In this book, Professor Terletskii does not avoid direct confrontations. Nor does he confine himself to the familiar so-called paradoxes of the reciprocal contraction of lengths, the journeying and stay-at-home twins, and the like. He goes much farther, illuminating, for example, the relativistic role of the speed of light by deriving the Lorentz transformation without using Einstein's postulate of the constancy of this speed. But what is most daring is his exploration not merely of the idea of particles of negative mass but also that of particles of imaginary mass.