Poul Anderson was one of the most productive SFF writers of the second half of the last century, publishing about a hundred books and winning seven Hugo and three Nebula awards. His first book was published in 1953 and his last fifty years later, two years after his death. Tau Zero was published in 1970 and was nominated for a Hugo. When it was selected for the reading list of the Classic Science Fiction discussion group I was surprised to discover that I had never read it, so I obtained a new copy (published by Gollancz under the SF Masterworks label).
The plot is simple: a colonisation ship with fifty people on board leaves Earth for another solar system where a probe had reported a habitable planet. En route, the ship runs into trouble and cannot decelerate. The only chance of survival is to keep accelerating closer and closer to light speed in order to maximise the time dilation effect and travel as far as possible; initially to find space empty enough to shut off the drive and protective screen in order to carry out repairs, and secondly to find a zone where the conditions are right for them to stop and find a place to live.
There are two threads running through this story: the first is a very technical, hard-science description of the functioning of the Bussard ramjet, the implications of the ship's velocity getting ever closer to the speed of light, and the structure and evolution of the universe. The second is the human story of the effect of their situation on the crew and scientists on board the ship, as time outside passes at an ever-increasing rate compared with time inside.
Like nearly all SF of the period, this story is about ideas more than people. If the title had not already been pre-empted by the famous short story, the novel might accurately have been called The Cold Equations. Having said that, the main characters are drawn well enough to carry the plot, while time has not been kind to the science. The effectiveness of the Bussard ramjet concept (very popular in the 1960s and 70s) has since been questioned and the future of the universe is now believed to be somewhat different from that shown in the book.
This story makes an interesting contrast with Niven's A World Out of Time published a few years after Tau Zero and reviewed on this blog recently. This also features a Bussard ramjet making an enormous journey to gain the benefit of time dilation, but Niven's story concerns itself much more with the social, genetic and technical changes which take place on Earth over the aeons and, to me at least, is all the more interesting and enjoyable as a result.
Tau Zero may not be the most enjoyable of tales, but it is deservedly a classic for its exploration of the science of relativity and its potential consequences.