Encouraged by the success of The Quantum Universe, we have tried to adopt a similarly pragmatic approach in this sister volume on Albert Einstein's relativity. Our goal is not only to present the essential ideas of both special and general relativity as simply as possible, but also to demonstrate how the predictions of these theories are verified by the results of experiments. Special relativity is concerned with uniform motion, and does away with Isaac Newton's notion of 'absolute time': it makes startling predictions for objects and observers moving at very high speeds. General relativity, on the other hand, is concerned with accelerations: it turns out to be a theory of gravity which has had a profound impact on our modem view of the universe.

Since our aim is to introduce as many people as possible to the strange world of relativity we have deliberately used a minimal amount of mathematics in the text. Some simple derivations requiring no more than high school maths have been relegated to an appendix for the curious. Needless to say, any book about both special and general relativity must also be to some extent about the physicist who almost single-handedly created these theories. Einstein's legacy is truly remarkable – both inside and outside of physics – and we hope to have captured some flavour of the man through the quotations and stories that accompany the text.