A prolific quantum mechanic: Aharon Casher 1941–2018

23 March 2018

Aharon (Rony) Casher was born in Haifa, Israel, and graduated from the Technion where he performed his thesis work on condensed bosonic systems under Micha Revzen. He then went to Yeshiva University in New York, where he wrote a well-known paper with Joel Lebowitz on heat flow in random harmonic chains. This is also where his longstanding collaborations with Yakir Aharonov and Lenny Susskind began.

The Aharonov–Casher effect, which is dual to the Aharonov–Bohm effect, is textbook material and also led to a beautiful result on the number of zero modes in 2D magnetic fields. With Lev Vaidman, Casher and Aharonov developed the mathematics underpinning weak measurements; and in a separate work with Shimon Yankielowicz they introduced the mechanism of magnetic vacuum condensation for confinement in QCD. The early suggestion by Aharon, Susskind and John Kogut that a vacuum polarisation mechanism can account for quark confinement was extremely influential. Additional, important joint papers on strong interactions, partons and spontaneous chiral symmetry breaking appeared in the early 1970s. The collaboration with Susskind also led to Aharons’ familiarity with string theories and to the early paper with Aharonov of a dual string model for spinning particles.

In the high-energy physics community, Aharon is best known for his work on spontaneous chiral symmetry breaking in QCD. In a singly authored paper he provided a beautiful insight into this subject, followed by a famous paper with Tom Banks that related such breaking to the enhanced density of the low eigenvalues of the Dirac operator. These topics dominated Aharon’s interest throughout the 1970s and early 1980s. His deep knowledge of topological field theory and understanding of non-perturbative effects enabled him to make key and long-lasting contributions.

Aharon often visited Brussels, where he worked with François Englert and others on supergravity, quantum gravity and studies of the early universe. Englert, in turn, became a frequent visitor at Tel Aviv University, and non-perturbative effects in quantum gravity and possible connections to the physics of black holes became a shared passion of both. Although Aharon gave a series of influential lectures on string theory at Tel Aviv shortly after the 1984 “string revolution”, and published with Englert, Nicolai and Taormina a paper showing that all superstring theories are contained in the bosonic string, he was critical of strings as the ultimate theory of nature. He was an independent thinker, uncompromisingly honest when analysing novel ideas in theoretical physics.

Aharon stayed at Tel Aviv for almost 50 years, his knowledge and remarkable talents enabling him to teach any subject in theoretical physics from memory alone. He was accessible to students and attracted many who subsequently had independent academic careers, including Neuberger, Nissan Itzhaki and Yigal Shamir. Aharon was an avid reader, interested in literature, history, science fiction, sports and politics. One could have an interesting conversation with him on any topic.

Aharon was highly negligent as a self- promoter and was in science for the sheer pleasure of doing it. He rarely gave talks about his work, preferring to think and calculate at his desk, and his collaborators and many others had the deepest respect for him. His ability to keep challenging us and to relentlessly pursue the subtleties that could harbour fatal flaws helped maintain our own scientific integrity. Aharon will be deeply missed.

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