Progress in physical concepts of string and superstring theory - Thirty years of string theory

In this paper we discuss the evolution of physical concepts which led to th e generation and development of string theories. The paper is conceived wit h the intention of summarizing and extending with new aspects the specific characteristics of strings which refer to the physical intuition and experi ment. We hope to present new insights into the physics of strings and make it understandable from the point of view of a non-string theorist. Even if there exist some opinions that the (super)string theory appertains to the t wenty-first or twenty-second century or that there are no concrete new pred ictions of string theory at low energies, we believe that string theory pre sents a rich field of research and a source of physical intuition not only for mathematicians but also for theoretical and experimental physicists. We offer as an example an atomic electron cloud which can also be interpreted in terms of a fixed point in a string theory We propose also an experiment to verify the fundamental hypotheses. Finally we deduce that the number of dimensions of spacetime must be infinite by virtue of the axiom of univers ality of motion. (C) 1999 Elsevier Science Ltd. All rights reserved.