The question of the form of the momentum-energy tensor of the electromagnetic field (in a medium) remains debatable to this day. The dilemma of whether the photon momentum in a medium is equal to nhf/c (Minkowski) or hu/nc (Abraham) therefore remains unresolved (n is the refractive index). Simple considerations based on the law governing the motion of the center of gravity of the "field + medium" system lead, however, to a unique choice of Abraham's tensor. The Jones-Richards experiments do not contradict this, although they do not lead to a solution of the problem. In principle, measurements of the Jones-Richards type (of the pressure of light in media) in the pulsed regime would yield the solution of the problem. Considerable space is allotted to an analysis of the question of the "rejection" of Abraham's tensor, a question advanced by Laue and supported by many authors. It is shown that the use of the Laue criterion is based on an error in the very formulation of the question. The arguments advanced in this connection are illustrated by using as an example analogous relations in the case of the motion of a simple static system, namely a charged capacitor. The conservations laws applied to a static electromagnetic field having angular momentum also lead to Abraham's expression for the field momentum density.

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A contribution ζ χ to the curvature perturbation will be generated during the waterfall that ends hybrid inflation, that may be significant on small scales. In particular, it may lead to excessive black hole formation. We here consider standard hybrid inflation, where the tachyonic mass of the waterfall field is much bigger than the Hubble parameter. We calculate ζ χ in the simplest case, and see why earlier calculations of ζ χ are incorrect.

The Russian text was published by Nauka Press in Moscow in 1969 for the Academy of Sciences of the USSR as Volume48 of the Proceedings (Trudy) of the P. N. Lebedev Physics Institute. The present translation is pub-Iished under an agreement with Mezhdunarodnaya Kniga, the Soviet book export agency.

We study the accuracy of the pion form factor, obtained with a local-duality version of dispersive sum rules. To probe this accuracy, we make use of a potential model, where the exact form factor may be calculated from the solution of the Schrödinger equation and confronted with the local-duality form factor. The deviation between these quantities is found to be below 20% in the region of momentum transfers Q > 2 − 3 GeV, independently of the specific form of the confining potential. We argue that the local-duality model for elastic form factors in QCD has at least this level of accuracy.

The XXIst International Workshop High Energy Physics and Quantum Field Theory

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