Time travel can be in either direction and can involve either circular causality or causality violation. These two directions of travel and two causality paradoxes are four aspects of time travel and have distinct literary sources -
pastward travel: Anon and Twain;
futureward travel: Wells;
circular causality: Wells and Heinlein;
causality violation: de Camp.
Uniquely, Poul Anderson addressed all four aspects in different works and also made five original contributions to the concept of time travel:
(i) corridors constructed in space, then rotated onto the temporal axis, so that it becomes possible to time travel by walking along a corridor;
(ii) mutants able to time travel by an act of will;
(iii) the Time Patrol with its milieu headquarters and branch offices in different periods linked by message shuttles and by Patrol members traveling on timecycles, its Specialists exploring undocumented periods and its Unattached agents acting at any point in history to prevent causality violations;
(iv) historical alterations caused not only by time travelers but also by random fluctuations in space-time-energy;
(v) a personal causal nexus, i.e., an individual whose world-line intersects with so many others that a small change in his life cause big changes in history.
In Anderson's Time Patrol series, mankind, instead of devolving into Morlocks or Eloi, evolves into the Danellians who found the Patrol whose members travel into the past in order to prevent causality violations sometimes by completing causal circles but, on one occasion, by responding to space-time fluctuations and neutralizing a personal causal nexus. This series incorporates the four aspects of time travel and also Anderson's contributions (iii)-(v). The Patrolmen use neither (i) time corridors nor (ii) a mutant ability but timecycles which resemble modernized, mass-produced, streamlined Time Machines. Thus, this series, while not fully comprehensive, is as comprehensive a synthesis of time travel fiction as we will probably get.
Pastward Travel
"Missing One's Coach" Anon
A Connecticut Yankee In King Arthur's Court by Mark Twain
"The Little Monster" by Poul Anderson
"The Man Who Came Early" by Poul Anderson
In each of these works, a man is transported to an earlier period with different consequences. The earliest literary time traveler merely visits the past, possibly just in a dream. The Yankee prospers. One Anderson character survives. The other does not.
Futureward Travel
The Time Machine by HG Wells
"Time Heals" by Poul Anderson
"Welcome" by Poul Anderson
"Flight to Forever" by Poul Anderson
In each of these works, a technological time traveler visits future periods, again with different results. Whereas Wells' Time Traveler sees the end of life on Earth and returns to the nineteenth century, one of Anderson's time travelers sees the end of the universe, then continues around the circle of time back to the twentieth century.
Circular Causality
"The Chronic Argonauts" by HG Wells
two stories and one novel by Robert Heinlein
The Technicolor Time Machine by Harry Harrison
three novels by Anderson
The Anubis Gates by Tim Powers
The Time Traveler's Wife by Audrey Niffeneger
Heinlein's time travelers experience the circular causality paradox in future periods and one of them emulates the Yankee by applying modern knowledge in a simpler period. Harrison's and Power's time travelers experience this paradox in historical periods whereas Niffeneger's characters experience it in their own lifetimes. Anderson's characters travel through past and future. Indeed, two future civilizations, using time corridors, wage war throughout history.
Causality Violation
Lest Darkness Fall by L Sprague de Camp
Bring The Jubilee by Ward Moore
De Camp's involuntary time traveler deliberately prevents the Dark Ages whereas Moore's time traveling historian accidentally alters the course and outcome of the Battle of Gettysburg.
Culmination
The Time Patrol series by Poul Anderson -
- a series of stories and novels about an organization of time travelers experiencing both causality paradoxes in various historical and prehistorical periods.
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