The book belongs to the pen of the outstanding Russian scientist Alexander Alexandrovich Fridman (1888-1925). This is the first popular statement of the theory of relativity in Russian, written more than forty years ago. During this time, many popular and unpopular works on Einstein's theory appeared, but A.A. Friedman's book did not lose its value. It will be read with interest and benefit by everyone who studies new physics and its history.