Aharon (Rony) Casher was born in Haifa, Israel, and graduated from the Technion in Haifa, where he did his thesis work on condensed bosonic systems with Micha Revzen. After graduation, he went to Yeshiva University in New York, where he wrote a well-known paper with Joel Lebowitz on heat flow in random harmonic chains.
At Yeshiva, Aharon met Yakir Aharonov and Lenny Susskind. He had long lasting collaborations with both.
The Aharonov–Casher effect, dual to the Aharonov–Bohm effect, is textbook material and the most famous result of the first collaboration, which also led to a beautiful result on the number of zero modes in two-dimensional magnetic fields. With Lev Vaidman, they developed the mathematics underlying weak measurements; and in a separate work with 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 polarization 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 led also to Aharon’s familiarity with string theories and to the early paper with Aharonov of a dual string model for spinning particles.
A paper with Herbert Neuberger and Shmuel Nussinov, suggesting that tunneling that breaks chromoelectric flux tubes can account for multiple particle production in high energy collisions, was later incorporated into the Lund Model. During a sabbatical at Princeton University, Aharon wrote the first comprehensive work on the subtle subject of gauge fields on the null plane.
In the high-energy 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. This was followed by a famous paper with Tom Banks, which in particular introduced the Banks–Casher criterion, relating such breaking to the enhanced density of the low eigenvalues of the Dirac operator.
All these topics at the scientific forefront dominated Aharon’s interest throughout the 1970s and early 1980s. His deep knowledge of field theory of topology and understanding of nonperturbative effects enabled him to make these key and long-lasting contributions.
Aharon visited Brussels often, where he worked with François Englert and others on supergravity, on quantum gravity, and on an early “quantum era” of the universe. François became a frequent and most welcome visitor at Tel Aviv University. Nonperturbative effects in quantum gravity and possible connections to the physics of black holes became a shared passion of both.
Aharon was familiar with the early string theory and gave a series of influential lectures on the subject at Tel Aviv shortly after the 1984 “string revolution.” Yet he was critical of strings as the ultimate theory of nature and did not follow the opinions of the majority. He was an independent thinker, uncompromisingly honest when analyzing novel ideas in theoretical physics.
Aharon stayed at Tel Aviv University for almost 50 years. His knowledge and remarkable talents enabled him to teach any subject in theoretical physics from memory, without notes. 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 Aharon on any topic.
A physicist’s renown often reflects publications and lectures on topical issues. 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 keep thinking and calculating at his desk.
Aharon was much more than his portfolio might indicate. His published works hardly do justice to the ongoing almost never-ending physics discussions and heated debates with us and many others, reflecting Aharon’s deep passion for physics. Also, vast amounts of work on quantum gravity, supersymmetry, instantons, large N, and high order physics remained unpublished.
His collaborators and many others, including, to our knowledge, ‘t Hooft and the late Brout, Symanzik, Wilson, and Gribov, had the deepest respect for him. His ability to keep challenging us and to relentlessly pursue the subtleties that could harbor fatal flaws helped us maintain our own scientific integrity. Aharon will be deeply missed.
