Table of contents

Plasma oscillations in gas discharges have been known and studied for nearly thirty years. The existence of similar oscillations in metals, i.e. longitudinal oscillations of the valence electron gas, analogous to sound waves, was suggested over twenty years ago, but only recently has their full significance been realized. The Coulomb interaction of the electrons can be divided into two parts: a long-range part, whose effect is described by the plasma oscillations; and a residual `screened' interaction, whose range is about 1 Å. Owing to the large excitation energy of a plasma oscillation, the long-range part can be ignored in many calculations, which explains the success of the independent particle model on which the band theory of metals is based. In the first half of this article, after a critical discussion of previous treatments of the Coulomb interaction, the elements of the plasma theory are made plausible by simple arguments. Then the quantum-mechanical theory of plasma oscillations in a free-electron gas, due to Bohm and Pines, is described, including an account of the calculation of the correlation energy. Experimental evidence of the existence of plasma oscillations, afforded by the energy losses of fast electrons in penetrating thin metallic films, is then discussed. Finally, going beyond the free-electron approximation, a semi-quantitative theory of the effects of the periodic lattice field and of the core electrons is given.

An account is given of the present state of the theory of liquid ⁴He. At low temperatures it is no longer possible to consider the motion of individual atoms, and a wave mechanical treatment becomes necessary. Such a treatment, as developed by Landau and Feynman, characterizes the thermal motion of the liquid by ` elementary excitations ', the so-called phonons and rotons. A consideration of these excitations then leads to the ` two-fluid ' model. It is shown that the treatment is adequate to account for nearly all the properties of the liquid, which are briefly reviewed. The principal problem still out-standing concerns the behaviour of the liquid at high rates of flow or when carrying a large heat current, when it is clear that some form of turbulence is induced.

Recent experiments with high-energy protons above 100 MeV are reviewed. Considerable data have been obtained from the study of proton-proton scattering below 400 MeV, especially where polarized beams have been used. It is now possible to extract the available information from such experiments by making phase shift analyses. The neutron-proton scattering experiments, although not so numerous, are reviewed where they are relevant to these analyses. The phase shifts have not yet been determined unambiguously but the analyses have indicated the general trend and make it possible to test various models of the nucleon-nucleon interaction. At energies above 400 MeV the character of the scattering changes and the production of mesons becomes noticeable. At the highest energies now available from the Bevatron at Berkeley antiprotons are produced.

Many of the experimental results in proton-nuclear scattering have been explained in terms of nuclear transparency, optical and statistical models, but careful measurements of inelastic scattering have suggested that more detailed models are required. The polarization in elastic scattering can be explained by an optical model with the addition of a spin-orbit coupling similar to that used in the shell model. The theoretical variation of the polarization with scattering angle displays sharp features near the diffraction minima but the observation of these tends to be obliterated by finite experimental angular resolution and detection of inelastically scattered particles.

A general discussion of the problems raised by systems of many interacting particles is given in the introduction. In the second section general methods are discussed for dealing with these problems, and special attention is paid to the method developed by Tomonaga. This method is applied to the theories of sound waves (§ 3), plasma oscillations (§ 4), nuclear surface modes (§ 6), and liquid helium (§ 7). A brief discussion is also given of some aspects of spin wave theory (§ 5).

This article surveys our empirical knowledge of the production processes, the nuclear interactions and the decay processes of K particles and hyperons, as this appeared at the close of 1956, and interprets these phenomena within the framework of the charge-independent classification scheme proposed by Gell-Mann and by Nishijima for these `strange particles'. The assignment of a `strangeness' quantum number to each strongly interacting particle is a deep-lying aspect of this scheme, and the main qualitative features of the empirical data allow a direct interpretation in terms of a law of conservation of strangeness through all strong interactions. Where appropriate, more detailed analyses of the phenomena are made on the assumption that charge independence holds for these strong interactions.

The evidence on decay processes for K particles and hyperons is discussed in some detail. These processes violate the strangeness conservation law and also involve changes in the total isotopic spin for those cases where this quantity is relevant for the final particles. The success of Gell-Mann's ΔT = ½ rule in accounting for certain aspects of the decay data is considered for each of the latter cases; this rule is given a simple origin in Schwinger's model for strange particle decay processes, which is discussed briefly. The implications of symmetry between particle and antiparticle, as pointed out by Gell-Mann and Pais for the neutral K particles and their decay modes, is also discussed in some detail, together with an account of the experimental data on long-lived K° particles and anomalous K° decay modes.

Indications in the data which bear on the spin and parity values for K particles and hyperons are stressed, especially for the decay modes associated with τ and θ particles. Although it is shown that the products of τ and θ decay necessarily differ in spin or in parity, a great similarity has been found concerning all other properties of the τ and θ particles, and the proposals which have been made to account for this situation are surveyed. For the Λ° particle, observations on bound Λ° hypernuclei provide much information relevant to the Λ°-nucleon interaction and the Λ° spin and parity, and this is also discussed in some detail.

A section is devoted to discussion of `anomalous events' for which no interpretation is known in terms of the interactions of the well-established particles. These events and the possibility that heavier unstable particles or perhaps resonance states may exist have an obvious importance, of special relevance for attempts to construct a theoretical foundation leading naturally to the Gell-Mann-Nishijima scheme. A brief discussion of the preliminary attempts to establish such a foundation is given.

An appendix has been added in proof to discuss the recent observations on the failure of parity conservation and of charge conjugation invariance in certain weak decay processes, and the effects which a failure of these symmetry properties also for strange particle decays would have on the arguments made and the conclusions reached in this survey.

This report discusses the application of electron magnetic resonance techniques to defect and impurity sites in solids, conduction electrons in metals, ferromagnetic, ferrimagnetic and antiferromagnetic materials. It also reviews the recent experiments on cyclotron resonance in semiconductors together with a brief account of some experiments involving the non-resonant absorption of microwaves by conductors at low temperatures and by ferrites.

The variety, range and precision of methods available for photographic recording of fast phenomena have been increasing rapidly. The capabilities of the newer techniques are considered, classifying them by the kind of record obtained. Streak records with drum cameras can give a time resolution of 10⁻⁷ sec; rotating mirror cameras at present approach 10⁻⁸ sec and may eventually achieve ¼×10⁻⁹ sec; deflecting image converters may go much further. Single flashes of light, bright enough for silhouette recording, can be as short as 10⁻⁸ sec, and for reflected-light recording 10⁻⁷ sec for near objects, or 10⁻⁶ sec for a field of view a metre square. Kerr cells can operate with an exposure of 10⁻⁸ sec, and image converter tubes 10⁻⁹ sec. Several pictures can easily be taken at short intervals superimposed on the one plate. Frames may be separated by intermittent film movement at rates less than 300 pictures per second (p.p.s.); or for speeds up to 10⁴ p.p.s. by using continuously moving film and short exposures, or by using some form of image movement compensation where exposures are a significant proportion of the interframe interval. With smaller pictures of lower resolution, higher speeds are possible. For higher speeds still one uses effectively separate cameras exposed in succession by mechanical means (such as a rotating slotted disc) up to 10⁵ p.p.s., by optical means (such as the use of a rotating mirror) up to 10⁷ p.p.s., or by electronic means (such as phased shutters or phased spark sources) up to 10⁷ p.p.s. Comparably high speeds with less elaborate equipment are made possible by image dissection. Simple dissection plates with clear lines or holes in an opaque ground allow recording rates of 10⁵ to 10⁶ p.p.s., but with low throughput of light. Using a rotating mirror camera to traverse the image elements rates of 10⁸ p.p.s. have been achieved for brilliant objects. Dissection by means of lenticular plates allows in many cases a considerably greater throughput of light. Simple cameras are now available that can take 300 pictures at 250 000 p.p.s. Lenticular plate image dissection has been applied to cinemicrography so that similar series of 300 pictures can easily be taken at 10⁵ p.p.s. at magnifications up to 2000 ×. Alternatively, the use of fibre light guides for dissection allows long series of pictures of low resolution at 10⁵ p.p.s. The combination of image dissection and deflecting image converter provides means for taking a series of 50 pictures at rates of the order of 10⁹ p.p.s.

Using one or another of these techniques, reasonable photographs may be taken of most macroscopic phenomena that are not too remote, though synchronization problems are sometimes of overriding significance. The difficulties are greater when the interest is in fine spatial detail. These problems are accentuated in cine-micrography where the image moves many times faster than the event. In such cases one must take advantage of the most advanced techniques, and there is urgent need for the continued development of better methods.

This paper reviews the experimental and theoretical work on photographic sensitivity and related topics which has been published during the past ten years. New experimental results, obtained to a large extent with model systems, have necessitated substantial revision and amplification of previously accepted theories. The role of crystal imperfections in all aspects of photographic sensitivity is now recognized and emphasis has shifted from the trapping of electrons, which characterized the Gurney-Mott theory, to the trapping of positive holes. The latent image consists of small groups of silver atoms formed by the combination of silver ions and electrons. This proceeds through intermediate stages in which first latent pre-image and then latent sub-image specks appear. The initiation of development by latent image specks of different sizes is discussed in the final sections. The approach which is followed in this paper leads to an integrated treatment of the whole range of photographic phenomena in which very few hypotheses with no direct experimental foundation have to be introduced.

In the last few years measurements have been made of the thermodynamic properties of solid argon which provide ideal material for the verification of the theory of crystal lattices. Starting from the basic problem of the crystal lattice, a review is given here of the dynamics and thermodynamics of the inert gas crystals. Satisfactory agreement is obtained in many cases between theory and experiment when the calculations are based on an anharmonic Einstein theory with a (12,6) interatomic potential. It is shown that further progress requires, in particular, the extension of experiments to lower temperatures and of theoretical calculations of the repulsion of argon atoms to a higher approximation.