Time travel, as Brian Clegg reminds us in his first chapter (sorry, first lesson), was a popular fictional subject long before it found its way into mainstream science. That it did is largely thanks to Einstein’s theory of general relativity, which is a notoriously abstruse area of modern physics. So it’s no easy thing to produce a popular-level book that really gets to grips with the serious science of time travel, and it’s to Clegg’s credit that he achieved just that in his brilliant How to Build a Time Machine (aka Build Your Own Time Machine) ten years ago. This new book is rather different, approaching the same subject in an altogether more lightweight way. Appropriately enough, it’s part of a series called ‘Pocket Einstein’. But the fact that Einstein keeps cropping up in it – with topics like quantum entanglement and Einstein-Rosen bridges as well as relativity – is largely coincidence. Other titles in the same series include Artificial Intelligence and Renewable Energy , whi