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 significance.
Apart from those small omissions, Nahin does a great job of getting through all the possibilities in a reasonably compact way. However, the way the book is tightly targeted means that it isn’t the ideal popular science book on time travel for the general reader. This is very much aimed at would-be writers of science fiction, and this comes through very strongly, both in Nahin’s remarks and in his approach which relies on many, many examples of science fiction stories featuring time travel. I’m quite a fan of science fiction, but I have to confess I found it heavy going with so many fictional references.
There are a couple of other tiny problems with the book. The science is mostly put across in a friendly fashion, but there are two times when Nahin goes into much too much detail, in a way that may well put off a lot of readers (particularly if they are looking at this as authors rather than would-be scientists). The other problem I had was a rather hectoring tone – at times he almost says that people with certain ideas (e.g. having problems with the killing-your-grandfather type paradox) are idiots because the paradox obviously won’t happen. I’m sorry, it isn’t obvious, and taking this approach is not particularly helpful.
Overall, then, a good if rather dated summary of the science of time travel, but because of the way it is put across only really of interest to serious SF fans or would-be writers (which, to be fair to the author, is exactly the audience that it is aimed at).