Tau Zero

by Poul Anderson

Tau Zero follows the crew of the starship Leonora Christine, a colonization vessel crewed by 25 men and 25 women aiming to reach the nearby star Beta Virginis. The ship is powered by a Bussard ramjet, which was proposed 10 years before Anderson wrote the book. This engine is not capable of relativity and time dilation: the crew will spend 5 years on board, but 33 years will pass on the Earth before they arrive at their destination. The ship accelerates at a modest constant rate for most of the first half of the journey, eventually achieving an appreciable percentage of the speed of light, and the goal is to decelerate at the same rate during the second half of the journey by rotating the ship 180° about its trajectory axis and continuing to fire the Bussard ramjet. However, the Leonora Christine passes through a small nebula before the half-way point, damaging the Bussard ram scoops which were to have been the deceleration module. Since the Bussard engines must be kept running to provide particle/radiation shielding, and because of the hard radiation produced by the engines, the crew can neither repair the damage nor turn off their ramjet.

The text consists of narrative prose interspersed with paragraphs in which Anderson explains the scientific basis of relativity, time dilation, the ship's mechanics and details of the cosmos outside.

As there is no hope of completing the original mission, the crew increase acceleration even more; they need to leave the Milky Way altogether in order to reach a region where the local gas density, and the concomitant radiation hazard, are low enough that they can repair the decelerator. The ship's ever-increasing velocity brings the time dilation to extreme levels and takes the crew further and further away from any possibility of contact with humanity. The initial plan is to locate and land on a suitable planet in another galaxy. Millions of years would have passed since their departure, and in any case they would be millions of light years from Earth. However, they find the vacuum of intergalactic space insufficient for safety; they must instead travel to a region between superclusters of galaxies to make repairs. They do, but the extremely thinly spread matter is then too dispersed to use for deceleration. They must wait, flying free but essentially without the ability to change course, until they randomly encounter enough galactic matter to try to decelerate enough to search for habitable planets. To make the waiting time shorter, they continue accelerating through the first several galaxies they encounter, more and more closely approaching the speed of light with tau, or proper time, decreasing closer and closer to zero.

Throughout the story, Charles Reymont, the ship's Constable, fights to keep hope alive in the confined community and at the same time maintain order and discipline, sometimes at great emotional cost to himself. He explains his system to his partner Chi-Yuen Ai-Ling: The storyline is similar to that of the long poem and later opera Aniara, in which the ship was unable to stop and doomed to travel endlessly, but Tau Zero has a more upbeat ending. By the time the ship is repaired, tau has decreased to less than a billionth and the crew experience "billion-year cycles which pass as moments". But by the time that they are ready to attempt to find a future home, they realize that the universe is approaching a Big Crunch. The universe collapses (a process the starship survives because there is still enough uncondensed hydrogen for maneuvering outside the growing singularity) and then explodes in a new Big Bang. The voyagers then decelerate and finally disembark at a planet with a habitat suitably similar to Earth, on which the vegetation has a vivid bluish-green color.