Edition: Gollancz, 1990 (Buy from Amazon)
Review number: 1109
Tau Zero is about a space ship carrying fifty people to a nearby star; massive acceleration to close to the speed of light and relativistic time distortion mean that the journey will take decades rather than centuries, at least as far as the crew and passengers are concerned. On this voyage, there are even veterans of previous trips to other stars.
What makes this particular jouney of interest is a disaster which occurs about half way through. The plan was to accelerate for half the trip, and decelerate for the rest; but at just about the mid way mark, an encounter with a comparatively dense region of gas damages the ship, making deceleration impossible until repairs can be carried out. Tau Zero is about how the crew of the Leonora Christina cope with the disaster - and in fact it is a science fiction equivalent of the disaster movie.
The physical measurement called tau by Anderson is incredibly important in the theory of relativity. (It was not given a name like this in the lectures I had on the subject as a student; the letter tau was used to indicate a time parameter.) It is a measure of the difference between an object's velocity and the speed of light, and is 1 at rest and approaches 0 at the speed of light. Its vital importance in this particular novel is shown not just by the title but because Anderson makes the extremely unusual step of putting the mathematical formula defining tau in the text. It describes the distortion of various measurements at high: length, time, mass.
In Tau Zero, Anderson has managed to combine hard science fiction with a study of character - not something for which the subgenre is known. It is also unusual because most of the science described in the novel is well-understood, unlike the speculations of writers like Niven and Forward. It is hard science fiction at its very best, presenting the science in an entertaining way as part of a real story. It is not surprising that Tau Zero has become one of the most famous of all novels in the genre.