The Impact of Dirac on 20th Century Science A. Zichichi CERN, Geneva Preserving a blackboard with its writing is A panel discussion held after the dedica­ one of many warmhearted gestures that tion ceremony allowed several prominent have been made by the Florida State Univer­ scientists to review Dirac's stupendous sity, Tallahassee in rememberance of Paul achievements. Among them was Professor Dirac. He supervised students and lectured Zichichi who compared Dirac's contribution at the University after his retirement from to our understanding of the world around Cambridge in 1971 until his death at 82 in us with that of Galilei. He also related a 1984. famous story on why the proton was consi­ The blackboard is kept in a special col­ dered for some time as the antiparticle of lection room in the University's new Paul the electron. A.M. Dirac Science Center Library that was dedicated to Dirac last December. The room, which is open to the public, houses a catalogued collection of materials: some The statue of Paul Dirac, unveiled in Decem­ were transfered from Cambridge and others ber 1989, that is placed in front of the Flori­ were donated by his wife who still lives in da State University library commemorating Tallahassee. his name. The useless particles : antielectron seemed to many scientists to today that there are more useless than use­ be merely science fiction. Why should we ful particles. The world familiar to us is from care for particles which do not exist as con­ made only with particles belonging to the Galilei to Dirac stituents of the matter around us? What so-called "first family": the useful one. can we expect to find from such specula­ Nevertheless, it is thanks to the "useless" tions, appearing as they do to be philosophi­ particles that we are able to trace our exis­ Albert Einstein provided us with the first cal and not directly related with the physical tence back to the origin of the universe. In conclusive synthesis of the enormous va­ world around us? fact, the world opened up by Paul Dirac exis­ riety of the world, as it appears to us now. Where is the antielectron ? Where is the ted only for a few instants after the big Thanks to him we now know that space and antiproton ? A few years after the discovery bang. At that time — 15-20 billion years time are not separate entities but mani­ of the Dirac equation, the antielectron was ago — there was a perfect mixture of parti­ festations of an unique, four-dimensional found in cosmic radiation by Anderson and cles and antiparticles of all families. Today, quantity. Moreover, energy and mass are Blackett. A further quarter of a century was after such a long interval of time, the equivalent. Finally, in order to understand needed before the discovery of the anti­ universe has evolved into a world where the structure of the universe where mass- proton. This long delay confirmed the belief only particles of the first family exist. energy plays a fundamental role, we do not that the world envisaged by Dirac represen­ The huge intellectual achievement of Paul need to worry. Mass-energy is not an addi­ ted an abstract and peculiar scientific spe­ Dirac is unprecedented in the history of tional entity as it is intrinsically contained in culation, not intimately connected with our science: when Galilei studied the way the structure and properties of space-time, world. stones run along an inclined plane, the i.e. its curvature. The recent discovery of the existence of stones were there; when Einstein succee­ With Einstein, the human intellect closed only three families of particles was made ded in his great synthesis he could see that a fascinating intellectual adventure, started using a supercollider in which billions upon space-time, mass-energy where there and by Galileo Galilei who wanted to understand billions of antielectrons collided with bil­ in front of everybody's eyes; but when the world around us. He completed Galilei's lions upon billions of electrons. If we now Dirac started to think of the existence of the adventure with his "superb synthesis" of know the number of fundamental particles antielectron and of antimatter, nothing of the basic quantities space, time, energy and it is due to these gigantic machines. But this sort was around. Neither antiparticles mass. none of them would exist if the elusive nor "useless" particles had ever been ob­ It appeared as if nothing else needed antiparticles predicted by the Dirac equa­ served. Paul Dirac's equation nevertheless to be understood until Dirac's equation tion were not there. tells us that if space-time, mass-energy and opened a new horizon towards a new world Dirac opened the way towards research matter are there, then antimatter and "use­ — a world which does not exist around us. for the "useless" particles and we know less" particles must also be there. On this planet where we live, there are no mines of antimatter. In the deepest corner of the ocean or on the highest summit of Why the proton came who pointed out that the antiparticle pre­ Everest, we shall never find a piece of anti­ dicted by the Dirac equation could not be matter. However, the existence of anti­ to be the antielectron : the proton. This is fairly self-evident since matter is as real as the existence of a stone. the true story the mass of the proton is 2,000 times larger The reason Paul Dirac will rank above than that of the electron and, in the Dirac Albert Einstein in the history of science is equation, the mass of the particle and the the radical novelty of his thinking. The The "useless" particles represented antiparticle must be the same. The problem meaning of the Dirac equation and of its mankind's greatest intellectual achieve­ was: how could it be that Dirac himself did consequences is profound, far-reaching and ment and Paul Dirac was the author. He not see the discrepancy? fascinating, much more so than Einstein's was also the "author" of an irreverent tale. When I had the great privilege of meeting "superb synthesis". As a student, one problem struck me. Paul Dirac, I tried to find out how this could Dirac's prediction of the existence of the Pauli, the text-books said, was the physicist have happened. It was for many years a 116 Europhys. News 21 (1990) hopeless task. Until 1982, when I met Piotr So let us take the proton as a good candi­ Kapitza who worked in Cambridge with date for being the antielectron." And so he Dirac, his close friend, at the Rutherford started spreading the rumour that he was Laboratory. seriously considering the proton as the anti­ After the discovery of the Dirac equation, particle predicted by his equation. As for its at every seminar no matter the topic, some­ mass, this was a detail that he was study­ one — often myself, Piotr Kapitza told me ing. Laughing at the end of each seminar — asked: "Dirac, where is the antielec­ stopped as soon as the rumour circulated. tron ?". Needless to say, a chorus of laughs No one any longer asked the question, closed the seminar. "Dirac, where is the antielectron?". Scientific Collaborators Paul Dirac eventually had had enough and It was great to hear the story told by decided to stop this. He knew that few col­ Piotr Kapitza, with his humour and ability. The Institute of micro- and leagues were able to follow his arguments, Paul Dirac was present and was laughing opto-electronics of the Swiss and that even fewer had seriously studied at Kapitza recollecting the fine hours in Federal Institute of Technology is his equation. Wanting to fool all these Cambridge. seeking applications for limited people, Dirac announced: "The proton has Books carrying Pauli's remarks should time research positions in the the opposite charge to that of the electron. perhaps be amended. areas of device physics and the properties of heterojunction field effect transistors. IOM Delegates to Council The positions would be suitable for candidates seeking to start a The two new delegates of the Indivi­ worked as a Research Fellow at the doctoral research program or for dual Ordinary Members to EPS Council, Fritz Flaber Institute in Berlin. A former recently graduated scientists. elected following the recent postal bal­ Director of the Laboratory and a former Candidates are asked to apply lot, are : Chairman of CMD, he remains a mem­ to : M. Balkanski, Paris (F) ber of MIT's Advanced Study Institute. Prof. M. Ilegems J. Fischer, Prague (CZ) Dr. Jan Fischer is a senior scientist at Institut de Micro- et Opto­ The delegates remaining in office are: the Institute of Physics, Czechoslovak électronique G. Chiarotti, Rome (I) Academy of Sciences, Na Slovance 2, EPFL P. Choquard, Lausanne (CH) CS-180 40 Prague 8. A Czech citizen CH -1015 Lausanne F. Netter, Paris (F) born in 1932, he has Doctorates from Switzerland H. Ryde, Lund (S) both the Institute and from Charles E. Skrzypczak, Warsaw (PL) University, Prague. He has worked as a D. Stacey, Oxford, (UK) visiting scientist at JINR (Dubna), ICTP Meeting Announcement W.T. Wenckebach, Leiden (NL) (Trieste) and CERN; and as a visiting 10-12 Sept. 1991 Lund, Sweden Professor Minko Balkanski is with the professor at the University of Langue­ Nuclear Physics at Storage Rings: Disassembly of Nuclei and Subthreshold Meson Emission Solid State Physics Laboratory, P. and doc, Montpellier. Chief Editor of the P. Arve, Dept. of Mathematical Physics, M. Curie University, 4, Place Jussieu, Czechoslovak Journal of Physics, he Lund Institute of Technology, POB 118, S-22100 Lund Tour 13, F-75230 Paris Cedex 05.