THIRTY-FIVE years ago this article on the theory of relativity, written by me at the rather young age of 21 years for the Mathematical Encyclopedia, was first published as a separate monograph together with a preface by Sommerfeld, who as the editor of this volume of the Encyclopedia was responsible for my authorship. It was the aim of the article to give a complete review of the whole literature on relativity theory existing at that time (1921). Meanwhile, the production of textbooks, reports and papers on the theory of relativity has grown into a flood, which rose anew at the 50th Anniversary of the first papers of Einstein on relativity, in the same year 1955 ,in which all physicists were mourning his death.
In this situation any idea of completeness regarding the now existing literature in a revised new edition of the book had to be given up from the beginning. I decided therefore, in order to preserve the character of the book as an historical document, to reprint the old text in its original form, but to add a number of notes at the end of the book, which refer to certain passages of the text. These notes give to the reader selected linformation about the later developments connected with relativity the.ory and also my personal views upon some controversialrquestions.