GJ Hyland: Associate Fellow of the University of Warwick, UK
Member of Int. Institute of Biophysics, Neuss-Holzheim, D

Fröhlich was born in the Black Forest town of Rexingen on 9th December 1905, and, after a brief period in commerce upon leaving school at the age of 15, entered the University of Munich as an undergraduate in 1927. There, under Sommerfeld's direction, he obtained his D.Phil (for a thesis on the Photoelectric Effect in Metals) after only 3 years, and without ever having taken a first degree! With the rise of Nazism, however, he was soon dismissed from his first post in Freiburg - where he was Privatdozent responsible for introducing modern physics - and in 1933 left Germany for Russia, to work (at Frenkel's invitation), as a 'Foreign Expert', in Joffe's Physico-Technical Institute in Leningrad (St. Petersburg). There he became acquainted with current work on semiconductors, and included a discussion of them in his now famous book, Elektronentheorie der Metalle (Springer, 1936, 1969). Being for some years the only textbook to contain a treatment of semiconductors, it later proved very influential when the technological potential of these materials started to be appreciated, particularly in the USA, where it was reprinted, un-translated, in 1943.

After only 2 years, the political situation in Russia forced him to flee again, and eventually, in 1935, he found himself in England - in Mott's department at the University of Bristol. Apart from a short stay in Holland in 1937, and a period of internment during the War, he remained in Bristol until 1948, rising to the position of Reader. Then, at Chadwick's instigation, he took up the first Chair of Theoretical Physics at the University of Liverpool. This he held with great distinction until his retirement in 1973, after which he was Professor Emeritus from 1976 until his death on 23rd January 1991, at the age of 85. Between 1973 and 1976, he was Professor of Solid State Electronics at the University of Salford, during which time he still maintained an office in Liverpool, spending there a total of 43 years.

Fröhlich was elected Fellow of the Royal Society in 1951, was awarded the Max Planck Medal of the German Physical Society in 1972, and received numerous Honorary Degrees worldwide. From 1974 until his death, he was a Foreign Member of the Stuttgart Max Planck Institute, where he regularly made extended visits, as he also did to many other parts of the world, lecturing and discussing physics.

During his long and illustrious career spanning some 60 years, Fröhlich made many contributions of fundamental significance to areas as diverse as meson theory and biology. Most influential of all was undoubtedly his introduction, around 1950, of the methods of quantum field theory into Solid State Physics, which completely revolutionised the future development of the subject. First came his work with Pelzer & Zienau on the motion of slow electrons in polar materials, from which emerged 'large' polaron theory. This was immediately followed by his fundamental contribution to the theory of superconductivity, which proved crucial to the eventual solution of the problem - namely, that the basic underlying interaction was a hitherto unrecognized aspect of the same interaction as is responsible electrical resistivity: the electron-phonon interaction. From field-theoretical considerations, with which he was already familiar with from his earlier work on the meson theory of nuclear forces with Heitler and Kemmer, he realized that this entailed an (attractive) interaction between electrons mediated by the exchange of virtual phonons. Consistent with the involvement of the ions in the phenomenon of superconductivity was, of course, the contemporaneous discovery of the isotope effect, for which his theory perfectly accounted.

1952 marked the start of a new era in Solid-State Physics, with his introduction of creation and annihilation operators for both electrons and phonons, in terms of which what is now known as the 'Fröhlich Hamiltonian' was first formulated, and from which he re-derived his phonon-mediated electron-electron interaction by canonical transformation.

He then succeeded in solving exactly a one-dimensional model of a superconductor, obtaining, for the first time, an energy spectrum with a gap, and one that exhibited an essential singularity in the electron-lattice coupling constant - a feature shared by the eventual BCS solution 3 years later.

Prior to these contributions, he was best known for his work (loc.cit.) on nuclear forces during the late 1930's, and for his many contributions - which were to continue for almost 30 years - in the field of dielectrics, where he was a world authority; of particular importance was his early work on dielectric breakdown, out of which later evolved the subject now known as 'hot' electrons. His second book, Theory of Dielectrics (OUP, 1949, 1958), immediately became the definitive work on the theory of the dielectric constant and dielectric loss, and was subsequently published in several languages, including Japanese. On its pages were also born such topics as ferroelectric 'soft modes' and 'polaritons', although these names were introduced somewhat later by others.

He was also active in many other areas, such as statistical mechanics, where he did much to elucidate, using reduced density matrices, the connection between microphysics and the physics of macroscopic systems near

thermal equilibrium, including not only 'classical' systems, but also those exhibiting quantum effects on a macroscopic scale, such as superfluids and superconductors, where Yang's concept of 'off-diagonal-long-range order' played a crucial role.

Nowhere, however, was Fröhlich's holistic outlook better illustrated than by his brilliantly daring introduction of concepts of modern theoretical physics - in particular, that of coherence - into biology. From the point of view of physics, living systems are highly non-linear, open, dissipative systems with remarkable dielectric properties, which are held far from thermal equilibrium by their metabolic activity. Using these facts, he showed in 1968 that, given a sufficient level of metabolic activity, the lowest frequency mode of a longitudinal electric polarisation field in such a system becomes strongly excited, attaining macroscopic significance as a 'coherent excitation', which is stabilised through elastic deformations.

In 1972, he went on to show that between two coherent systems of almost equal frequency is an attractive interaction (stronger than that of van der Waals) proportional to the inverse cube of their separation, via which the specificity of the attraction between enzymes and their substrates, for example, becomes immediately understandable. This attractive interaction later played a central role in his model of electrical brain-wave activity based on self-sustaining (limit cycling) oscillations.

The importance of his pioneering work on coherent excitations in living systems is that it directed attention from (static) biological structure to dynamic biological functionality. It continues to generate considerable interest because of the variety of possibilities it offers for understanding the ultra-sensitivity of living systems to very weak electromagnetic radiation at specific frequencies, in which deterministic chaos was later found to be implicated. Quite unexpected, was the role that macroscopic quantum effects apparently play in living systems - a role that has been subsequently invoked in consciousness studies.

These ideas - for which there is now some experimental support - stimulated much other work, both theoretical and experimental, and led to the establishment of series of international conferences, such as those at l'Institute de la Vie in Paris, which continued for many years. The situation as of 1988 was summarised in the book Biological Coherence & Response to External Stimuli (Springer, 1988), which he edited at the age of 82.

It is perhaps not generally appreciated that throughout his life Fröhlich maintained a profound interest in elementary particle physics. In 1960, he developed an ingenious treatment of space reflections as continuous (rotational) transformations in a 4-dimensional space, which not only accounted for all mesons known at the time, but also predicted a further 4 particles with properties identical to those of subsequently discovered vector mesons. During his later years, eschewing contemporary approaches based on interactions, he focused on attempting to understand the separation of elementary particles into leptons and quarks in terms of a novel bilocal extension of the conventional Dirac theory. This programme, which sadly remained incomplete at the time of his death, made unexpected contact with his earlier work on continuous reflections.

Outside of physics, Fröhlich's interests included hiking, skiing, music and abstract art - an interest he shared with his wife, herself an artist.

For all his eminence, FRÖHLICH remained always accessible to the two generations of researchers who studied under him, and who benefited so much from his wise counsel, always so generously given; on them his magnetic personality made an indelible impression. His enthusiasm for physics was infectious, and his incisive, critical insight legendary. His holistic outlook and constant alertness to the possibility that certain concepts might well have relevance to fields other than those in which they had first arisen helped to resolve some of the most enigmatic mysteries of the physics of his era.

His most heroic attribute, however, was undoubtedly a courage to entertain an unusually wide range of novel ideas and to have the conviction to express them without fear of possible refutation - an attribute not uncommon amongst physicists of his generation, but one that is sadly conspicuously absent today.