II. THE CONFERENCES ON THE SEARCH OF THE LAST CONSTITUENTS OF MATTERS
The participants in the 1st Solvay Conference in Physics are shown in the, perhaps, most famous photograph of physicists of any time.
Hendrik Antoon Lorentz, from Leiden (Holland), presided the conference, whose general theme was the "Theory of Radiation and the Quanta." The conference (5) was opened with speeches by Lorentz and Jeans, one on "Applications of the Energy Equipartition Theorem to Radiation," the other on the "Kinetic Theory of Specific Heat according to Maxwell and Boltzmann." In their talks, the authors explored the possibility of reconciling radiation theory with the principles of statistical mechanics within the classical frame. Lord Rayleigh, in a letter read to the conference, stressed again the difficulty he had brought out in his masterly analysis (6) of 1900 and added: "Perhaps one could invoke this unsuccess as an argument in favor of the opinion of Planck and his school, that the laws of Dynamics (in their usual form) can not be applied to the last constituents of bodies. But I should confess that I do not like this solution of the difficulty. I do not see any inconvenience, of course, in trying to follow the consequences of the theory of the elements of energy (i.e., quanta). This method has already brought interesting consequences, due to the ability of those who have applied it. But it is difficult for me to consider it as providing an image of reality."
Two papers, one by E. Warburg and the other by H. Rubens, summarized the experimental measurements of the blackbody radiation.
These contributions were followed by an extensive presentation of the "Law of the Black Body Radiation" by Max Planck, who discussed, among other aspects, the physical nature of the constant h. Does this "quantum of action," he said, possess a physical meaning for the propagation of electromagnetic radiation in vacuum, or does it intervene only in the emission and absorption processes of radiation by matter?
The first point of view had been adopted by Einstein in the frame of his hypothesis of the "light quanta."(2)
Views of the second kind had been adopted by Marmor and Debye, (7) who conceived the quantum of action h as an elementary domain of finite extension in the space of phases intervening in the computation of the probability W(E) for the energy density to have the value E.
In his contribution Nernst dealt with "the application of the Theory of Quanta to a few Physico-Chemical Problems," in particular the connection between Nernst theorem (8) and the quantization of energy.
Sommerfeld applied the theory of quanta to the emission of X- and gamma - rays, to the photoelectric effect, and sketched the theory of the ionization potential. At the beginning of his paper he discussed in some detail the relationship observed in the emission of X- (or gamma - rays) by cathode (or beta) rays and arrived at the conclusion that "large quantities of energy are emitted in shorter times and small quantities of energy in larger times."(9). According to Sommerfeld this empirical result speaks in favor of the central role played in atomic and molecular phenomena by the quantum of action h introduced by Planck, the dimensions of which are energy multiplied by time.
The problem of specific heats, treated by Jeans from the classical point of view, as I said above, was discussed by Einstein in the case of solids, with special regard to the discrepancy observed at low temperature between the measured values and those deduced from the theory he had constructed in 1907 by quantizing the mechanical oscillators (3) as Planck had quantized the radiation oscillators.
Knudsen reported on the experimental properties of ideal gases, Kamerlingh Onnes on the electrical resistance of metals at low temperature in particular on superconductivity he had discovered in Leiden in 1911, (10) and Langevin on the kinetic theory of magnetism and the central role played by the magneton, that is the magnetic moment of the elementary magnets which had been introduced in different approaches by Weiss and Langevin himself.
Finally Jean Perrin presented an extensive (97 pages) "Rapport sur les Preuves de la Realite Moleculaire" in which he summarized his famous experiments on the Brownian motion of emulsion droplets suspended in a liquid and discussed the fluctuations, the determination of the elementary charge, the ( decay of some radioactive nuclei, and the corresponding production of helium. The last section of the paper contains a comparison of the values of Avogadro's number deduced by completely different methods. The very satisfactory agreement between all these values provides the proof of molecular reality announced in the title of the paper.(11)
Nothing was said during the conference about the structure of the atom except in a short remark to Jeans' report by Rutherford in which he pointed out that the atom can be divided into two parts, an external part and an interior part and that the generalized coordinates, which according to Jeans appear not to contribute to the specific heat, could be those connected with the internal part of the atom.
The 2nd Solvay Conference took place in 1913 and its theme was "The Structure of Matter," (12) The meeting was opened with a long contribution (44 pages) by J.J. Thomson on the "Structure of the Atom" in which he tried to explain in a qualitative way from the classical point of view many general properties of matter.
In his long paper, however, there is no mention at all of the Rutherford model which appeared in the Philosophical Magazine of 1911 (13) or of the papers by Geiger published in 1908 and 1910 (14) and by Geiger and Marsden, (15) which appeared shortly before the meeting, on the scattering of ( particles by atoms, or of the theoretical paper in which Niels Bohr had quantized the circular orbits of the electron of the Rutherford model. (16) Only in the discussion that followed J.J. Thomson's paper, did Rutherford mention the recent results of Geiger and Marsden which "bring to the conclusion that the atom consists of a positive nucleus, surrounded by a collection of electrons whose number is equal to one half the atomic weight..." In a second intervention, in the same discussion, Rutherford added some more detail and said: "An accurate comparison of the theory with the experiments has been made by Geiger and Marsden and the conclusions of the theory have been found in perfect agreement with the experimental results."
Then Langevin pointed out that the central nucleus mentioned by Rutherford had to contain electrons in order to explain the emission of beta-rays by radioactive atoms and Marie Curie elaborated the idea that within the atom there should be "two kinds" of electrons. The peripheral electrons responsible for the processes of absorption and emission of radiation and for conductivity of metals and, in addition, the electrons emitted in the ( decay of some radioactive nuclei.
The successive report to the conference was presented by Marie Curie who, in 5 short pages, dealt with "The Fundamental Law of Radioactive Transformations." She discussed the exponential law of decay and its interpretation in terms of a probability for an atom to decay independently from its previous life. This point of view had been already recognized at the beginning of the century. However, on this occasion she added a short but adequate review of the idea of her pupil and collaborator Debierne (17) insisting on the existence of a "disorder inside the central part of the atom (the nucleus) where its constituents should move with very high velocity judging from that of the emitted particles." Madame Curie examined even the possibility of defining a kind of temperature internal to the nucleus, much higher than the external temperature.
The discussion that followed, in which Nernst, Rubens, Brillouin, Wien, Llundman, and Langevin took the floor, will remain forever one of the moments of highest interest in the history of the gradual infiltration of the probability law into physical sciences which foreran the advent of quantum mechanics and its statistical interpretation.(18)
The rest of the conference was dominated by the recent discovery of Friedrich, Knipping, and Laue (19) of the diffraction of X-rays in crystals and reviews of the first steps in X-ray spectroscopy and in the investigation of crystal properties.
The 3rd Solvay Conference in Physics took place in 1921, after a long interruption due to the First World War. Its theme was "Atoms and Electrons."(20) It was centered on the Rutherford model of the atom and Niels Bohr's atomic theory. Bohr, however, was not able to attend the conference because of illness.
In a speech, 20 pages long, Rutherford discussed "The structure of the Atoms." After a detailed presentation of the results of Geiger and Marsden on the scattering of alpha particles by atoms, Rutherford discussed the simple relation obtained by Moseley between the frequency of the X-ray lines of the different elements and their number of order in the periodic system,(21) which was then recognized to be identical with the ratio of the electric charge of the nucleus and the absolute value of the charge of the Electron.
Rutherford gave an estimate of the dimensions of the nucleus and then discussed the passage of beta and alpha particles through matters, the transmutation of nitrogen into oxygen he had observed 2 years before,(22) the existence of isotopes of both radioactive and stable elements, their separation, and finally the structure of nuclei for which a reduction of the mass had been established with respect to the sum of the masses of their constituents, which were assumed to be protons and electrons, in appropriate number.
Maurice de Broglie discussed the relation E=hv in various phenomena, such as the photoelectric effect, the production of light and X-rays in collisions of electrons against atoms; Kamerlingh Onnes, the paramagnetism at low temperature and the superconductivity; and de Haas, the angular moment of a magnetized body. At the end of the conference Paul Ehrenfest read a paper sent by Niels Bohr on "The Application of the Theory of Quanta to Atomic Problems" and added a survey on "The Correspondence Principle." Also, a paper by Millikan on "The Arrangement and Movements of the Electrons Inside the Atoms" not presented to the conference was added at the end of the proceedings.
The next Solvay Conference along the same line of thinking was the 5th held in October 1927 on "Electrons and Photons."(23) Later Langevin said that this was the occasion in which "la confusion des idée's atteignit son maximum!" Sir William L. Bragg commented "I think it has been the most memorable one which I have attended..." Heisenberg and Bohr also expressed their highest satisfaction.(24)
Quantum mechanics had exploded between 1923 and 1927. A.H. Compton, in 1923, had discovered the change in frequency of X-rays scattered from the electrons (the Compton effect).(25) Compton and, independently, Debye had underlined the importance of this discovery in support of the Einstein conception of light-quanta or photon propagation in space.(26)
The paper of Louis de Broglie, associating a wavelength to any particle, had appeared in 1925.(27) The five notes of Schrodinger on wave mechanics appeared in 1926.(28) The various papers on the representation of physical quantities by matrices by Born, Heisenberg, Jordon, Dirac, fand Pauli were published between 1925 and 1927.(29) The results of diffraction experiments by Davisson and Germer (30) of electrons scattered by a single crystal of nickel and by G.P. Thomson and Reid (31) of electrons transmitted by celluloid films were in 1927.
The paper by Max Born on "Quantummechanik des Stossvorgange," in which he had proposed the statistical interpretation of the wave function, had appeared in 1926. (32) Niels Bohr had presented his principle of complementarity at the Como Conference in September 1927 (33) and Heisenberg had formulated the uncertainty principle shortly before the Solvay Conference.(34)
The Conference was opened on October 24 with reports by Lawrence Bragg and Arthur Compton about the new experimental evidence regarding scattering of X-rays by electrons exhibiting widely different features when firmly bound in crystalline structures of heavy substances and when practically free in atoms of light gases.
All other contributions regarded various aspects of the new quantum mechanics.
Louis de Broglie discussed the relations between energy, momentum, and wavelength for photons as well as for electrons and examined the results of the Davisson-Germer experiment on the diffraction of electrons by crystals, which were in perfect agreement with theory.
Born and Heisenberg presented the foundations of the quantum mechanics in which the classical kinematic and dynamical variable are replaced by operators obeying a noncommutative algebra involving Planck's constant and showed that the introduction of these operators in the Hamiltonian gives rise to the Schrodinger equation. Schrodinger summarized the main features of wave mechanics and applied it to radiation, showing that the electric moment deduced in this approach is equivalent to a matrix of the theory of Born and Heisenberg.
Finally, Niels Bohr gave a report on the epistemological problems involved in the new quantum mechanics and stressed the viewpoint of complementarity. In the very lively discussion that followed, differences in terminology gave rise to great difficulties for agreement between the participants. The situation was humorously expressed by Ehrenfest, who wrote on the blackboard the sentence from the Bible, describing the confusion of languages that disturbed the building of the Babel tower.
The discussions, started at the sessions, were continued during the evenings within smaller groups, in particular, among Bohr, Ehrenfest, and Einstein, who, as is well known, was reluctant to renounce the deterministic description.
The discussion on these matters between Einstein and Bohr went on for years. It was taken up again at the 1930 and 1933 Solvay Conferences as well as in other places. It has been summarized by Niels Bohr in a chapter of the well-known book Albert Einstein: Philosopher-Scientist, which appeared for the first time in 1949.(35)
At any occasion Einstein proposed a new argument or a new "thought experiment" designed to illuminate some paradoxical consequences of quantum mechanics. On some occasions immediately, on other some time later, Bohr always was able to give an answer fully satisfactory in the language of quantum mechanics. Bohr's arguments were accepted by the majority of the physicists but did not satisfy Einstein, or a few others (36) who still were reluctant to renounce a view of the world they considered obvious and natural. These world views, called by some authors "local realistic theories of nature,"(37) are based on three assumptions or premises which, in their opinion have the status of world established truths, or even self-evident truths.(38) This definition of "reality" is what we are used to ascribing to systems of macroscopic particles and differs from the "reality" of quantum states of atomic or subatomic systems.
The progress undergone in recent years toward a solution of the problem raised by Einstein 1927 has become possible because of two developments; (I) A few experimental techniques, in particular, the methods for measuring very short times with good accuracy, have permitted in recent years the execution of several experiments which in their essence are practical versions of the "thought experiment" proposed and discussed in 1935 by Einstein, Podolski, and Rosen.(39) (II) A procedure of analysis of their results has been made possible by the work of Bell (40) who has derived in the frame of "local realism" a relation (the Bell inequality) obeyed by "local realistic theories" but violated by quantum mechanics.
Although not all the results of the experiments carried on until now are consistent with one another, most of them not only violate the Bell inequality but are also in good agreement with the prediction of quantum mechanics. The experimental errors are still rather large but in a few years this matter will be definitely settled by the results of further, more accurate experiments. It is rewarding that the final word even in this philosophical debate started in 1927, when quantum mechanics was in its infancy, will be definitely settled by accurate results of experiments designed and carried on with the most refined techniques of half a century later.
The 7th Solvay Conference in Physics was held in 1933, and was entitled the "Structure and Properties of Atomic Nuclei."(41) It was chaired by Langevin who, after the death of Lorentz (4 February 1928), had become President of the Scientific Committee. It took place at a time when the field had recently undergone extremely rapid developments. The conference was really marking the beginning of modern nuclear physics.
Cockcroft presented an extensive paper (56 pages) on the "Disintegration of Elements by Accelerated Particles" in which he reviewed the various types of accelerators available in those days (Greinacher or Cockcroft and Walton voltage multiplier, van de Graaf electrostatic accelerator, cyclotron, etc.) and the reactions that he had discovered, in collaboration with Walton, not long before in a few light elements (Li, F, Be, C, N) irradiated with protons and deuterons accelerated up to energies of the order of a few hundred kiloelectronvolts.(42)
In the discussion that followed, Rutherford added information about the work he was carrying on, in collaboration with Oliphant, on various reactions produced in lithium by bombardment with protons and deuterons, (43) and Ernst Lawrence described in more detail the cyclotron he had invented and first constructed with a few collaborators in 1932.(44)
The next speech was by Chadwick who treated (in 32 pages) the anomalous scattering of the ( particles, the transmutation of the elements, and the evidence for the existence of the "neutron" he had discovered in 1932.(45)
Frederick Joliot and Irene Curie discussed the gamma-rays emitted in association with neutrons by berillium irradiated with alpha-particles and reported to have observed under the same conditions also the emission of fast positrons. The origin of these particles was not yet clear at the time of the Solvay Conference. It was understood by the same authors a few months later when they discovered the artificial radioactivity induced by ( particle bombardment which normally takes place by emission of positrons.
In the discussion that followed a number of participants took the floor: Lise Meitner, Werner Heisenberg, Enrico Fermi, Maurice de Broglie, Wolfgang Bothe, and Patrick Blackett.
In his intervention Blackett treated the discovery of the positron in cosmic rays by C.D. Anderson in 1932 (46) and its confirmation by Blackett and Occhialini,(47) who had introduced, for the first time, the technique of triggering a vertical cloud chamber by means of the coincidence between two Geiger counters, one placed above, the other below the chamber. Blackett also discussed a number of papers by Meitner and Philipp, Curie-Joliot, Blackett, Chadwick and Occhialini, and Anderson and Neddermeyer,(48) all appearing almost at the same time, on the production of positrons in various elements irradiated with the gamma-rays of 2.62 MeV energy of Thc. These were the first observations of electron-positron pair production. He also pointed out that the observed production of positrons has a cross section larger than the nuclear dimensions, and therefore, most probably, does not originate from a nuclear process.
Then Blackett examined the nature of showers observed in cosmic rays starting from Bruno Rossi's curve showing the dependence of the number of observed showers as a function of the thickness of the layer of lead in which they are produced.(49) Then he presented the results he had recently obtained in collaboration with Occhialini by means of their new experimental technique mentioned above.(47)
The next paper was by Dirac on the "Theory of the Positron." In the following discussion Niels Bohr made a long intervention on the correspondence principle in connection with the relation between the classical theory of the electron and the new theory of Dirac.
Then Gamow talked about the "Origin of gamma-Rays and the Nuclear Levels" and Heisenberg, on "The Structure of the Nucleus," discussed the exchange forces of the two types that Heisenberg (50) himself and Majorana(51) had proposed not long before.
In the discussion that followed, Pauli again brought up the suggestion he had already made in June 1931 on the occasion of a Conference in Pasadena. In order to explain the continuous spectrum of the beta-rays emitted in the decay of many radioactive nuclei the emission of the electron had to be accompanied by the emission of a neutrino, that is, a neutral particle of a very small mass, possibly zero mass, and spin 1/2.
Before the end of 1933 Fermi developed his theory of ( decay,(52) in January 1934 Joliot-Curie discovered the radioactivity induced by ( particles, (53) and Fermi that induced by neutrons;(54) Neutron physics was at the beginning of a new unexpected fast development.
Rutherford's comment to the 7th Solvay Conference was: "The last conference was the best of its kind that I have attended."
For a first time the theme selected for the 8th Conference was "Cosmic Ray and Nuclear Physics," but along period of illness of the President of the Scientific Committee, Paul Langevin, imposed a first adjournment. Later it was decided that the conference would deal with the problems of elementary particles and their mutual interactions and that it would be held in October 1939. Even the list of speakers was prepared but World War II started on 3 September 1939 and the conference was postponed to an indefinite date.
The next Solvay Conference in Physics, the 8th from the beginning, took place in October 1948, that is, 15 years since the previous one. The Scientific Committee was now chaired by Sir Lawrence Bragg. The general theme of the conference was "Elementary Particles."(55)
Already in 1936 Anderson and Neddermeyer (56) had shown that the penetrating component of cosmic rays, first discovered in 1932 by Bruno Rossi, (57) consisted of charged particle of mass intermediate between those of the electron and the proton. Shortly later it was shown that this particle was unstable with a mean life on the order of 10-6 sec.
Both the mass and the mean life of this particle were in qualitative agreement with those suggested about 1 year before by Yukawa for the mediator of the nuclear forces.(58)
The picture, however, had changed rapidly after the end of the Second World War. The experiment of Conversi, Pancini, and Piccioni(59) had shown that this particle had an interaction with nuclei much weaker than that expected for the Yukawa mediator. At the beginning of October of the same year 1947, Lattes, Occhialini, and Powell(60) in Bristol had discovered in cosmic rays a new particle, that they called pi-meson. It is unstable and decays, with a mean life of approximately 10-8 sec, into a neutrino and the particle of Anderson and Neddermeyer that was called m-meson or muon.
Almost at the same time Rochester and Butler (61) at Manchester observed in a cloud chamber triggered by counters two "V events" identified later as decays of a thita0-meson and a Lamda0-hyperon.
These were the first examples of "strange particles," the adjective "strange" referring to the following anomalous property. Their decay takes a time of the order of 10-10 sec instead of the much shorter time (approximately 10-23 sec) expected from the time observed to be involved in their production (approximately 10-23 sec).
In Brussels the 8th Solvay Conference was opened with two speeches on mesons, one by Cecil F. Powell who reported on the results of work made in cosmic rays, and the other by R. Serber who presented the results on the production of "artificial mesons" obtained in Berkeley by Burfening, Gardner, and Lattes by bombarding matter with ( particles accelerated with the cyclotron.(62)
The properties of particles contained in large atmospheric showers were discussed by Auger and the problem of the nuclear forces by Rosenfeld from two points of view. In a purely phenomenological approach by means of a nuclear potential or in terms of a nuclear field mediated between any two interacting particles by particles of intermediate mass.
Bhabha treated the general relativistic wave equations and Tonnelat presented the idea of Louis de Broglie of trying to describe a photon as an object composed of two neutrinos.(63)
Heitler spoke on the quantum theory of damping, which is a heuristic attempt to eliminate the infinities of quantum field theory in a relativistic invariant manner, Peierls spoke of the problem of self-energy, and Oppenheimer gave "an account of the developments of the last years in electrodynamics" in which he discussed the problem of the vacuum polarization and charge renormalization with special reference to the recent work of Schwinger and Tomonaga.
A few reports on different topics were also presented to the same conference. Blackett discussed "The Magnetic Field of Massive Rotating Bodies" and Teller presented a paper prepared jointly with Maria Goeppert Mayer, on the original formation of the elements.
The conference was closed with some general comments by Niels Bohr "on the present state of atomic physics" in which he referred to the satisfactory situation in quantum electrodynamics, but pointed out that the dimensionless coupling constant alpha=e2/hc which equals approximately 1/137 cannot be computed within the framework of the theory itself, indicating the need for a future more comprehensive theory.
The 12th Solvay Conference on "Quantum Field Theory" was held in 1961, just 50 years after the first Solvay Conference.(64) This circumstance was celebrated by Niels Bohr, who opened the conference with a speech on "The Solvay Meetings and the Development of Quantum Physics."
Once more the development undergone between 1948 and 1961 by the experimental techniques as well as by the theoretical interpretation of subnuclear particles was amazing. In 1952, at the Brookhave National Laboratory, the first proton synchrotron, the Cosmotron, entered into operation. It produced protons of energies up to 3.2 GeV, and became immediately a controlled source of pions and strange particles of much higher intensity than cosmic rays.
In the same year, Pais (65) suggested that the anomalous property of strange particles could be explained by assuming that they possess an internal degree of freedom, specified by a quantum number, and that various selection rules based on the conservation or nonconservation of this quantum number are operating in the production and decay.
Within 1 year, at the Cosmotron, Fowler et al.(66) discovered a new phenomenon, the associated production in the same collision of two different strange particles: for example, a Lamda0 and a Kappa0. These observations were in agreement with Pais' approach and prompted its full development: in 1954 Gell-Mann and Pais and, independently, Nishijima (67) were able to define the new quantum number, called strangeness, and, from a detailed analysis of the already rich harvest of new processes, to assign its value to all known particles.
In 1954, a larger accelerator, the Bevatron, producing protons up to 605 GeV, went into operation at Berkeley, and marked the end of cosmic rays as a tool for the investigation of subnuclear particles.(68) About 1 year later with this machine, Chaimberlain, Segrè, Wiegand, and Ypsilantis (69) observed the production of antiprotons generated in proton-nucleon collisions. This result provided the long-awaited confirmation of the generality of the 1931 Dirac forecast: his relativistic equation providing an adequate description of the electron and its antiparticle, is valid in general (apart from some correction) for any particle of spin 1/2.
A further step of fundamental importance was made in 1961 by Gell-Mann and Ne'eman who proposed a classification scheme based on the Sophus Lie group of symmetry SU(3) (70) for the more than 100 particles endowed with strong interactions (hadrons).
About 2 years before the 1961 Solvay Conference two new accelerators producing protons of 28-30 GeV went into operation, one at CERN, near Geneva, and the other at the Brookhave National Laboratory. Pretty soon, both of them started to enrich even more our knowledge of subnuclear phenomenology.
At the 1961 Solvay Conference the situation reached by quantum field theory was reviewed by Heitler, with special regard to the problem of renormalization. The successive speech on the same general theme was given by Feynman who started with a comparison of the predictions of the quantum field theory with experimental results. He discussed the scattering of high-energy electrons by protons, with special emphasis on the proton form factors and the search for deviations from the photon propagator, the comparison between the measured and computed values of the Lamb shift, of the magnetic moment of electron and muon, and of the hyperfine structure of spectral lines.(71) As a general conclusion on the first part of his talk, Feynman stated: "All this may be summarized by saying that no error in the prediction of quantum electrodynamics has yet been found. The contributions expected from various processes envisaged have been found again, and there is very little doubt that in the low-energy region, at least, our methods of calculation seem adequate today." These remarks are valid even today in spite of the fact that the precision reached in the measurements in the low-energy experiment is increased by two or three orders of magnitude (72) and the exploration of high-energy phenomena has been extended from values of the cut-off parameters Lambda of 0.6 GeV to more than 100 GeV.(73)
The second part of Feynman's speech dealt with theoretical questions. The first one was the problem of the renormalization of the mass of the electron as well as of particles such as the pion and the kaon which exist in charged and neutral states and therefore provide a direct indication of the contribution originating from the electromagnetic field.
A second question was the interaction of photons with other particles which do not allow a sharp separation between quantum electrodynamics and other interactions. "It will not do to say that Q.E.D. is exactly right as it stands, because virtual states of charged baryons must have an influence. Two sufficiently energetic photons colliding will not do just what Q.E.D. in that limited sense supposes; they also produce pions. Or, more subtly, a sufficiently accurate analysis of the energy levels of positronium would fail, for the vacuum polarization from mesons and nucleons would be omitted."
A third and last theoretical item treated by Feynman was "Dispersion Theory," discussed in greater detail by other participants in the Solvay Conference.
"Weak Interactions" were treated by Pais who, starting from Fermi's original theory, discussed the discovery by Lee and Yang,(74) almost 5 years before, of the parity violation by weak interactions, its experimental confirmation, (75) the muon-electron universality, (76) the idea of an intermediate boson as a mediator of weak interaction, and the "two-neutrinos question."(77)
In the following speech, on "Symmetry Properties of Fields," Gell-Mann discussed exact symmetries as well as approximate symmetries: the conservation of the x and y component of the isotopic spin I, broken by electromagnetism and weak interactions, the conservation of Iz, or strangeness, broken by weak interactions and the conservation of C and P separately, also broken by weak interactions. Then, using various arguments, among them the conserved vector current, already recognized in 1955-1958,(78) he stressed the central interest of charge operators and of the equal-time commutation relations among them. "The mathematical character of the algebra which the charge operators generate is a definite property of nature," Gell-Mann said.
He then considered the currents in the Sakata-Okun model for which he derived the expressions for the electromagnetic and weak currents.
At this primitive stage of development of current algebra quarks had not yet appeared on the scene.
Other speeches were by Kallen on some aspects of the formalism of field theories, Goldberger on single variable dispersion relation, Mandelstam on "Two-Dimensional Representations of Scattering Amplitudes and Their Application" and finally by Yukawa on "Extensions and Modifications of Quantum Field Theory."
The appearance of the current algebra in the speeches and discussions of this conference really mrks the beginning of one of the most fruitful lines of development that has brought us to present views.
The 14th Solvay Conference on "Fundamental Problems in Particle Physics" was held in October 1967.(79) The Conference was presided over by Christian Moller who opened the meeting with a "Homage to Robert Oppenheimer," who had died on 18 February of that year.
Many speeches were of a theoretical nature: Dürr spoke on "Goldstone Theorem and Possible Applications to Elementary Particle Physics," Haag on "Mathematical Aspects of Quantum Field Theory," Källen on "Different Approaches to Field Theory. Especially Quantum Electrodynamics," and Sudarshan on "Indefinite Metric and Nonlocal Field Theories." Heisenberg gave a "Report on the Present Situation in the Non-linear Spinor Theory of Elementary Particles."
Already in 1964 Zweig and Gell-Mann had postulated the existence of quarks as building blocks of hadrons, thus establishing the premises necessary for a dynamical interpretation of the SU(3) already well-established symmetry.(82) At the time of the Solvay conference, however, the quark hypothesis was considered only as a convenient model.
A report by Gell-Mann, also on elementary particles, unfortunately does not appear in the proceedings of the Conference because it was not available at the time of publication. From the contributions to its discussion by W. Heisenberg, C.F. Chew, F.E. Low, S. Mandelstan, R. Brout, R.E. Marshak, S. Weinberg, N. Cabibbo, S. Fubini and others, it appears that the talk by Gell-Mann touched upon the SU2 and SU3 groups and their connections with bootstrap mechanism, chiral symmetry, and partial conservation of axial currents (PCAC).(83)
This is the last of the Solvay Conferences on Physics devoted to subnuclear particles in spite of the fact that the progress undergone by this field since 1967 has been amazing. The main reason was that many conferences were held each year on this general subject as well as on various parts of it, so that the Scientific Committee for Physics felt it would be better to turn its attention to other subjects less frequently dealt with in international conferences.
Now it is clearly time to devote yet another Sovlay Conference to the last constituents of matter.
III. THE CONFERENCES ON THE INQUIRE AFTER LAWS IN COMPLEX SYSTEMS
Let me now consider the conferences devoted to solid-state and statistical mechanics.
The first conference of this group is the 4th Solvay Conference on "The Electrical Conductivity of Metals" held in April 1924.(84) The conference was in some way premature. It took place just before the advent of quantum mechanics, in particular 2 years in advance of the first formulation by Fermi of the antisymmetric statistics and the consequent concept of the degenerate electron gas.
The conference was opened with a speech by Lorentz on the theory of electrons he had developed about 20 years before, followed by papers by Joffe on the electrical conductivity of crystals, Kamerlingh Onnes on superconductivity, and Hall on the metallic conduction and the transversal effects of the magnetic field. This last speech was followed by a discussion in which Langevin and Bridgman injected a few interesting remarks.
The 6th Solvay Conference on "Magnetism," held in 1930,(85) was opened by a contribution by Sommerfeld on "Magnetism and Spectroscopy" in which he discussed the angular momenta and magnetic moments of the atoms which had been derived from the investigation of their electronic constitution.
Van Vleck reported on the experimental data of the variation of the magnetic moments within the group of the rare earths and its theoretical interpretation, and Fermi discussed the magnetic moments of the atomic nuclei and their determination from the splitting of hyperfine structure. Pauli treated the "Quantum Theory of Magnetism," with special regard to the paramagnetism of a degenerate Fermi gas of electrons, Weiss dealt with the equation of state of ferromagnets, Dorfman with ferroelectric materials, and Cotton and Kapitza reported on the study of the magnetic properties of various materials in very intensive magnetic fields. The phenomenological theory of ferromagnetism by Weiss is still of interest today, mainly because the microscopic mechanism that gives rise to the dipole-dipole interaction is not yet understood in all its detail.(86)
The 9th Solvay Conference on "Solid State took place in 1951.(87) The question of "interface between crystals" was discussed by C.S. Smith, grain growth observed by electron optical means by G.W. Ratenau, recrystallization and grain growth by W.G. Burgers, crystal growth and dislocation by F.C. Franck, the generation of vacancies by moving dislocations by Seitz, dislocation models of grain boundaries by Shorckley, and diffusion, work-hardening, recovery, and creep by Mott.
It was at that time that Franck and Seitz proposed mechanisms for the multiplication and generation of vacancies by intersection of dislocations (88) explaining the observed softness of crystals and providing models that were subsequently verified by the technique of decoration of dislocations.(89)
The 10th Solvay Conference on "The Electrons in Metals" was held 3 years later, in 1954.(90)
Pines examined the collective description of electron interaction in metals; Löwdin, an extension of the Hartree-Fock method to include correlation effects; Mendelson, the experiments on thermal conductivity of metals; Pippard, the methods for determining the Fermi surface; Kittel, resonance experiments and wave functions of electrons in metals; Friedel, primary solid solutions in metals; Fumi, the creation and motion of vacancies in metals; Shull neutron diffraction from transition elements and their alloys; Neel, antiferromagnetism and metamagnetism; and Frölich, superconductivity with special regard to the electron-electron interaction carried by the field of lattice displacements, which paved the way to the theory that Bardeen, Cooper, and Schrieffer developed between 1955 and 1957.(91) Finally, Mathias dealt with the empirical relation between superconductivity and the number of valence electrons per atom.
The last Solvay Conference that I have rather arbitrarily put in this class is the 17th on "Order and Fluctuations in Equilibrium and Nonequilibrium Statistical Mechanics" held in October 1978.(92)
The Conference was divided into four parts to each of which a full day was devoted: the first one treated: "Equilibrium Statistical Mechanics," with special regard to "The Theory of Critical Phenomena"; the second part regarded "Nonequilibrium Statistical Mechanics. Cooperative Phenomena"; the third one, "The Macroscopic Approach to Coherent Behavior in Far Equilibrium Conditions"; and the fourth and last, "Fluctuation Theory and Nonequilibrium Phase Transitions."
Two methods appear to be very powerful for the study of critical phenomena: field theory as a description of many-body systems, and cell methods grouping together sets of neighboring sites and describing them by an effective Hamiltonian. Both methods are based on the old idea that the relevant scale of critical phenomena is much larger than the interatomic distance and this leads to the notion of scale invariance and to the statistical applications of the renormalization group technique.(93)
As pointed out by van Hove in his concluding remarks, the common methodology between high-energy physics and critical phenomena is striking although the conceptual basis is quite different in the two cases. In high-energy physics approximate scale invariance and its calculable breaking are characteristic of the large momentum scale regime (i.e., small space and time intervals), whereas in statistical physics scale invariance and renormalization group methods are applicable to a domain of large space and time intervals.
The approximate methods of renormalization for the investigation of phase transitions in degenerate states (94) were presented to the conference by Kadanoff and by Brezin. The nonequilibrium statistical methods were discussed by Prigogine, (95) followed by Hohenberg who treated critical dynamics. In the third part, Koschmieder discussed the experimental aspects of hydrodynamic instabilities (96) followed by Hohenberg who treated critical dynamics. In the third part, Koschmieder discussed the experimental aspects of hydrodynamic instabilities (96); Arecchi, the experimental aspects of transition phenomena in quantum otpics (97); and Sattinger the bifurcation theory and transition phenomena in physics.(98)
The fourth part included a paper by Graham on the onset of cooperative behavior in nonequilibrium states. Suzuki talked about the theory of instability, with special regard to nonlinear Brownian motion and the formation of macroscopic order, and P.W. Anderson developed a series of interesting considerations of very general nature around the question: "Can broken symmetry occur in driven systems?"
The question in the title can be reformulated by asking how much can be dug out of an analogy between broken symmetry in dissipative structures (such as the ripple marks generated by wind, i.e., an external perturbation, in an otherwise flat surface of sand) and broken symmetry defined as phenomena of condensed matter systems of the kind observed near the critical points. The value of Anderson's discussion is to be seen more in the deepening of the question itself thtn in the answer that cannot yet be final, and for the moment, according to the author, appears to be more on the negative side.
The contributions presented by Prigogine and by Sattinger to the 17th Solvay Conference on Physics appear as a natural introduction to some of the problems that will be examined at the present Solvay Conference in Chemistry.
Exact symmetries and broken symmetries were the central theme of the 15th Solvay Conference held 8 years before, that is, in 1970 on "Symmetry Properties of Nuclei;"(99) This was the second conference on nuclear structure, the previous one being the conference held in 1933 immediately after the discovery of the neutron. In the 37 years that have passed between the first and the second Solvay Conference on nuclear structure the subject has undergone an extraordinary development although the naive hope, generally shared by physicists until 1935, for a full understanding of nuclear dynamics in terms of nucleons interacting with two-body forces, has been completely deluded. A number of models have been developed which, although very different from each other, are not contradictory. Each of them, in some way, emphasizes a particular aspect of some category of nuclei and/or nuclear phenomena, and thus allows an adequate interpretation of a set of their static and/or dynamic properties.
The "Symmetry of Cluster Structures of Nuclei" was discussed by Brink who showed contour plots of nucleon density obtained from Hartree-Fock calculations for simple nuclei such as 8Be, 12C, and 20Ne.
Much attention was devoted to collective models: Mottelson reviewed "Vibrational Motion in Nuclei"; Judd, the use of Lie groups; Lipkin, the SU(3) symmetry in hypernuclear physics; Radicati, Wigner's supermultiplet theory(100); Fraunfelder, "Parity and Time Reversal in Nuclear Physics"; Wilkinson, the isobaric analogue symmetry; Aage Bohr, the permutation group in light nuclei; and J.P. Elliot, the shell model symmetry.
The conference was closed by a few concluding remarks by E.P. Wigner, not completely free from critical lines.
At the time of the conference the study of nuclear reactions produced by intermediate energy protons or pions as well as the investigation of collision between two nuclei of Z >= 3 were still in a rather prilitive stage whereas today they constitute a rich field of empirical knowledge and phenomenological interpretation.
IV. THE CONFERENCES ON EXPLORATION OF OUR ENVIRONMENT AT LARGE
I come now to the last group of Solvay Conferences: The three regarding astrophysical problems. The first one, devoted to "The Structure and Evolution of the Universe" was held in June 1958. It was the 11th Solvay Conference in Physics.(101)
The conference took place in a moment of extraordinary expansion of general interest for astrophysical problems dur to the first steps made in new observational techniques such as radio signal reception and observations from space vehicles.
This was also the first Solvay Conference in which Einstein's Theory of General Relativity started to be quoted and used as a conceptual structure of fundamental importance for the interpretation of large-scale phenomena.
The theme of the conference was divided into three parts: the first one concerned "General Statements of Cosmological Theory." It was introduced by speeches by Lemaitre, on the "Primaeval Atom Hypothesis and the Problem of Clusters of Galaxies," by Oscar Klein who developed "Some Considerations Regarding the Earlier Development of the System of Galaxies," and by Hoyle on "The Steady-State Theory." This was followed by a talk by Gold, on the "Arrow of Time" and another by Wheeler on "Some Implications of General Relativity for the Structure and Evolution of the Universe."
In a talk, very important even today, J.H. Oort discussed the "Distribution of Galaxies and the Density of the Universe." Lovell presented "Radio Astronomical Observations Which May Give Information on the Structure of the Universe."
In the third part of the conference, on the "Evolution of Galaxies and Stars," Hoyle presented the then recent and still important work by the Burbidges, Fowler, and himself on the origin of elements in stars.(102)
In the general discussion Bondi asked for tests that could decide between the evolutionary and steady-state universe. The question was premature because the arguments, which later allowed the exclusion of the steady-state theory, are based on the analysis of the blackbody cosmological radiation and of the distribution of the number of radio sources versus flux. The cosmological radiation was discovered only in 1965 (103) and the distribution of the radio sources was still highly controversial at the time of the conference. Indeed, in the discussion of this point Lovell stated: "At present the whole of the cosmological interpretation of the radio sources is based on a half a dozen identifications of the Cygnus type." Furthermore, the mechanism leading to radio emission was misunderstood: the prevailing view attributed it to collisions between galaxies.
From the small amount of information I have given it appears clear that in 1958 astrophysics was still in its classical stage. The conference, however, marks a historical step in modern astrophysics because for the first time the physics of neutron stars and collapsed objects was reproposed by Wheeler since 1939 when Oppenheimer and Snyder first presented the existence of black holes.(104)
The 13th Conference on the "Structure and Evolution of Galaxies" took place in October 1964.(105) It was presided over by Robert Oppenheimer, who had succeeded Sir Lawrence Bragg.
The Conference was opened by Ambartsumian who spoke "On the Nuclei of Galaxies and their Activity." He presented new observational data that in his opinion supported the idea, which he had already submitted to the 1958 Conference, that most of the processes connected with the formation of new galaxies and their structure start from the nuclei. This interpretation, however, is shared only by a minority of astrophysicists.
Oort reported on "Some Topics Governing the Structure and Evolution of Galaxies" and described the latest knowledge concerning our Galaxy. Woltjer discussed the "Galactic Magnetic Field."
The structure and evolution of the stars were the subject of the second part. Spitzer discussed the "Physical Processed in Star Formation," a subject tht was further developed by Salpeter with special regard to the birthrate function of the stars. W.A. Fowler and Bierman discussed the evolution toward the main sequence and R. Minkowski discussed the data available concerning the supernovae.
Finally, an important survey of the findings about extragalactic radio sources was presented by J.G. Bolton and, in the discussion that followed, Bruno Rossi presented the first observations made by Giacconi et al.(106) in 1962 and by Friedman et al. In 1963 of localized X-ray sources, in particular, Scorpio X-1.(107)
One of the most exciting contributions to the conference was the speech by M. Schmidt who discussed the "Spectroscopic Observations of Extragalactic Radio Sources," with special regard to the interpretation of the red shift of the quasars, discovered about 2 years before. (108) His conclusion, still accepted today by the majority of astrophysicists, was that most likey these red shifts are cosmological.
G.R. Burbidge and E.M. Burbidge, in their report on "Theories and the Origin of Radio Sources," summarized the understanding that had been achieved in the creation of radio waves.
In the final discussion G.R. Burbidge stressed that the problems of energy conversion from a primary energy source to the form of relativistic particles needed for the radio emission are entirely unsolved. In particular, the production of cosmic ray particles requires an acceleration mechanism, of an efficiency at least a few orders of magnitude larger than that of the best man-made accelerators.(109)
In this connection Alfven proposed the annihilation of matter and antimatter as a possible source of energy, but also other mechanism, in particular, some form of release of gravitational energy were examined. The problem is still open today, but the consensus is indeed that strong gravitational fields should be involved.
The 16th Solvay Conference in Physics, held in September 1973, was entitled "Astrophysics and Gravitation."(110) The progress undergone in many fundamental chapters of astrophysics with respect to the previous conference, held in 1964, was really striking. In particular, X-ray astronomy had won a status comparable with other conventional branches of astronomy.
The Conference was opened with a progress report on pulsars by Pacini, followed by speeches on the observational results on compact galactic X-ray sources by Giacconi, on the optical properties of binary X-ray sources by the Bahcalls, and a review on the physics of binary X-ray sources by Martin Rees.
Among the many communications and invited talks essentially on the same subject, I recall the speech by Pines on "Observing Neutron Stars," by Pandharipande on "Physics of High Density and Nuclear Matter," and by Cameron and Canuto on "The General Review on Neutron Stars Computations."
Black holes were extensively discussed by J.A. Wheeler and the search for their observational evidence by Novikov, followed by communications by Rees and by Ruffini.
Woltjer gave a general talk on "Theories of Quasars," and Martin Schmidt discussed "The Distribution of Quasars in the Universe."
G.R. Burbidge gave a review paper on the "Masses of Galaxies and the Mass-Energy in the Universe" and Hofstadter presented the information available at the time of the recent discovery of bursts of gamma-rays.(111)
I am now at the end of my series of flashes on the Solvay Conferences in Physics. I hope that, in spite of its shortness and incompleteness, it may help in stimulating two kinds of considerations. Those of the first kind regard the extraordinary development undergone during the last 70 years by our views on the physical world, many parts of which in present days appear to be dominated by a few general concepts, such as those of exact and approximate symmetry, and to be treatable by mathematical procedures such as the application of the renormalization group. The other kind of considerations concerns the role that the Solvay Conferences in Physics have played in the development of physics during the last 70 years, and the unique value they will maintain, even in the future, as sources of information for the historians of science.
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108. As announcement of the discovery of quasars one can take No. 4872 of Nature (London), appeared on March 16, 1963 (Vol. 197), where a set of four articles touch on a few fundamental properties of two radio sources:3C 273 and 3C 48. The second of the four articles, due to Martin Schmidt, is the core of the whole argumentation. The first article by C. Hazard, M. B. Mackey, and J.J. Shimmins (p. 1037) presents the "Investigation of the Radio Source 3C 273 by the method of Lunar Occultation." In the second paper, by Martin Schmidt (p. 1040) entitled "3C 273: A Star-like Object with Large Redshift", the author shows that 6 lines (four of the Balmer series, one of MgII, the other of O III) can be explained with a redshift of 0.158. The third paper by J.B. Oke (p. 1040), is entitled "Absolute energy distribution in optical spectrum of 3C 273" and the fourth, by J.L. Greensstein and T.A. Matthews (p. 1041), "Redshift of the Unusual Radio Source 3C 48", for which they give a value of 0.3675 as weighted average of six relatively sharp lines.
109. The efficiency of man-made accelerators, defined as the output power in the beam devided by the total power supply is: Berkeley Bevatron, appr. 3 x 10-4; 28 GeV CERN-PS, appr. 3 x 10-4; Frascati 1 GeV electrosyncrotron, appr. 1 x 10-4.
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