Abstract:
Ten years ago, the author published an article with the same title in this journal [Usp. Fiz. Nauk, 103, 87 (1971), Sov. Phys. Usp. 14, 21 (1971)]. The aim of the present paper is to consider the changes that have occurred during the last decade with regard to the problems considered in the earlier paper. Naturally, some new problems have also arisen, and they too are considered in the present paper.