Tau Zero

Tau Zero

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Tau Zero is a hard science fiction
Hard science fiction
Hard science fiction is a category of science fiction characterized by an emphasis on scientific or technical detail, or on scientific accuracy, or on both. The term was first used in print in 1957 by P. Schuyler Miller in a review of John W. Campbell, Jr.'s Islands of Space in Astounding Science...

 novel by Poul Anderson
Poul Anderson
Poul William Anderson was an American science fiction author who began his career during one of the Golden Ages of the genre and continued to write and remain popular into the 21st century. Anderson also authored several works of fantasy, historical novels, and a prodigious number of short stories...

. The novel was based upon the short story "To Outlive Eternity" appearing in Galaxy Science Fiction
Galaxy Science Fiction
Galaxy Science Fiction was an American digest-size science fiction magazine, published from 1950 to 1980. It was founded by an Italian company, World Editions, which was looking to break in to the American market. World Editions hired as editor H. L...

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
Hugo Award for Best Novel
The Hugo Awards are given every year by the World Science Fiction Society for the best science fiction or fantasy works and achievements of the previous year. The award is named after Hugo Gernsback, the founder of the pioneering science fiction magazine Amazing Stories, and was once officially...

 in 1971.


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
Bussard ramjet
The Bussard ramjet is a theoretical method of spacecraft propulsion proposed in 1960 by the physicist Robert W. Bussard, popularized by Larry Niven in his Known Space series of books, and referred to by Carl Sagan in the television series and book Cosmos....

, which was proposed shortly before Anderson wrote the book. This engine is not capable of faster-than-light
Faster-than-light communications and travel refer to the propagation of information or matter faster than the speed of light....

 travel, and so the voyage is subject to relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 and time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

: 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 collides with a nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 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 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). 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
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...

. The universe collapses into a cosmic egg
Cosmic Egg
Cosmic Egg is the second studio album by Australian rock band Wolfmother, released on 23 October 2009. It is the first album by the second lineup of the band, featuring vocalist, songwriter and lead guitarist Andrew Stockdale, bassist and keyboardist Ian Peres, rhythm guitarist Aidan Nemeth and...

 (which the starship survives because there is still enough uncondensed hydrogen for maneuvering, outside the monobloc) and then explodes in a new big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

. 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 (), where and v is the velocity as a fraction of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

. At a given velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

, 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
Proper time
In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events. The proper time depends not only on the events but also on the motion of the clock between the events. An accelerated clock will measure a smaller elapsed time between two...

, so Anderson's "tau factor" is what would conventionally be written d/dt. Physicists also prefer to use gamma (γ) to represent the Lorentz factor
Lorentz factor
The Lorentz factor or Lorentz term appears in several equations in special relativity, including time dilation, length contraction, and the relativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol γ . It gets its name from its earlier...

 in time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

, which in Anderson's terminology would be 1/. Incidentally, a real spaceship flying this close to the speed of light would not observe time dilation in this manner. Instead, they would, counter intuitively, observe the world slowed down by the same factor. The effect of time dilation (as in the twin paradox) would only be apparent upon slowing down the ship.


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
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 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 Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...

n 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.