This book deals with the simplest of questions: What is space and what is time? It is one of the most difficult questions, and scientific progress measures the revolutions that have provided new answers to it.

For the moment, we find ourselves in the midst of such a revolution, during which not one but several new ideas about space and time are being examined. The new theory that will provide us with answers to questions concerning space and time is not yet complete, but it already has a name: quantum theory of gravity or simply quantum gravity.