Wolfhart Zimmermann: Life and Work
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JID:NUPHB AID:14268 /FLA [m1+; v1.280; Prn:28/02/2018; 9:46] P.1(1-11) Available online at www.sciencedirect.com ScienceDirect Nuclear Physics B ••• (••••) •••–••• www.elsevier.com/locate/nuclphysb Wolfhart Zimmermann: Life and work Klaus Sibold Institut für Theoretische Physik, Universiät Leipzig, Postfach 100920, D-04009 Leipzig, Germany Received 19 December 2017; received in revised form 17 January 2018; accepted 22 January 2018 Editor: Hubert Saleur Abstract In this report, I briefly describe the life and work of Wolfhart Zimmermann. The highlights of his scien- tific achievements are sketched and some considerations are devoted to the man behind the scientist. The report is understood as being very personal: at various instances I shall illustrate facets of work and person by anecdotes. © 2018 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction The present report is based on a colloquium talk given at the end of a memorial symposium to honour Wolfhart Zimmermann: Max Planck Institute for Physics, Munich (May 22–23, 2017) I borrowed freely from the following obituaries: Physik-Journal 15 (2016) Nr. 12 S.50 W. Hollik, E. Seiler, K. Sibold Nucl. Phys. B193 (2016) 877–878 W. Hollik, E. Seiler, K. Sibold IAMP News Bulletin, Jan. 2017 p. 26–30 M. Salmhofer, E. Seiler, K. Sibold 2. The beginning Wolfhart Zimmermann was born on February 17, 1928 in Freiburg im Breisgau (Germany) as the son of a medical doctor. He had an older sister with whom he liked to play theater and, when E-mail address: [email protected]. https://doi.org/10.1016/j.nuclphysb.2018.01.022 0550-3213/© 2018 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. JID:NUPHB AID:14268 /FLA [m1+; v1.280; Prn:28/02/2018; 9:46] P.2(1-11) 2 K. Sibold / Nuclear Physics B ••• (••••) •••–••• visiting Gymnasium, to talk to in “Giganisch” – a language which he invented, because he found Latin too easy. In 1946 he entered the university in Freiburg to study mathematics and physics. As far as lectures and seminars are concerned WZ was somewhat sceptical about their useful- ness for him: “Either they were too fast or too slow for me. Either I had to think about the new content – then I was too slow. Or I understood it instantly, then I was too fast and the lecture boring.” A good measure to evaluate this statement and to put it in the right perspective is to look at the facts. Already in 1950 he finished with a doctoral degree in mathematics. His thesis was devoted to topology [1], published in [2]. He once told that he had written an earlier dissertation, but abandoned it because he found out that the main result could be proven in a much simpler way, hence considered this work as inadequate for a doctoral degree. He published a further article on topology [3]. These papers were written in style and spirit of BOURBAKI, on which he commented privately “I can read BOURBAKI like the newspaper.” 3. LSZ – the first highlight In 1952 Wolfhart Zimmermann joined the group of Werner Heisenberg at the Max-Planck- Institut für Physik in Göttingen as a Research Associate, a position which he held until 1957. Remarkably, his first physics paper [4] did not deal with quantum field theory (QFT), but with the thermodynamics of a Fermi gas in interaction. He bought his ticket to Heisenberg’s group, later called “der Feldverein”, with a paper on the bound state problem in field theory [5]. It was co-authored by Vladimir Glaser, a Croatian physicist, who was to accompany Wolfhart Zim- mermann on his scientific way for a long time. Truly famous, however, were three papers [6–8] written together with Harry Lehmann and Kurt Symanzik. They contained what later was coined the “LSZ formalism” of quantum field theory. Based on the principles of Lorentz covariance, unitarity and causality of Green functions and the S-matrix they provided the first axiomatic for- mulation of quantum field theory. Lehmann, Glaser and Zimmermann [9] then, conversely, gave sufficient conditions which a set of functions has to fulfil such that these functions give rise to a field theory in the sense of LSZ. LSZ did not refer to perturbative expansions, but nevertheless proved to be greatly successful in its perturbative realization and thus extremely powerful in practice. Until the present day it serves as the most efficient description of scattering amplitudes in particle physics. The key idea is the following: in the remote past and future a scattering experiment (idealized) deals with free particles, whereas interaction takes place only in a finite region of spacetime. The respective fields are related by the asymptotic condition: √ φ(x) −→ zφ out (x), (1) x0→±∞ in where z is a number and φ out are free fields which satisfy the field equations in 2 2 + m φ out (x) = 0, (2) in whereas φ(x) is an interacting field. The limit is understood in the weak sense, i.e. it is valid only for matrix elements. (In parentheses we note that this notion of weak convergence which is of utmost importance has presumably been introduced by LSZ for the first time in the theory of particle physics.) JID:NUPHB AID:14268 /FLA [m1+; v1.280; Prn:28/02/2018; 9:46] P.3(1-11) K. Sibold / Nuclear Physics B ••• (••••) •••–••• 3 In a scattering experiment ni particles are prepared in an initial state and go over into nf particles in the final state. The transition is described by the S-operator with matrix elements Sfi. Those are given by the famous LSZ-reduction formula = | ≡ | Sfi f i p1 ...pnf q1 ...qni (3) + nf n −1 nf ni i = √ lim (p2 − m2) (q2 − m2)G(˜ −p , ..., −p ,q , ..., q ) (4) z k j 1 nf 1 ni k=1 j=1 n nf +ni ni f i − = √ dx e iqkxk (2 + m2) dx eipj yj (2 + m2) × z k xk j yj k=1 j=1 | G(y1 ...ynf ,x1 ...xni ) 0= qk ωk 0= pj ωj 2 → 2 2 → 2 0 0 (with lim: pk m , qj m , pk > 0, qk > 0). Here G(y1, ..., ynf , x1, ..., xni ) denote the Green functions = G(y1,...,ynf ,x1,...,xni ) Tϕ(y1)...ϕ(xni ) , (5) the vacuum expectation value of the time ordered product of field operators. They thus permit to calculate the S-matrix and are themselves determined by equations of motion. An important generalization of this reduction formula expresses the matrix elements of a (composite) operator O in terms of the Green functions with an insertion (details see below). + −1 n l n l p , ..., p |O(x)|q , ..., q = √ lim (p2 − m2) (q2 − m2) × 1 n 1 l z k j k=1 j=1 ˜ × GO(x)(−p1, ..., −pn,q1, ..., ql) GO(x)(y1, ..., yn,x1, ...xl) =T O(x)ϕ(y1)...ϕ(xl) (0) (0) (0) (0) i L T O(x)ϕ(y1)...ϕ(xl)=RT : O (x) : ϕ (y1)...ϕ (xl)e h¯ int Historical remark: Another axiomatic formulation of QFT was initiated by Wightman (1956). The relation of the LSZ-scattering theory to those axioms and clarification of the role of funda- mental fields were given by Haag (1958, 1959) and in particular by Ruelle (1962). As mentioned above these formulae for Green functions and the S-matrix can be satisfied in a perturbative manner. In practice the most often employed technique is in terms of diagrams as introduced by Feynman. With every elementary interaction one associates a vertex (symbolizing the specific interaction) and emanating lines which stand for particles propagating in spacetime. x1 x2 x4 x3 A scattering process is described by diagrams where these elementary vertices are linked by lines. The vertices and lines are interpreted in terms of mathematical prescriptions, the “Feynman JID:NUPHB AID:14268 /FLA [m1+; v1.280; Prn:28/02/2018; 9:46] P.4(1-11) 4 K. Sibold / Nuclear Physics B ••• (••••) •••–••• rules”, which eventually permit to calculate Green functions and the S-matrix. In the most naive version diagrams are ordered according to the number of vertices they are made of; this results into a power series of coupling constants: the perturbation series. Since the diagrams mimic physical processes, they are intuitively appealing and, therefore were overwhelmingly used in particle physics (and beyond: e.g. in condensed matter physics). But, of course, the mathematical expressions for Feynman rules have to be derived by a consistent algorithm. Usually it exploits the Gell-mann-Low formula for Green functions G(x1, ..., xn) =T(φ(x1)...φ(xn)) (6) (0) (0) i L(0) T φ (x1)...φ (xn)e int = (7) i L(0) e int For the S-operator this is tantamount to the Dyson formula L S = Tei int (8) On this level it is (almost) evident that the fundamental axioms – Lorentz covariance, unitar- ity and causality – are satisfied. For the subsequent discussion it is crucial to observe that the propagator for a (real, spinless) field i ˜ (p) = (9) c p2 − m2 + iε is, mathematically speaking, not a function, but rather a distribution. This can be seen by cal- culating the product at the same point c(x − y)c(x − y). The result is infinite, hence apriori meaningless. It implies that many of the Feynman diagrams containing closed loops are mean- ingless. Therefore one faces the problem to give mathematical meaning to such expressions and at the same time not to violate the fundamental axioms in doing so.