Time travel is an absolutely fascinating aspect of physics, if only because few people realize that it’s not fantasy – not only does time travel fail to break the laws of physics, we even have to deal with (very small scale) time travel on the part of GPS satellites to make them work properly. Paul Nahin doesn’t point this out, quite possibly because this is a surprisingly old book – although reissued in 2011 with a new preface, it dates back to 1997. GPS might not have been part of everyday life back then, but most of the time travel science has survived pretty well unchanged. There are a few time-based omissions. No mention of superluminal tunnelling experiments or laser-based frame dragging, for instance. But the biggest omission in the science due to the age of the book has nothing much to do with time travel – it’s the casual dismissal of the cosmological constant, showing a predating of the discovery of dark energy. But luckily this is more a side-comment than of any great...