Table of contents

Computer simulation results on diffusion in metal and some nonmetal amorphous materials are summarized. Methods for producing and analyzing disordered systems as well as for modeling diffusion processes in such systems are considered. Certain features of diffusion in disordered systems are demonstrated. Possible mechanisms of diffusion in amorphous metals are analyzed.

A review of the main directions of development, the most promising new techniques, and urgent applications of coherent four-photon methods of investigation of excited and ionized gas media is presented. It is demonstrated that the methods of coherent four-wave mixing (FWM) provide important spectrochronographic information on atoms and ions, allow selective studies of separate components of nonstationary gas and plasma systems with a complex composition using polarization-sensitive four-photon spectroscopy and coherent ellipsometry, and permit nonlinear-optical imaging of the spatial distribution of atoms and ions in excited gases and plasmas.

About thirty years ago, the author published a paper [1] under the same title which, mainly educational in its nature, aimed primarily at widening the scientific horizons of the young generation of physicists. For this purpose, a list of the top agenda problems of the day was given and commented on, admittedly subjective and unavoidably inexhaustive and limited (one cannot do the impossible, after all). In the author's later articles, additions both to the list itself and the comments were made (see [2] for the latest version). In the present paper the author takes on an end-of-the-century perspective in addressing this theme once again.

Of the two definitions of 'information' given by Shannon and employed in the communication theory, one is identical to that of Boltzmann's entropy and gives in fact a measure of statistical uncertainty. The other involves the difference of unconditional and conditional entropies and, if properly specified, allows the introduction of a measure of information for an open system depending on the values of the system's control parameters. Two classes of systems are identified. For those in the first class, an equilibrium state is possible and the law of conversation of information and entropy holds. When at equilibrium, such systems have zero information and maximum entropy. In self-organization processes, information increases away from the equilibrium state. For the systems of the other class, the equilibrium state is impossible. For these, the so-called 'chaoticity norm' is introduced and also two kinds of self-organization processes are considered and the concept of information is appropriately defined. Common information definitions are applied to classical and quantum physical systems as well as to medical and biological systems.

Departures from Coulomb's law displayed by two electrostatically interacting conducting balls are examined in detail. By computing forces on the balls as a function of the ball separation, it is found that at short separations a switch from repulsion to attraction occurs in the general case of arbitrary, likely charged balls. The only exception is the case in which the charges of the balls are related as the squares of their radii: such balls always repel each other. For identical balls of equal charges in magnitude, asymptotic short-separation relations for energies and forces are found. For most of the results obtained the self-similarity property is shown to apply.