Max Planck Institut in Stuttgart, where he was a visiting professor from 1980. He died of cancer of the bladder in Liverpool on 23 January 1991, and was cremated there a week later. He was survived by his wife.
- Bodl. Oxf., corresp. relating to Society for Protection of Science and Learning
- F. Fröhlich, sketch, repro. in Mott, Memoirs FRS, 161
- photograph, repro. in The Times
- photograph, repro. in The Independent
Wealth at Death
£164,767: probate, 21 March 1991, CGPLA Eng. & Wales