# Tau Zero

For the Tau Zero Foundation, see Breakthrough Propulsion Physics Program#Legacy.
Tau Zero

Cover of first edition (hardcover)
Author Poul Anderson
Cover artist Anita Siegel[1]
Country United States
Language English
Genre Science fiction novel
Publisher Doubleday
Publication date
1970
Media type Print (Hardcover & Paperback)
Pages 208 pp
ISBN ISBN 1-56865-278-X
OCLC 37202159

Tau Zero is a hard science fiction novel by Poul Anderson. The novel was based upon the short story "To Outlive Eternity" appearing in Galaxy Science Fiction in 1967. It was first published in book form in 1970.

The book is regarded as a quintessential example of "hard sci-fi", as its plot is guided by technology until the dramatic conclusion. It was nominated for the Hugo Award for Best Novel in 1971.[2]

## Plot

Tau Zero follows the crew of the starship Leonora Christine, a colonization vessel crewed by 25 men and 25 women aiming to reach a distant star system. The ship is powered by a Bussard ramjet, which was proposed shortly before Anderson wrote the book. This engine is not capable of faster-than-light travel, and so the voyage is subject to 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 during the first half of the journey and decelerates during the second. However, it flies through a nebula before the half-way point, damaging the deceleration module. Since the 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 decelerator nor turn off the accelerator.

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.

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 (albeit one that does not conform to modern thinking on the evolution of the universe).[citation needed] By the time the ship is repaired, tau has decreased to less than a billionth and the crew experience "billion-year cycles which passed 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 monobloc) and then explodes in a new big bang. The voyagers then decelerate, examining potential star systems. They eventually disembark at a planet with a habitat suitably similar to Earth, on which the vegetation has a vivid bluish-green color.

## Origin of the title

The novel's title is derived from the value of the time contraction factor Tau ($\tau$), where $\tau = \sqrt{1-v^2/c^2}$ where v is the velocity and c the speed of light. At a given velocity, the duration that is experienced on the non-accelerating Earth may be multiplied by tau to yield the duration experienced on board the ship. Therefore, as Anderson writes, "the closer that [the ship's velocity] comes to [the speed of light], the closer tau comes to zero", and the longer the time that passes outside the ship for a duration inside. The ship in the story intended to attain a tau of 0.015, but as they continue to accelerate beyond the original schedule, it decreases.

This usage of tau is somewhat idiosyncratic. In physics, tau is more usually used to represent the total elapsed time of the moving clock, so Anderson's "tau factor" is what would conventionally be written d$\tau$/dt. Physicists also prefer to use gamma (γ) to represent the Lorentz factor in time dilation, which in Anderson's terminology would be 1/$\tau$.[citation needed]

## Themes

Much of the novel deals with the crewmembers' reactions to being the last remnants of humanity, and the prospect of being confined with their colleagues indefinitely. Though they were prepared to "lose" twenty Earth years during their journey and spend five on board the ship, the knowledge that they are being carried further and further into the future has various effects on the psychology of the characters. The novel describes the changing and extreme time dilation effects as well as events from the perspective of both the ship and an external observer.

Incidental to the main themes is the political situation on the Earth from which the protagonists set out: a future where the nations of the world entrusted Sweden with overseeing disarmament and found themselves living under the rule of the Swedish Empire. This sub-theme reflects the great interest which Anderson, an American of Danish origin, took in Scandinavian history and culture. In later parts of the book, characters compare their desperate situation to that of semi-mythical characters of Scandinavian legend, with the relevant poetry occasionally quoted.