Introduction

This is the popular account of relativity that Einstein himself wrote for the general reader; in his preface he says it is meant for those 'interested in the theory but not conversant with the mathematical apparatus of theoretical physics', and requires only school mathematics. Starting from everyday experience, with vivid thought experiments of trains, embankments, lightning flashes, lifts and rotating discs, he leads the reader step by step to two basic principles: the laws of physics are the same in all inertial frames, and the speed of light is the same for all observers. From these follow astonishing conclusions: simultaneity is relative, moving rods contract, moving clocks run slow, and mass and energy can be converted into each other. The second part turns to general relativity, showing that gravitation can be understood as the curvature of space-time, and the book ends by considering whether the universe is finite or infinite. Translated into dozens of languages, it remains the most authoritative introduction to relativity.