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11 Feb - "Beyond Perturbation theory: Integrability from fundamentals to applications" Lecture by Changrim Ahn, Ewha Womans University, South Korea

The visit of Changrim Ahn is organised in collaboration with Francesco Ravanini from the Department of Physics and Astronomy.

Feb 11, 2020 from 05:30 PM to 07:00 PM

Where Sala Rossa, Palazzo Marchesini, via Marsala 26, Bologna (first floor)

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Perturbation theory is an approximation method to solve dynamical equations in physics. As physical systems get more and more complicated, even with infinite degrees of freedom, perturbation theory has been a universally powerful theoretical tool. It is a series expansion of a small parameter in a similar way as an elementary mathematical approximation sin(x) ~ x when x is small. Amazing successes of the 20th century physics can be attributed to perturbation theory. It has been applied to all the 4 fundamental forces in nature, including the strong force since its strength gets weak when quarks are very close. Therefore, it may seem that perturbation theory is enough to understand most of physical phenomena. Quite on the contrary! Thanks to amazing advances of both experimental technologies and theoretical methods, a lot of “non-perturbative” physics has been discovered. One of the most important challenges in physics is to establish a new paradigm for this. I will present integrability, which appears when a quantum system has an infinite number of conserved charges. I do it by taking two examples. The first is an excellent experiment with a cobalt compound which is a magnetic spin system described by Ising model with an extenal magnetic field. It realizes a huge E8 group symmetry predicted by Zamolodchikov. The other example is a gauge/string duality between string theories moving in 10D curved space and gauge theories in 4D. In both cases integrability is crucial.


Changrim Ahn is a professor of theoretical physic at Ewha Womans University in Seoul during the last 25 years. After graduating from Seoul National University with Bachelor degree, he received a Ph.D. degree at Stanford University in Califonia, U.S.A. with Prof. Michael Peskin as a thesis advisor. The thesis topic covered one-loop corrections in the Standard model and string theory in terms of modular invariance. His first research position was at a theory group in Conell University, where he collaborated with Andre LeClair on exact S-matrices of general coset conformal field theories with integrable relevant perturbations. Based on these results, Changrim Ahn has found a complete set of exact S-matrices of supersymmetric sine-Gordon model for the first time. Then, he moved to ICTP in Trieste, Italy where he collaborated with G. Mussardo on certain integrable models. After he found a permanent position in Korea, he introduced the integrability research to Korean physics community and organized many intenational workshops ever since. Also, he started intenational collaborations with Prof. F. Ravanini at Univ. of Bologna on finite-size effects from nonlinear integral equations, with Prof. R. Nepomechie at Univ. of Miami on exact worldsheet S-matrix of 3 dimensional AdS/CFT duality, with Prof. P. Pearce at Melboune, Australia on integrable lattice models, and with Prof. Z. Bajnok at Budapest, Hungary on S-matrix of deformed AdS/CFT and many others. He coauthored over 90 published papers with over 3000 accumulated citations with h-index 28. A review article on the AdS/CFT duality coauthored with other leading scientists has recorded over 1000 citations so far. As a P.I. of World Class University project sponsored by Korean Science Foundation with the largest grant at that time for 5 years, he has led a group of 30 researchers and professors including G. Smoot, a nobel laureate who became a chaired professor at Ewha. In addition to academic activities, he is serving as a member of Govenment Evaluation committee chaired by Prime Minister of Republic of Korea, for which he is selected as a recipient of a honorary medal of Korea.