APH N.S., Heavy Ion Physics 3 (1996) 131 - 134 HEAVY ION PHYSICS �9 Akad› Kiad£

Elementary Constituents Fields or Particle States?

Yuval N e ' e m a n

Wolt~on Distinguished Chair in Theoretical Physics Raymond and Beverly Sackler Faculty of Exact Sciences Tel-Aviv University, Tel-Aviv, Israel, and Center for Particle Physics, University of Texas, Austin, USA

Received 30 September 1995

1. Eugene Wigner, in Memoriam

The Hungarian Jewish Community in the first half of the XXth Century was remarkable for its vitality and creativity. XIXth Century emancipation (under Austria-Hungary) opened higher education to them, even if sometime only half way. In two fields, the num- ber density of brilliant individuals, who emerged from just these groupings, is outstanding. One such field is that of the humorists, the best known outside Hungary being George Mikes in England ("How to be an Alien", etc.) and Efraim (Ferenc) Kishon and "Dosch" in Israel. I shatI not pursue my discussion of the humorists, except for mentioning that my wife and I did introduce the Kishons to the Wigners (or vice versa) at a party in our Tel- Aviv home, on one of the Wigners visits to Israel in the Seventies. On the scientific side, we have Wigner, Szil• Teller, von Neumann and von Karman, tire individuals (G. Marx' "Martians") who left a deep mark on our world. This is perhaps a minor com- pensation for the fact that some ninety percent of this community some six hundred thou- sand souls--were gassed at Auschwitz.

It was also on that same visit at our home in Tel-Aviv that a very relaxed Eugene told me the story of his failed immigration to Israel. Ir was (I think) the academic year 1934/35 of 1935/36, which Eugene Wigner spent in Jerusalem, at Hebrew University. Most of the scholars were newly arrived refugees from Hitler's Germany. The university was small and poor, and at the end of the academic year, with two candidates, both from Hungary. One was Wigner, the other was Ladislaus Farkas, a brilliant physical chemist (1904- 1948). Eugene was a bachelor, whereas Farkas was married with a child. Eugene decided not to compete with his friend and left for the USA. Farkas hada great impact on a gener- ation of chemists in Israel, until he was killed in ah air crash on his way to the USA to pur- chase equipment. Wigner edited the Farkas Memorial Volume in 1952 [1].

Perhaps one Wigner visit to Israel which "made" the history books was that in the spring of 1901 (I was still in London at Imperial College at the time). He was visiting the

0231-4428/96/$ 5.00 �9 1996 Akad› Kiad£ Budapest

132 Yuval Ne'eman

Weizmann Institute (and taking part in a meeting of the Institute Board or some commit- tee) as the guest of Amos DeShalit (nuclear physicist and director of the W.I.) at the same time as I.I. Rabi. This was the time when J.F. Kennedy, newly in office, tried to stop the construction of a Nuclear Research Reactor by Israel at Dimona, in the Negev. There was no Non-Proliferation Treaty at the time. France had recently become the fourth nuclear power, China was on its way to become the fifth. Kennedy was alarmed by the possibility of Israel becoming the sixth and exerted pressure on Ben-Gurion to freeze the project. In his correspondence with Kennedy, Ben-Gurion had stressed that the reactor was aimed primarily at a scientific and technological research program and was only naturally and in- directly creating the additional potentiality of being used to develop a defense nuclear op- tion some day in the future, should circumstances ever require it. With Israel's geographic isolation in a region where its neighbors were staunchly refusing to recognize its right to exist, this was a welcome spin off, but there was no intention to go beyond the construc- tion of the reactor, as far as this aspect was concerned. One day while Wigner and Rabi were in Rehovot, DeShalit (asa result of a conversation with Ben-Gurion), took his distin- guished guests on a visit to Dimona, where they could view the research program and rel- evant equipment and testify to the validity of this aspect [2]. The visit did assuage Kennedy's fears for a while.

2. Quantum Mechanics

We met, Wigner and I, at various conferences and colloquia, from 1964 on (I first visited the USA in June 1963). Our bonds developed around physics and through the common friendship with Edward Teller (who first visited Israel in 1966, as my guest, but has since come almost yearly). In 1968 Salam organized a one month session on "Contemporary Physics" at Trieste, which we both attended. So did the Dirac's, and we formed mutual friendly ties with both brothers-in-law--who one day played a joke on the meeting by exchanging nametags. Physics-wise, after agreeing about many topics, we also discovered one disagreement, which we thoroughly analyzed together. Wigner believed that the non- Schr6dinger-like dynamical evolution upon the performance of a measurement, i.e. the collapse of the state-vector, was due to quantum mechanics being incomplete in that it does not take into account the mind ofan intelligent observer. This was, for Wigner, just one aspect of something missing in physics, some additional component of the physical description which would represent life. My own position was that (1) the collapse has nothing to do with intelligent observers. Any interaction of the system with a macroscopic system--leading to irreversibility will do a sa quantum measurement. In addition, I have since come to believe (2) that human intelligence is involved in the non-intuitive aspects of quantum mechanics, but in the negative sense. Darwinian evolution has evolved in us a "macroscopic intuition" in which space-time, determinism and collapsed state vectors appear to us as inherent parts of reality, whereas the true facts about existence are limited to Hilbert space and amplitudes. You can imagine that we thrived on these discussions-- which I later continued with another common friend, John A. Wheeler, who for a while shared some of Eugene's positions, but definitely later changed his mind on point (1), coming close to the interpretation I espoused.

Elementary Constituents--Fields or Particle States? 133

There was another aspect of the interpretation of quantum mechanics where Wigner was one of the first to adopta very modern view. Zeh [3] had just suggested the idea of the decoherating processes, which can be assumed to generate classical islands (e.g. the mea- surement apparatus) in a quantum universe. The Copenhagen interpretation could thereby be regarded as an "effective" theory, with primordial physics being fully of the quantum type. Wigner liked the idea and made various estimates which confirmed him in his confi- dence in the new interpretation (Hartle and Gell-Mann have become active propagators of this view lately).

3. Particles and Fields

In the Sixties, SU(3) (now known asflavor-SU(3)) symmetry, my first contribution in phys- ics, proved a success in classifying the strongly interacting particles (later named hadrons). Wigner had been awarded the Nobel Prize in 1963 for his application of group theory in physics. Two months later, the Omega Minus experiment confirmed our version of SU(3). When he published the text of his Nobei lecture in Physics Today, several months later, Wigner added footnotes stressing this new victory of the theory of Lie groups.

For the next thirty years, Group Theory was another bond I was awarded the "Wigner Medal" in 1982, after Wigner himself and V. Bargmann received the award in 1978 and I.M. Gelfand in 1980. There was one discussion topic, though, which we pur- sued from meeting to meeting. Wigner had, a s a matter of fact, defined what a particle was, by classifying the unitary irreducible representations of the Poincar› group, espe- cially those of positive mass (time-like) and massless (null). His approach was from phe- nomenology and his criterion of elementarity was stability. He l"egarded the electron, the neutrino and the proton as the only elementary particles, in an inwitively well-founded judgment. The neutron, for instance, was a compound state made, through the weak inter- action, from a proton, an electron and an anti-neutrino. With SU(3) [4], I was putting the unstable neutron, and the even less stable A ~ Z +, E ~ Z-, E ~ E- on the same footing as the stable proton. Somehow, he felt very uneasy about this feature, even though he greatly ap- preciated the action of the symmetry group and its applications. When the picture started being reinterpreted in terms of constituent quarks [5], he breathed freely. He only had to replace the proton by the lightest quark, in his list of elementary states.

However, this was soon to become part of a new confusion, from his basically solid phenomenological point of view. After 1973, particle physicists adopted the Standard Model. This was a local gauge theory of color SU(3)|174 Wigner had no prob- lem with the gauge group, but he was highly suspicious of Relativistic Quantum Field Theory and its renormalization process. Like Dirac, he was unhappy about the mathemati- cal foundations. As to the list of elementary constituents, this was now a list offields. Quark fields, according to the new doctrine, do not materialize as quark states in Hilbert space; only compound systems, each made of three "valency" quarks ( o r a quark-anti- quark valency pair) plus a "'sea" of gluons and quark-antiquark pairs, end up making a (hadron) state in Hilbert space. Quarks also do not existas asymptotic states at large dis- tances. How could these entities fit into Wigner's ,boxes? You do not have to be an ortho- dox positivist, to be suspicious of constituents which are permanently confined.

134 Yuval Ne'eman

Asymptotic freedom [6] made ir somewhat easier to accept the new theory. J. Fried- man, H. Kendall, and R. Taylor were awarded the Nobel Prize in physics in 1990 for ah experiment which amounted to an observation of the quarks at the high-energy (short dis- tance) end of the spectrum, in the SLAC 1967-70 deep-inelastic eleetron-nucleon seatter- ing. Wigner agreed that it is the theory which tells us what we ate measuring. Rutherford's scattering of aluminium sheets by alpha particles--using bis calcul• and models in- deed revealed that the atom is empty, except for a (105 times sma]ler) nucleus. Similarly, using Feynman's and Bjorken's calculations made it clear that the deep-inelastic experi- ments were "seeing" the quarks. This was a concrete result and fitted with Wigner's phi- losophy. He was still unhappy about having six kinds of quarks and three sets of leptons, but he could well believe that this would some day be further resolved in terms of more basic units, such as the Harari-Shupe rishons, for example. He was also unhappy about the growing separation between Hilbert space states and fundamental fields. Asymptotic Freedom could reinstate quarks as Hilbert space states at the high-energy limit. But what about gluons? Would there also be a regime in which they would become observable states and warrant inclusion in Hilbert space? I had no answer to that one.

Dedicated to the memory of E.P. Wigner

References

1. A. Farkas and E.R Wignet, L. Farkas Memorial Volume, 1952. 2. see for example S.M. Hersh, The Samson Option, Random House, NY (199t), p. 102.

This book is rather weak on the factual side, but Wigner's visit is r› to in Kennedy's documentation.

3. H.D. Zeh, Found. ofPhys. 1 (1970) 69. 4. Y. Ne'eman, Nucl. Phys. 26 (1961) 222; M. Gell-Mann, Phys. Rev. 125 (1962) 1067. 5. H. Goldberg and Y. Ne'eman, Nuovo Cim. 27 (1963) 1; M. Gell-Mann, Phys. Lett. 8

(1964) 214; G. Zweig, CERN reports TH401-402 (1964), unp. 6. G. t 'Hooft, Marseilles Conference on Renormalization of Yang-Mills Fields and Appli-

eations to Particle Physics (June 1972, unp.); D.J. Gross and F. Wilczek, Phys. Rev. Lett. 30 (1973) 1343; H.D. Politzer, Phys. Rev. Lett. 30 (1973) 1346.