Through Time With Mark Twain, HG Wells And Their Successors...

...mainly including Poul Anderson, of course.

The anonymous "Missing One's Coach" presents a visit to a historical period that might be a dream.

Mark Twain's Connecticut Yankee experiences instantaneous "transposition of epochs" to Arthurian Britain and returns to the nineteenth century by Merlinian suspended animation.

In The Time Machine by HG Wells, the Time Traveler and his dinner guests discuss "time travelling" before the Time Traveler demonstrates his model Time Machine which does not move anywhere but, remaining stationary on the Earth's surface, undergoes extreme time dilation in either temporal direction while also unaccountably becoming both invisible and intangible.

In "The Dark Tower" by CS Lewis, five men discuss "time travelling" and agree that it is impossible before one of their number demonstrates his chronoscope which displays scenes not, as they initially think, from the past or future but from an alternative timeline.

In the Time Patrol series by Poul Anderson, timecycles, resembling modenized Time Machines, instantaneously change their spatiotemporal coordinates, thus neither moving nor dilating but disappearing and appearing.

In The Corridors Of Time by Poul Anderson, "time travelers" really do travel through time because they walk or drive down corridors whose lengths extend along the temporal axis.

In The Dancer From Atlantis by Anderson, a space-time vehicle does not remain stationary on the Earth's surface but moves across it "...while traveling through time." (London, 1977, p. 32)

In There Will Be Time by Anderson, some mutants can "time travel"/dilate invisibly and intangibly without needing a time machine and one of their number gives Wells the time travel idea.

In The Time Traveler's Wife by Audrey Niffenegger, mutant time travel is instantaneous but involuntary.

In Bid Time Return by Richard Matheson and two Time novels by Jack Finney, travel to the past and back to the present is a learned skill.

In "The Flight of the Horse" by Larry Niven, time travel is scientifically impossible so it takes explorers into a fantasy past where they find not extinct species but mythical beasts.

This is not and cannot be a complete list of time travel fictions but it does cover most of the possibilities and presents a conceptual sequence:

time travel is possible with machines - but there are different kinds of machines;
time travel is possible without machines - but in different circumstances;
time travel is impossible but time viewing is possible with a machine;
time travel is impossible so it is fantasy, not sf.

Time Travel and Poul Anderson

The concept of time travel raises at least three questions. Is it logically possible? Is it physically possible? How is it treated in fiction? I have read (a lot of) time travel fiction and studied philosophy, including logic, but not physics. I discuss the logic of time travel here and here but, in the current article, it is mentioned only to introduce the fiction.
"Travel" just to the future is time dilation or temporal stasis. Genuine time travel necessarily involves at least the possibility of pastwards travel, from the present to the past or from the future back to the present. Thus, Wells’ Time Traveller returns; his Sleeper doesn’t.
Time travellers may:
disappear at one time and appear at another;
experience futurewards or pastwards time dilation;
rotate around a massive gravitational field.
The first case is the easiest and simplest. The third is a contribution from later physical theories. In the second, Wellsian, case, it is almost always assumed that time travellers, while travelling, are undetectable by others. This assumption, despite Wells’ defence of it, is questionable but simplifies story telling. It is part of the "treatment in fiction", not of the "logic". The assumption enables a time traveller to arrive in the future without being expected or to arrive in the past without his journey from the future then becoming visible to others.
A pastwards time traveller’s arrival precedes his departure. Thus, an effect precedes its cause and might either cause or prevent it so the causality paradoxes are circular causality and causality violation. A "paradox" is either an apparent or a logical contradiction. The occurrence of apparent contradictions is unexpected but the occurrence of logical contradictions, or "contradictions in terms", is impossible. An event that occurs, because it has an effect, and does not occur, because it is prevented by its effect, is a logical contradiction – unless it occurs in one timeline and does not occur in another. Alternatively, within a single timeline, an uncaused event might prevent the event that would have caused it. Uncaused events, like causal circles, are counterintuitive but not contradictory. Causal laws are empirical generalisations, not logical necessities.

Causality Violation

The idea of causality violation developed gradually.

(i) Implicit

The earliest account of travel to the past that I know of, although I have not managed to read it, is "Missing One’s Coach", in which a modern man is mysteriously transported to a historical period. (1) Any account of travel to the past implicitly raises the question whether the time traveller would be able to affect (cause, change or prevent) past events.

(ii) Attempted?

Mark Twain’s Connecticut Yankee, a modern man mysteriously transported to a, different, historical period, tries unsuccessfully to change past society without reflecting that success might have been paradoxical. (2) Perhaps he thought that his social revolution, even if successful in Arthurian Britain, would have been unrecorded and forgotten by the nineteenth century?

(iii) Implied

In The Time Machine by H. G. Wells, the Medical Man suggests that a time traveller observing the Battle of Hastings might attract attention as an anachronism while the narrator comments that time travelling "suggests curious possibilities of anachronism and of utter confusion." (3) (4). Pastwards travel is "anachronism". Causality violation would be perhaps the most extreme form of "utter confusion" but Wells did not elaborate.

(iv) Successful

In Lest Darkness Fall by L. Sprague de Camp, a modern man, transported by lightning to yet another historical period, tries successfully to change the course of history. He prevents the Dark Ages. His success is explained by comparing time to a branching tree but also by comparing world lines to a tough web which it is difficult but not impossible to distort. (5) The second comparison continues the Wellsian conceptualisation of time as a fourth dimension.

(v) Accidental

In Bring the Jubilee by Ward Moore, a time traveller observing the Battle of Gettysburg attracts attention, thus unintentionally changing the outcome of the battle and the subsequent course of history. (6)

(vi) Prevented

In "Time Patrol" by Poul Anderson, a man from our future travels to post-Roman Britain in order to change the course of history but is apprehended by the Time Patrol, a time travelling organisation that prevents other time travellers from either accidentally or deliberately changing the past. Causality violation is explained by de Camp’s second comparison. (7)
In "Delenda Est" by Poul Anderson, two time travellers intervene in a Roman-Carthaginian battle, thus deliberately changing the outcome of the battle and the subsequent course of history. Time Patrollers returning from the Pleistocene to 1960 find that they are in an alternative timeline, study history to locate the moment of divergence and restore their preferred timeline by counter-intervening in the pivotal battle. (8)
In Year of the Ransom, Time Patrollers prevent extra-temporal intervention in a later battle. (9) In "Women and Horses and Power and War", they lure time criminals to a particular period with false intelligence about the historical significance of an ancient battle. (10)
In "Amazement of the World", a random fluctuation in space-time energy changes the outcome of a medieval battle and the subsequent course of history. (11) Correcting the course of the battle does not restore the preferred timeline because the hero of the battle is a "personal causal nexus" whose world-line intersects with so many others that small changes in his career disproportionately affect the outcome of the medieval church-state conflict. (12) His actions prevent the birth either of a powerful Emperor or of a powerful Pope. By eliminating this knight, the Patrol restores the familiar history in which a stalemate between theocracy and autocracy allows the development of freedom and science.
The Time Patrol series presupposes a single discontinuous timeline or a single mutable timeline or multiple successive timelines, although I argue here that there is only a terminological difference between "single mutable" and "multiple successive". "One timeline changes into others" and "many timelines succeed each other" mean the same thing.
A discontinuous timeline entails, counter-intuitively, that some events described in the stories do not occur. Mutable/successive timelines entail that deleted timelines existed in the past of a second temporal axis, not that they never existed, as the characters believe. However:
we can appreciate fictitious characters’ adventures without sharing their beliefs;
Anderson, more than any other time travel writer, avoids overt contradictions and conceals subtler ones;
his accounts of historical periods and analyses of historical processes make this series, like Jack Finney’s two Time novels, a culminating point of time travel fiction even if its logical basis is, partly, questionable.

(vii) Other

Although Finney’s Time novels are primarily about the experience of being in the past, each also culminates in a dramatic causality violation, although I argue here that these excellent novels are marred by inconsistencies when they address this paradox. (13) I also argue here that other authors’ accounts of causality violation are unacceptably inconsistent and that they contribute nothing to the literary development from Twain and Wells via de Camp and Moore to Anderson and Finney.

Using Modern Knowledge in the Past

The question whether a time traveller can change the past is a corollary of the simpler question: would a time traveller be able to use modern knowledge, techniques or equipment to his advantage while in the past? Any major application of modern knowledge etc in the past would have to change it. Using machine guns in an important ancient battle would, almost certainly, change the outcome of the battle and the subsequent course of history. However, guns might be used in an unimportant and unrecorded battle without any longer term consequences. The Time Patrol might allow or even instigate this for some reason as it does with other anachronistic equipment in "The Sorrow of Odin the Goth". (14) Perhaps the Connecticut Yankee thought that his attempted social changes would be unrecorded and forgotten.
A time traveller arriving in an earlier period carries within himself his modern knowledge and technical skills. However, if his means of time travel do not allow him to transport equipment, then it will be difficult or impossible to duplicate modern equipment in an earlier, especially a pre-industrial, period. Hank Morgan, the Connecticut Yankee, arrives with only the clothes that he wears but, impossibly, produces an endless supply of cables, telephones and machine guns and a printing press. He single-handedly and effortlessly masterminds a mostly clandestine industrial revolution in Arthurian Britain. Twain either did not realise or, more probably, did not care that this was impossible, and even had most of it occur off-stage, whereas his successors have paid more attention to details and practical difficulties.
De Camp’s Martin Padway struggles to innovate, e.g., to persuade Romans to accept Arabic numerals, and still has not synthesised gunpowder by the end of the novel, yet it is he, not Morgan, that accomplishes enough to divert history. Anderson’s "The Man who Came Early" is a direct reply to Lest Darkness Fall because its hero, like Padway, is transported to the past by a flash of lightning but he comments that this "…happens only once in many thousands of times." (15) Further, he not only does not introduce modern techniques but dies because he is unskilled in Viking customs and techniques. Against this, a boy in Anderson’s "The Little Monster" uses modern knowledge, techniques and a knife to survive in the Pliocene. (16)
In "Time Patrol", Stane’s easily transported energy weapon and his future sociological knowledge win him a place at court in fifth century Britain but, in "Delenda Est", a Time Patroller asked to manufacture a temporal vehicle in a pre-industrial period "…didn’t have the tools to make the tools to make what was needed…" (17)

Using Modern Knowledge in the Future

Affecting the future does not raise the same logical problem as affecting, and possibly altering, the past. However, we usually expect future societies to be too powerful or complex for a surviving modern man to influence them. This is the case in "Welcome" and "Time Heals" by Poul Anderson. (18) (19) On the other hand, the Time Traveller fights Morlocks with matches and explains them with Darwinism.
Bob Wilson in "By His Bootstraps" by Robert Heinlein is a literary successor not only of the Time Traveller but also of Morgan and Padway because he not only visits a future paradisal society but also uses modern knowledge and equipment to take control of it. (20) Martin Saunders in "Flight to Forever" by Poul Anderson uses the time travel technique to win a space battle. (21)

Circular Causality

"The Chronic Argonauts" by H. G. Wells introduced but understated the circular causality paradox in a contemporary British setting. (22) The central character moves into an empty and shunned house, then travels through time while remaining within the house. On arriving in the past, he is attacked as an intruder by the previous occupants of the house but defends himself and returns to his present. In the absence of any evidence of an intruder, two brothers are convicted of the murder of their father killed in self-defence by the time traveller. Consequently, the house is empty and shunned. "A Stitch in Time" and "The Chronoclasm" by John Wyndham present uncomplicated accounts of the same paradox, also in contemporary British settings. (23) (24)
"By His Bootstraps", The Door into Summer and "-All You Zombies-" by Robert Heinlein present more complicated accounts of circular causality in future American settings. (25) (26) In "…All You Zombies", a sex-changed time traveller is both of his own parents. In "By His Bootstraps", Bob Wilson returns to 1952 to persuade his younger self to visit the future only to be interrupted by his older self trying to prevent him. The nuisance phone caller who interrupts their three-sided argument and the older man who sent Wilson back to 1952 are both himself.
In Beyond the Barrier by Damon Knight, a couple depart to seek a time traveller stranded in the past a moment after seeing themselves return safely with him. Their child does not accompany them but is not left unattended. (27)

The Technicolor Time Machine by Harry Harrison, The Anubis Gates by Tim Powers and three novels by Poul Anderson (The Corridors of Time, There Will Be Time and The Dancer from Atlantis) place circular causality in historical settings. (28) (29) (30) (31) (32)
A circle, once closed, is complete whereas anything that has been changed once can be changed again. Therefore, circular causality stories do not have sequels whereas Finney wrote a sequel to Time and Again and "Time Patrol" became a series. All the works mentioned so far in this section could be set in a single, continuous timeline. Such a timeline can accommodate causal circles but not causality violations because the latter require either a single, discontinuous timeline or several timelines.
Anderson incorporated circular causality into his causality violation scenario. In "Brave to be a King", "The Only Game in Town", "The Sorrow of Odin the Goth" and "Star of the Sea", the Patrol finds that it must intervene in history not to preserve but to cause its own past. (33) (34) (35) Sometimes the Patrol prevents causality violations by closing causal circles. After killing Stane, they retrieve the time vehicle that he had stolen but leave its fuel chest to be buried with him because it was "the singular contents of an ancient British barrow", mentioned in a work of literature, that had alerted the Patrol to unauthorized extra-temporal activity in post-Roman Britain. (36)
In "Ivory and Apes and Peacocks", Time Patroller Manson Everard, seeking time criminals in ancient Tyre, uncovers, by detective work, an account of how the Patrol had apprehended the criminals decades earlier. He then leads the expedition to apprehend them. (37)
I do not know whether other authors learned the circular causality paradox from "The Chronic Argonauts". However, this idea, having appeared in Wells, almost certainly came to Anderson from Heinlein, if not also from others. The idea of time travellers on historical battlefields passed from Wells to Moore and Anderson. Attempted historical causality violation passed from Twain via de Camp to Anderson. Thus, the Time Patrol series incorporates and perfects three strands of time travel fiction, including both causality paradoxes.
(Since writing this article, I have read The Time Traveller's Wife by Audrey Niffenegger, a major circular causality novel that is not genre sf (see below) but a novel of relationships with time travel integral to the plot. (38) Any novel can reflect on the passage of time and on the distance travelled from childhood but only time travel fiction can return a character physically to his remembered past. The Time Traveller's Wife is a third culmination of time travel fiction, following Anderson’s Time Patrol and Finney’s Time novels.) (Later: I have just caught up with Richard Matheson's Bid Time Return/Somewhere In Time, a romantic novel involving time travel. Matheson has some good moments of circular causality and of what it would be like to be in the past. His means of time travel (see the next section) is identical with Finney's but Matheson's novel is ambiguous as to whether the central character, whose account we read, had imagined his chronokinesis.)

Means of Time Travel

Wells coined the terms "time machine", "time traveller" and "time travelling", later shortened to "time travel". Twain had used the phrase "transposition of epochs". Later authors coined the terms "chrononaut" and "chronokinesis" but Wells’ terminology remains in common use. "The Chronic Argonauts", which became The Time Machine, introduced the idea of a temporal vehicle.
The Time Traveller sits on the Time Machine and Time Patrollers sit on their timecycles but the BBC character Dr Who travels inside the TARDIS and Martin Saunders in "Flight to Forever" by Poul Anderson travels inside his time projector. Simon Morley in Jack Finney’s Time novels, Jack Havig in Poul Anderson’s There Will Be Time and Henry DeTamble in Audrey Niffenegger’s The Time Traveller's Wife do not need vehicles. Henry’s uncontrolled chrono-displacements are genetically based, therefore affect only his body, leaving behind even his clothes. Henry’s daughter, a second generation time traveller, sometimes controls when she travels to.
Despite his unaccountable sensation of "headlong motion," the Time Traveller remains spatially stationary while, effectively, fast forwarding, then rewinding, everything else. There is, therefore, no basis for his claim that the Machine, while "travelling," is undetectable because of its speed. In fact, he began his exposition by arguing that material bodies do not move along the fourth dimension but merely extend in that direction. If they did move along "time," then we would have to ask: what do they move in relation to? and: surely the Time Traveller, by accelerating, would leave everything else behind? - whereas he is instead described as seeing objects around him while travelling and then as finding them waiting for him on arrival. Further, motion along the fourth dimension would take time, therefore this dimension would not be time. In any case, a bullet or propeller is invisible because of its speed but we cannot safely occupy the same space as it.
Wells contradicts himself. In "The New Accelerator", he describes a drug which makes the bodies and minds of its inventor and the narrator function "many thousand times" faster than usual. (39) Even when they stand still, their visual and mental processes accelerate. Thus, they see a glass falling so slowly that it appears to them to be suspended in mid-air. Then, when they walk through Folkestone, too swiftly to be seen by anyone else:
"A purple-faced gentleman was frozen in the midst of a violent struggle to refold his newspaper against the wind…that had no existence so far as our sensations went…To see all that multitude changed to a picture, smitten rigid, as it were, into the semblance of realistic wax, was impossibly wonderful." (40)
Next, the inventor:
"…hopes to find a Retarder…it should enable the patient to spread a few seconds over many hours of ordinary time…" (41)
(The Time Traveller spreads a few minutes of his experience over many millennia of history.)
"…and so to maintain an apathetic inaction, a glacier like absence of alacrity, amidst the most animated or irritating surroundings." (41)
For many millennia, the Time Traveller, by comparison with his surroundings, is inactive, glacierlike, "smitten rigid" into the semblance of wax, and therefore should not be described as "going…fast" and blurred into invisibility. (42) His Machine is a Retarder, not an Accelerator, although, of course, what he notices and describes is not his own inaction but the comparative speed of everything else:
"The slowest snail that ever crawled dashed by too fast for me." (42)
Despite this, the Psychologist, one of the Time Traveller’s dinner guests, discussing the recently departed model time machine, says:
"We cannot see it, nor can we appreciate this machine, any more than we can the spoke of a wheel spinning, or a bullet flying…If it is travelling through time fifty times or a hundred times faster than we are, if it gets through a minute while we get through a second, the impression it creates will of course be only one-fiftieth or one-hundredth of what it would make if it were not travelling in time…" (43)
and, to demonstrate this, he passes his hand through the space in which the model time machine had been.
However, his account is incoherent. First, he says that the machine is travelling through time faster than the dinner guests, then he speaks as if it alone is travelling. Secondly, his "while" cannot mean "during the same period of time as". If the machine "gets through" a minute and the guests ":get through" a second, then, at the end of these two processes of "getting through", they are not at the same time. The Psychologist means something like, "a time traveller would get through an objective minute in a subjective second whereas we get through only an objective second in a subjective second" or "a time traveller experiences a second while, during the same period of time as, everyone else experiences a minute." Thus, the time traveller is "Retarded", not "Accelerated".
The Psychologist further argues that this stretching (dilation) of the traveller’s duration causes an attenuation of his substance (although it is the Time Traveller who introduces the term "attenuated" shortly afterwards). (44) The material substance that usually fills a second is supposedly stretched out over a longer period to such an extent that it becomes invisible and even intangible. Such attenuation is not usually regarded as a consequence of relativistic time dilation and, even if it were, it would not be the same thing as the "headlong motion" that the Time Traveller says he experiences. Later writers usually accept the undetectability of time travellers in transit but not Wells’ explanation of it.
The static four-dimensional continuum expounded by the Time Traveller is presented more consistently in The Quincunx of Time by James Blish. Blish’s character, Thor Wald, explains:
"…if time is a linear dimension, just like length, height, and width, then the entity that I see before me as Captain Robin Weinbaum, is only a second-by-second section through a much larger entity, one of whose extensions is invisible to me." (45)
Weinbaum and Wald receive messages from their future and thus gain some knowledge of future events but cannot experience those events any sooner than anyone else. One of these messages links Quincunx to Midsummer Century by Blish. (46) In this second short novel, John Martels’ personality, though not his body, is projected from 1985 to 25,000. Personalities are semi-stable electromagnetic fields. Martels’ field is projected accidentally and received by a preserved brain. He could return to 1985, although he opts not to, so his "time-projection" is genuine time travel. His experience is consistent with the Time Traveller’s theory that it is our "consciousness" or "mental existences", not our bodies, that move along time. Wald adds:
"And the consciousness of Robin Weinbaum is moving along that entity in that invisible direction." (45)
There are problems with this theory. First, it contradicts the Time Traveller’s practice, though not Wald’s. Secondly, the Time Traveller describes "mental existences" not, e. g., as "fields" but as immaterial and dimensionless which suggests that they are undetectable and even non-existent. (47) Thirdly, as before, motion "along the Time-Dimension with a uniform velocity" would take time so this dimension would not be time. (47) Fourthly, the theory suggests that, before or after the passage through human bodies of their minds, these bodies are mere automata although indistinguishable from conscious organisms.
This radical mind-body dualism leads to solipsism. Each of us has no way of knowing whether other organisms are really conscious. My neighbour’s mind, if he has one, may move along the fourth dimension more slowly or quickly than mine. Thus, it may not be present in his body when I am speaking to him. Indeed, it is unnecessary:
"He is not free in any way to change the shaping of the ultimate creature; all he can do is observe…" (45)
"…the shaping of the ultimate creature…" includes the movements of his own body and even of its vocal organs. I remember Blish suggesting in a conversation at a science fiction convention that minds might move along time at different rates but this is surely an absurd conclusion to arrive at. If the conclusion is valid, then I cannot be sure that Blish’s consciousness was present when it was suggested.
Although I have tried to distinguish the literature from the logic, Wells in particular presents both. His introductory conversation, imitated by C. S. Lewis in "The Dark Tower" (although Lewis argues against time travel and for time viewing) is in the literary-philosophical tradition of Plato’s dialogues. (48) We can only do justice to The Time Machine by simultaneously appreciating it as fiction and arguing with its philosophy.
Returning to Blish, one message from the future comes from a ship that is somehow travelling physically backwards in time, from 8873 to 8704, but the recipients of the message do not know enough about the context to understand this. (49) Blish had hoped to base a time travel novel on the "finite, spinning universe" theory that allows world-lines to curve backwards through space-time. Perhaps the "world-line cruiser" mentioned in The Quincunx of Time was an early intimation of this.
Wells beautifully describes the fast forwarding universe:
"…night followed day like the flapping of a dark wing…the palpitation of night and day merged into one continuous greyness…the jerking sun became a streak of fire, a brilliant arch…The landscape was misty and vague…I saw trees growing and changing like puffs of vapour…I saw huge buildings rise up faint and fair, and pass like dreams…I saw great and splendid architecture rising about me, more massive than any buildings of our own time, and yet, as it seemed, built of glimmer and mist." (50)
Anderson partially matches this with his character Jack Havig in There Will Be Time:
" ‘I’m in a shadow world while I time-travel. Lighting varies from zero to gray. If I’m crossing more than one day-and-night period, it flickers. Objects look dim, foggy, flat.’ " (51)
But Havig wills himself backwards or forwards in time without needing a vehicle. He senses resistance, as if swimming against a high tide. Thus, his experience is more like a form of motion than is the Time Traveller’s. However, a four dimensional observer, with sufficiently keen perception, would see not Havig’s three dimensional body moving along the fourth dimension but his elongated, and attenuated, world-line stretching along it. Even if the attenuated world-lines of Havig and his fellow time travellers are undetectable, they must be statically present throughout space-time.
Unlike the Time Traveller or Havig, Martin Saunders sees only featureless greyness through the porthole of his time projector while it fast forwards the rest of the universe.
A Time Patroller on a timecycle experiences an instantaneous jump to a different set of spatiotemporal co-ordinates (although where does the energy for this come from?). The TARDIS is also a space-time vehicle, not just a time machine, but it and its occupants exist for a short period in another realm, the "time vortex", between disappearing from one place and time and appearing at another. Heinlein’s Bob Wilson steps instantaneously through a disk of nothingness called a Time Gate that he projects from the future after stepping through it.
Jack Finney’s Simon Morley enters earlier decades by self-hypnosis while wearing period costume in unchanged locations like old buildings. Finney’s fictitious physicist, Danziger, theorises that this is possible because, according to Einstein, all times are real and the past still exists. However, Einstein’s space-time is the rigid four-dimensional continuum already mentioned. Its earlier decades are not only years ago in time but also light years away in space, not somehow just around the corner or out of sight. Finney’s Time novels would have had a sounder theoretical basis if Danziger, like Blish’s Adolph Haertel (Wald’s predecessor) had been presented as superseding, not implementing, Einstein’s ideas. Morley’s self-hypnosis gets him into the researched past and back to the remembered present but cannot get him into the unknowable future. However, Finney’s sf is nostalgic, not anticipatory, so the future is of no interest to him.
In Bring the Jubilee by Ward Moore, The Door into Summer by Robert Heinlein, "Wildcat" by Poul Anderson, "The Little Monster" by Poul Anderson, "The Face in the Photo" by Jack Finney and "The City That Was The World" by James Blish, a machine sends a character to another time but does not accompany him. (52) (53) (54) In "A Stitch in Time" by John Wyndham, a machine in the present lifts a character from the past, then puts him back.
In "Singularities Make Me Nervous" by Larry Niven and The Avatar by Poul Anderson, space travellers become time travellers by rotating around massive gravitational fields but Niven’s field is a black hole whereas Anderson’s is generated by a large artefact called a T machine, named after its theoretical inventor, F. J. Tipler. (55) (56) In "Rotating Cylinders and the Possibility of Global Causality Violation" by Larry Niven, named after a Tipler paper, some disaster always prevents the completion of a T machine because its completion in a single, continuous timeline would lead to the logical contradiction of causality violation. (57) A way to destroy your enemies is to let them steal the secret of T machines, unless a nova kills you first.
Olaf Stapledon’s Last Men and the characters in "The Long Remembering" by Poul Anderson observe past events through the brains and nervous systems of previous generations. (58) (59) The Last Men sometimes influence past minds, for example by inspiring Stapledon to write what he thinks is fiction.
In "Welcome" by Poul Anderson, Barlow moves forward five hundred years in a superenergy state. In "Time Heals" by Poul Anderson, Hart moves forward almost nine hundred years in a low-entropy field. By my original criterion, these cases are not time travel because they are not reversible but Anderson does not need reversible techniques in stories that are only about what it would be like to get into the future.
In The Corridors of Time by Poul Anderson, warring factions called Wardens and Rangers walk or drive along corridors that have been rotated onto the temporal axis. Before entering a corridor, time travellers leaving different periods are separated by temporal intervals. After entering the corridor, they are, theoretically, separated only by spatial distances. Some will be near enough to see each other. Others will pass when moving along the corridor. Therefore, they should encounter and interact with each other and with their later and older selves on their first attempt to use the corridor.
However, such an outcome would complicate the story uncontrollably, especially since the factions are at war. An instant pitched battle inside the corridor would ensure that most individuals did not survive their first attempt to travel along the corridor and therefore never did re-enter it as older versions of themselves. Usually, however, the characters use the corridors without meeting each other. The only explanation given is:
"Duration occurs there too, but on a different plane…" (60)
The twentieth century protagonist, Lockridge, standing in a corridor with a companion, thinks:
"At any moment, someone might enter through some other gate and spy them. (Just what did that mean, here in this time which ran outside of time? He’d think about it later.)" (61)
If he does think about it, he does not do so in the novel. When Lockridge, pursued by Rangers, enters one of the corridors, his pursuers arrive in the corridor not a short distance away from him but a few moments after him. Thus, the order of events in the time "on a different plane" in the corridor follows the order of events in the familiar time outside the corridor. Again, this is convenient for story telling purposes. Lockridge would have been apprehended if his pursuers had arrived simultaneously with him.
If, in Lockridge’s experience, his first journey along a particular corridor starts from the twentieth century and his second journey along the same corridor starts from the fourteenth, which of the journeys will an observer inside the corridor witness first? We must imagine not an observer moving steadily along the corridor from the past towards the future but instead a stationary observer able to perceive the entire length of the corridor simultaneously.
As in the Time Patrol series, Anderson is good at writing his way around potential complications so that the reader is rarely troubled by them. The question in the preceding paragraph occurred to me only when writing about The Corridors of Time. Another potential problem arises when we are told that:
"Emergence cannot be precise, because the human body has a finite width equivalent to a couple of months. That was why we had to hold hands coming through – so we would not be separated by weeks." (60)
The Einsteinian space-time equivalence is not one body width to two months or "thirty-five days per foot" but one hundred and eighty six thousand miles to one second. (62)
Thor Wald, explains:
"Assuming, to keep the figures simple, that Robin lives to be a century old, he would then be roughly a foot thick, two feet wide, five feet five inches in height, and five hundred and eighty-six trillion, five hundred and sixty-nine billion, six hundred million miles in duration." (45)
Thus, Anderson’s corridors that endure for six thousand years would have to extend for six thousand light years in space before they were rotated into time but, in this novel, Anderson avoids mentioning Einsteinian space-time. If space-time equivalence is valid, then the simplest theory is that one of our three space dimensions becomes a corridor’s time dimension. It should be possible to construct a spatial corridor that allows entry to any period of the internal history of one of the corridors. This is another complication that Anderson does not need for the story that he wants to tell so it is not mentioned.
Time Patrol timecycles resemble the Time Machine. Jack Havig’s time travelling resembles the Time Traveller’s. The equation of time with length in The Corridors of Time recalls the discussion at the beginning of The Time Machine. Time travel to historical periods is not Wellsian (such a passage was cut from The Time Machine) but does continue the tradition of Twain and de Camp.
Anderson presents more varied and more imaginative means of time travel than anyone else. In particular, the Time Patrol and There Will Be Time are opposites: instantaneous vehicular space-time travel with causality violation as against lifespan-consuming psychic time travel without causality violation. Some means are mere formulaic premises for short stories but the temporal corridors and Havig’s power are original ideas and major features of the novels in which they appear.

Limits on Time Travel

Limits prevent paradoxes. For example, if time travel is limited to the remote past, then time travellers cannot intervene in recent history. If time travel is possible only in interstellar spaceships, then an individual inventor cannot make a machine that will take him to yesterday and return him to today while remaining in his laboratory. Thus, again, he cannot easily intervene in recent events.
The imprecision of emergence from a time corridor is one such limit. A time traveller who sets out to change or prevent a known event will invariably arrive after the event, if he encounters no mishaps en route. Even travelling pastwards to warn a colleague of an impending danger may be impossible:
"Such as could be sent have doubtless been baffled by the uncertainty factor; they emerged too early or too late." (63)
Time travellers know of some events in their own futures but not others:
"Lockridge was certain to reach Brann…That fact was in the structure of the universe. "However, details were unknown. (Like the aftermath…Did he or did he not get back alive? The margin of error in a gate made it unfeasible to check that in advance.)" (64)
Anderson imagines limits, then invents ways around them. In "Flight to Forever", Saunders travels from 1973 to 2073, then finds that he would need infinite energy to travel back more than about seventy five years but returns to 1973 by moving forwards around the circle of time.
In "Wildcat", it is only possible to travel forwards in hundred year hops and backwards by about 100,000,000 years. Therefore, it seems to be impossible to get information from the near future. However, when Team A, visiting the twenty first century, finds that Earth was sterilised by a nuclear war a year after the twentieth century base date, that Team places this data where it will be found by Team B which visits 100,000,000 A. D. and returns home. Team A joins a Jurassic base a century after its establishment. It is known that life will be re-evolving in 100,000,000 A. D. and hoped that descendants of the Jurassic settlers built spaceships.
In There Will Be Time, a few individuals from different periods can time travel by an act of will. They need no spatial journey or temporal vehicle. There seems to be no limit to their time travel ability. However, there are practical limitations. They can carry only a few pounds, including clothes. They are immobilised if attached to a larger mass (seized by an opponent, handcuffed to a captor or chained to a wall). They cannot breathe while travelling. (The Time Traveller did not encounter this difficulty. Perhaps the Time Machine carried some air along with it.) When ice covers the Earth’s surface for extended periods, unwary time travellers cannot emerge and risk suffocation. When the Earth is covered with water, they can emerge but only to drown. Jack Havig’s group carries miniature oxygen tanks when necessary but not all time travellers are so well equipped or organised.
A time traveller in There Will Be Time soon loses count of days flickering past, especially over centuries or millennia and when storms or buildings conceal the dawns. Casting about to zero in on the target date consumes lifespan. Havig’s "chronolog", small and light enough to be carried through time, detects heavenly bodies even through an overcast and flashes a light at his destination. Another time travel group, planning their return journey from the day of the Crucifixion but lacking a chronolog, erects, in the twenty first century ruins of Jerusalem, a big billboard with the date corrected daily.
The dates of some important events are not known. Anyone seeking the Crucifixion aims for Passover, 33 A. D., whether or not this is accurate, but they first have to travel to Jerusalem by ordinary means which did not include air travel until the twentieth century.
Despite the limits to his power, it would seem to be easy for Havig to change the past. The simplest experiment would be this: the time traveller remains in a room for two minutes; if his older self has not appeared after one minute, he travel pastwards for one minute; if he has appeared, he does not travel pastwards. A simple way to intervene in public events would be if he set out to appear in front of a television news reader on the previous day. However, Havig’s first attempt at causality violation is not a carefully planned experiment but an expedition to prevent his father’s death. He is prevented from setting out first by an accidental injury, then by business responsibilities, then by family commitments.
Eventually, like other time travellers, he stops trying. When a captured enemy escapes, they do not waste their efforts in trying to prevent the escape, which has happened, but attempt an early recapture, which may occur in their immediate future. Therefore, most events are as they are either because no time traveller wants to change them or because, knowing that some accident, possibly a fatal injury, will prevent any attempted change, they do not try. In these cases, there need not be any accident waiting to happen to interfering time travellers. The fact that time travellers do not attempt to change an event sufficiently explains why the event is not changed.
If an event occurs, then we know either that no one has tried to prevent it or that someone has tried and failed, whether or not the someone in question is a time traveller. But it is difficult to imagine what could thwart the "simplest experiment" suggested above, especially if it were attempted simultaneously in different places by a team of hundreds of experimentally minded time travellers. Would they all have heart attacks or be struck by lightning or meteors immediately before starting to travel into the past? Must we choose between a logical contradiction and a statistical impossibility?
(Added on 1 April 2007: Thanks to Colin G. Mackay for pointing out that there is no such thing as a "statistical impossibility". My argument should be re-phrased: Must we choose between a logical contradiction and a statistical improbability?)
I can think of only two explanations: either there are no time travellers (this may be the world that we live in) or there are very few and none of them is inclined to conduct this kind of experiment. In There Will Be Time, the group called the Eyrie is intent on exercising power and will do nothing to risk losing its power. Its members are unscientific and even superstitious. Havig’s rival group is morally responsible, intent only on overthrowing the Eyrie and then on doing something useful with time travel (although I still think that a few experiments would be appropriate).
Although Havig cannot change known events, he can sometimes change their significance. This starts on the day of the Crucifixion. Havig goes there hoping to meet other time travellers. The Eyrie goes there to recruit. Thus, for Havig and the Eyrie, Crucifixion day is merely a convenient rendezvous point. On that day, one of Havig’s fellow recruits is called Boris. Later, having defected from the Eyrie and formed a rival group, Havig will recruit Boris to that group, then send him to Jerusalem to infiltrate the Eyrie. Thus, we later learn, Boris must have recognized Havig on the day of their recruitment to the Eyrie but feigned ignorance. Thus also, Havig cannot prevent the Eyrie from recruiting Boris but can cause it to recruit him in circumstances that favour Havig’s cause, not the Eyrie’s.
The Sachem of the Eyrie cannot prevent the Maurai Federation from dominating international politics for three or four centuries after the nuclear war but does found and guide an American-based nation that he hopes will supplant the Maurai and restore white supremacy. He expects to succeed because he periodically visits the future Eyrie and sees that it prospers. He does not know that Havig’s group will defeat the Eyrie and fake the evidence of its success.
When an Eyrie recruiter prevents a thirteenth century time traveller from trying to rescue the Saviour en route to Golgotha, Anderson adds that a Roman soldier would have killed the would-be rescuer a second later. This addition is unnecessary. Eyrie intervention is sufficient to explain why the Passion was not interrupted. This would, of course, involve a causal circle: the Eyrie would be able to recruit on the day of the Crucifixion because, while recruiting there, they ensured that the event occurred as described in the Gospels.
Of course, the crucifixion victim whom the time traveller had tried to rescue might not have been the historical Jesus. The Eyrie recruiter asks:
"How do you know that person really was your Lord?" (65)
I took this question to be philosophical or metaphysical but it may be simply historical. There were a lot of men called Jesus and a lot of crucifixion victims.

What would it be like to visit the past?

Any serious account of travel to the past raises this question. L. Sprague de Camp in Lest Darkness Fall and Tim Powers in The Anubis Gates evoke the experiences of modern men having to survive in earlier periods. The single most sustained answer is given by Jack Finney who perfected nostalgic time travel focused on turn of the century New York. His characters immerse themselves in images and records of an earlier year, then experience that year directly. They walk along streets that they have seen in old photographs.
However, Poul Anderson vividly describes the sights, sounds and smells of many historical periods. When Everard enters the Pasargadae of Cyrus the Great, we hear the street cries. (66) When he enters "Tyre of the purple" in 950 B. C., we see the docks and "the costly colors". (67) When he enters Bactra in 209 B. C.:
"…the scene was eerily half-familiar. He had witnessed its like in a score of different lands, in as many different centuries. Each was unique, but a prehistorically ancient kinship vibrated in them all." (68)
When Everard travels on foot through medieval Sicily, four pages describe the countryside that he walks through. (69) When Havig enters Jerusalem in 33 A. D., Anderson lists the smells of horses, men, smoke, bread, leeks, garlic, grease, animal droppings, musk, roses, lumber and leather. (70) His time travellers move not through abstract time but through real history. Time Patrollers are immersed in ancient civilisations, Roman legions, medieval battlefields and barbarian settlements in European forests. Two Patrollers on a mission to the first century Low Countries camp on a hill surrounded by woods:
"They could have sprung back to Amsterdam’s comforts, but it would have wasted lifespan, not in the shuttling but in the commuting to and from quarters there, the shucking and redonning of barbarian garb, perhaps most the changes back and forth of mind-set. Let them rather dwell in this archaic land, become intimate not only with its people but with its natural world. Nature - the wilderness, the mysteries of day and night, summer and winter, storm, stars, growth, death – pervaded it and the souls of the folk. You could not really understand them, feel with them, until you had yourself entered into the forest and let it enter into you." (71)
A Patrol ethnographer who has lived for twelve years in the first century remarks, when visiting his colleagues’ camp:
"Coffee…I often drink it in my dreams." (72)
A Patroller staying with an Italian knight in 1146 A. D., must eat "…the usual meager, coffeeless breakfast." (73)
In some passages, Anderson follows the experiences not of the Patrollers but of their acquaintances in the past and thus writes pure historical fiction. When a Time Patroller, mistaken for Wodan, has a son in the fourth century:
"Dagobert stayed unrestful…folk said that was the blood of his father in him, and that he heard the wind at the edge of the world forever calling. When he came back from his trip south, he brought news that a Roman lord hight Constantine had finally put down his rivals and become master of the whole Empire." (74)
Chapters recounting only the experiences of permanent dwellers in the past often begin with the weather: "Winter brought rain…" (75); "Wind rushed bitter…" (76); "Suddenly springtime billowed over the land." (77)
The concluding two sections of "The Sorrow of Odin the Goth", and the opening three sections of the following story, "Star of the Sea", cover:

time travellers in 43 A. D.	science fiction
the death of a king in 374 A. D.	historical fiction
a divine betrothal	mythological writing
the siege of a first century Roman camp	historical fiction
a time traveller in 1986 A. D.	science fiction

One Time Patroller, in 1990, specifically rejects Finney’s nostalgic approach to time travel:
"The Midwest of his boyhood, before he went off to war in 1942, was like a dream, a world forever lost, already one with Troy and Carthage and the innocence of the Inuit. He had learned better than to return." (78)
Anderson writes poetically not only about past times but also about variable reality. Everard in Amsterdam in 1986 knows that "…all the life that pulsed in the city…" is:
"…a spectral flickering, diffraction rings across abstract, unstable space-time, a manifold brightness that at any instant could not only cease to be but cease ever having been." (79)
(Only the last clause is problematic. Causality violation in an earlier century might prevent Amsterdam from coming into existence but how could causality violation allow the city to exist until 1986, then annihilate it? Nothing can exist until a particular time, then, at that time, cease having existed until that time. The deletion of a four-dimensional space-time continuum might be an observable event in a fifth dimension functioning as a second temporal dimension but it cannot be an event within that same continuum. Analogously, if a book is thrown into a furnace, then it does not make sense to ask at what point in the text the entire text is destroyed. It is all destroyed together.)

What would it be like for a modern man to be transported to the remote future?

The classic answer remains The Time Machine. Wells describes his future as colourfully as Anderson and Finney describe their pasts. The modern man need not be transported by time travel specifically. Three successors to The Time Machine are "Flight to Forever" by Poul Anderson (time travel), A World Out Of Time by Larry Niven (time dilation) and Midsummer Century by James Blish ("time-projection"). (80) All four works not only describe a future period but also summarise a future history.
The Time Traveller fast forwards the growth of civilisation, deduces the devolution into Morlocks and Eloi and witnesses the extinction of life before returning to the nineteenth century. Martin Saunders in "Flight to Forever" stops often enough to learn the course of galactic history before passing through heat death and re-creation into the twentieth century of the next identical cosmic cycle. In A World Out Of Time, Jerome Corbell’s identity survives for three million years by freezing, memory transfer, time dilation, cold sleep and rejuvenation. He reconstructs most of what has occurred in the Solar System in his absence. In Midsummer Century, when Martels’ consciousness has been projected from 1985 to 25,000, he learns of four successive future civilisations and helps to build a fifth.

Literature and Genre

A Connecticut Yankee in King Arthur’s Court is an American fantasy about a modern entrepreneur mysteriously transported to a historical period. The Time Machine is a British science fiction novel about a modern scientist technologically travelling to a remote future. They were the first major works on time travel. Like the first science fiction novel, Frankenstein by Mary Shelley, they predate the literary ghetto of genre science fiction and are generally recognised as contributions to world literature. (81)
In passing from Twain and Wells to de Camp and Heinlein, we pass not only from the original ideas to their fuller development but also from literature to genre. The genre includes:

action-adventure fiction with extraterrestrial, futuristic or other exotic settings;
conceptual speculation, for example about the technology and paradoxes of time travel;
serious fiction, for example about the predicaments of travellers to the past or future, the literary successors of the Yankee and the Time Traveller.
To illustrate this third point:
"It was a strange thing to meet her at intervals of months which for Havig were hours or days. Each time, she was so dizzyingly grown. In awe he felt a sense of that measureless river which he could swim but on which she could only be carried from darkness to darkness." (82)
In that paragraph, Anderson expresses universal experience, "…carried from darkness to darkness", and Wellsian imagination, "…that measureless river which he could swim…"

The Time Machine, originally described as a "fantastic and imaginative romance", combines conceptual speculation with serious fiction while incorporating action-adventure in the fights with the Morlocks and the encounter with giant crabs. Much genre sf emphasises action-adventure and/or speculation at the expense of characterisation and even of competent writing but other genre writers are worthy successors of Shelley, Twain and Wells.
In particular, Anderson’s sf ranges from effective action-adventure fiction to serious thematic and speculative novels, sometimes featuring the same characters. Dominic Flandry stars in mere space operas with the colourful setting of the Terran Empire, then in more substantial novels about imperial decline. Another reader described Anderson to me as a popular writer with literary pretensions. I think that it is the other way around. Anderson was a serious writer who enjoyed action-adventure so much that he sometimes introduced it into stories that would have been better without it. Thus, The Boat of a Million Years has an unnecessary few pages when the small group of immortals practises a deception in order to rescue one of its members from a hospital. (83) This could have happened off stage, if it even had to happen.
By contrast, the Time Patrol police actions and battle scenes, when they do occur, are integral to the narrative and secondary to the descriptions of past eras and the predicaments of the characters. The Time Patrol places technological time travel in historical settings, thus synthesising the legacies of The Time Machine and A Connecticut Yankee. Uniquely, the Time Patrol is not a single story or novel about an individual time traveller experiencing one of the causality paradoxes in a particular historical period but a series of stories and novels about an organisation of time travellers experiencing both causality paradoxes in many historical periods. Because it is a long sf series set in many historical periods, the Time Patrol is a more effective synthesis of sf with historical fiction than is Asimov’s celebrated Foundation series which merely projects the Roman Empire onto the Galaxy.
By contrast, Anderson:
projects the Roman Empire less implausibly onto a smaller volume of interstellar space and a shorter period of time in one of his eight future histories;
speculates more credibly about interstellar travel in later future histories;
shows the real Roman Empire in The Golden Slave (historical fiction), The King Of Ys tetralogy (historical fantasy, with Karen Anderson) and the Time Patrol series (historical sf); (84) (85)
depicts the Viking period in a historical trilogy and in several fantasies, including a retold Saga; (86) (87)
depicts the fourteenth century in historical fiction, historical fantasy and sf; (88) (89) (90)
also synthesizes sf with historical fiction in a long novel about an immortal man who lives from before Christ into an indefinite future, thus experiences but cannot revisit historical periods (his single volume could have been two, a fictitious history with a futuristic sequel); (91)
depicts alternative histories in the Time Patrol series and in several short stories and fantasy novels; (92) (93) (95) (95)
imagines an inn between the worlds where characters from different periods, histories and fictions meet, although the Time Patrol excludes itself even from this multiverse by disallowing the peaceful coexistence of alternative timelines. (96)
Jules Verne’s fiction is collectively called "Extraordinary Voyages". Anderson’s could be called "Past and Future Histories".
Heinlein linked his ingenious time travel fiction to his Future History when a major character of the History, Lazarus Long, revisited the period of his own childhood in the early twentieth century. (97) Unfortunately, Heinlein wasted this excellent premise by focusing on a sentimentalised sexual relationship with the added shock value of incest: Long with his, at that time unknowing, mother. Asimov linked his logically inconsistent causality violation novel, The End of Eternity, to his Galactic Empire series when the prevention of time travel allowed the development of the Empire. See here. (98) Anderson linked his ingenious circular causality novel, There Will Be Time, to his Maurai future history when his time traveller, Jack Havig, visited the Maurai period.
In the Time Patrol series, we read of the Temporal language which enables Patrollers from different periods to communicate without being understood by others and which has tenses for time travel. In There Will Be Time, time travel is discussed, briefly, in Latin: "Es tu peregrinator temporis?" (65)
What Anderson presents is both more and better. His Time Patrol, originally four stories, collected and re-arranged to present a beginning, a middle and a culmination, was already a major contribution to time travel fiction. Then, over a decade later, further stories, usually longer and more complicated and making original contributions to the time travel concept, began to be added so that the series is now several times its original length with possibilities for further growth although, sadly, Anderson is no longer alive to continue it. It is unique in time travel fiction and superior among sf series. Whereas other writers develop particular aspects of the time travel concept – historical veracity, futuristic speculation, the technology or one of the paradoxes – Anderson alone addresses every aspect.

End-notes

1. Anonymous, "Missing One’s Coach" (Dublin: Dublin Literary Magazine, 1838).
2. Mark Twain, A Connecticut Yankee in King Arthur’s Court, 1889.
3. H. G. Wells, The Time Machine (London: Pan Books, 1973), p. 11.
4. Wells, The Time Machine, p. 17.
5. L. Sprague de Camp, Lest Darkness Fall (Henry Holt & co, 1941) reprinted as Lest Darkness Fall (New York: Pyramid Books, 1963), pp. 5-6.
6. Ward Moore, Bring the Jubilee (London: Four Square Books, The New English Library, 1965).
7. Poul Anderson, "Time Patrol" (The Magazine of Fantasy and Science Fiction, May 1955), reprinted in P. Anderson, Guardians of Time (London: Gollancz, 1961), (London: Pan Books, 1964) and P. Anderson, The Time Patrol (New York: TOR Books, October 1991), pp. 1-33).
8. Poul Anderson, "Delenda Est" (The Magazine of Fantasy and Science Fiction, December 1955), reprinted in Anderson, Guardians of Time and The Time Patrol, pp. 105-139.
9. Poul Anderson, "Year of the Ransom" (Byron Press Visual Publications, 1988), reprinted in Anderson, The Time Patrol, pp. 399-458.
10. Poul Anderson, "Women and Horses and Power and War" in The Shield of Time (New York: TOR Books, 1990), pp.7-123.
11. Anderson, "Amazement of the World" in The Shield of Time, pp. 267-436.
12. Anderson, "Amazement of the World", p. 408.
13. Jack Finney, Time and Again (New York: Simon & Schuster Inc, 1970) and From Time to Time (New York: Simon & Schuster Inc, 1995).
14. Poul Anderson, "The Sorrow of Odin the Goth" in P. Anderson, Time Patrolman (New York, TOR Books, 1983), pp. 117-254, reprinted in The Time Patrol, pp. 207-289.
15. Poul Anderson, "The Man Who Came Early" (Fantasy House Inc, 1956), reprinted in D. Knight, ed., 100 Years of Science Fiction (London: Pan Books, 1972), pp. 185-212.
16. Poul Anderson, "The Little Monster" (Roger Elwood, ed., Way Out, 1974), reprinted in P. Anderson, Past Times (New York: TOR Books, 1984), pp. 142-163.
17. Anderson, "Delenda Est", p. 118.
18. Poul Anderson, "Welcome" (The Magazine of Fantasy and Science Fiction, 1958), reprinted in P. Anderson, Past Times, pp. 58-70.
19. Poul Anderson, "Time Heals" (Astounding Science Fiction, October 1949), reprinted in P. Anderson, Dialogue with Darkness (New York: TOR Books, February 1985), pp. 165-191.
20. Robert Heinlein, "By His Bootstraps" (Astounding Science Fiction, October 1941, by Anson MacDonald), reprinted in R. Heinlein, The Menace From Earth (Gnome Press, 1959) and (London: Corgi Books, 1983), pp. 40-87.
21. Poul Anderson, "Flight to Forever", (Super Science Stories, 1950), reprinted in P. Anderson, Past Times, pp. 207-288.
22. H. G. Wells, "the Chronic Argonauts" (The Science Schools Journal, 1888).
23. John Wyndham, "A Stitch in Time" in Consider Her Ways (London: Penguin Books, 1961).
24. John Wyndham, "The Chronoclasm" (Science Fantasy, No. 10, 1954), reprinted in J. Wyndham, The Seeds of Time (London: Penguin Books, 1959), pp. 9-31.
25. Robert Heinlein, The Door into Summer ( The Magazine of Fantasy and Science Fiction, October, November and December 1956) reprinted as The Door into Summer (London: Pan Books, 1974).
26. Robert Heinlein, "-All You Zombies-" (The Magazine of Fantasy and Science Fiction, March 1959), reprinted in The Unpleasant Profession of Jonathan Hoag (Gnome Press, 1959) and (London: New English Library, November, 1980), pp. 7-111.
27. Damon Knight, Beyond the Barrier (London: Victor Gollancz Ltd, 1964).
28. Harry Harrison, The Technicolor Time Machine (London: Faber and Faber Ltd, 1968).
29. Tim Powers, The Anubis Gates (London: Triad Grafton, 1986).
30. Poul Anderson, The Corridors of Time (Amazing Stories, May, June 1965), reprinted and expanded as The Corridors of Time (London: Victor Gollancz Ltd, 1966) and (London: Panther Books, 1968).
31. Poul Anderson, There Will Be Time (New York: Signet, 1973).
32. Poul Anderson, The Dancer from Atlantis (London: Sphere Books Ltd, 1977).
33. Poul Anderson, "Brave To Be A King" (The Magazine of Fantasy and Science Fiction, August 1955), reprinted in Anderson, Guardians of Time and The Time Patrol, pp. 34-68.
34. Poul Anderson, "The Only Game in Town" (The Magazine of Fantasy and Science Fiction, January 1960), reprinted in Anderson, Guardians of Time and The Time Patrol, pp. 79-104.
35. Poul Anderson, "Star of the Sea" in The Time Patrol, pp. 291-398.
36. Anderson, "Time Patrol", p. 11.
37. Anderson, "Ivory, and Apes, and Peacocks" in Time Patrolman and The Time Patrol, pp. 141-205.
38. Audrey Niffenegger, The Time Traveler’s Wife (London: Jonathan Cape, 2004; London: Vintage, 2005).
39. H. G. Wells, "The New Accelerator" (The Strand, December 1901) reprinted in The Complete Short Stories of H. G. Wells (London: A & C Black, 1987), p. 932.
40. Wells, Complete Short Stories, p. 937.
41. Wells, Complete Short Stories, p. 941.
42. Wells, The Time Machine, p. 24.
43. Wells, The Time Machine, p. 16.
44. Wells, The Time Machine, p. 26.
45. James Blish, The Quincunx of Time (New York: Dell, October 1973), pp. 115-116.
46. James Blish, Midsummer Century (The Magazine Of Fantasy and Science, April 1972) and (New York: Doubleday, 1972).
47. Wells, The Time Machine, p.10.
48. C. S. Lewis, "The Dark Tower" in The Dark Tower and other stories, (London: Fount Paperbacks, 1983), pp. 15-91.
49. Blish, The Quincunx of Time, p. 96.
50. Wells, The Time Machine, pp. 24-25.
51. Anderson, There Will Be Time, p. 37.
52. Poul Anderson, "Wildcat" (The Magazine of Fantasy and Science Fiction, 1958), reprinted in P. Anderson, Past Times (New York: TOR Books, 1984), pp. 7-57.
53. Jack Finney, "The Face in the Photo" in I Love Galesburg in the Springtime (New York: Simon & Schuster, 1962) and About Time (New York: Simon & Schuster, 1986), pp. 130-147.
54. James Blish, "The City That Was The World" (Galaxy Magazine, July 1969), pp. 69-97.
55. Larry Niven, "Singularities Make Me Nervous" in L. Niven, Convergent Series (New York: Ballantine Books, March 1979)
56. Poul Anderson, The Avatar (London: Sphere Books, 1985).
57. Larry Niven, "Rotating Cylinders and the Possibility of Global Causality Violation" in Niven, Convergent Series, pp. 183-187.
58. Olaf Stapledon, Last and First Men (London: Penguin Books, October 1930), reprinted June 1937.
59. Poul Anderson, "The Long Remembering" (The Magazine of Fantasy and Science Fiction, November 1957), reprinted in P. Anderson, Homeward and Beyond (New York: Berkley Publishing Corporation, July 1976), pp. 22-36.
60. Anderson, The Corridors of Time, p.
61. Anderson, The Corridors of Time, p. 88.
62. Anderson, The Corridors of Time, p. 35.
63. Anderson, The Corridors of Time, p. 151.
64. Anderson, The Corridors of Time, p. 132.
65. Anderson, There Will Be Time, p. 62.
66. Anderson, "Brave To Be A King", p. 42.
67. Anderson, "Ivory, and Apes, and Peacocks", pp. 143-145.
68. Anderson, "Women and Horses and Power and War", p. 24.
69. Anderson, " Amazement of the World", pp. 318-322.
70. Anderson, There Will Be Time, p. 61.
71. Anderson, "Star of the Sea", pp. 336-337.
72. Anderson, "Star of the Sea", p. 351.
73. Anderson, "Amazement of the World", p. 419.
74. Anderson, "The Sorrow of Odin the Goth", p. 239.
75. Anderson, "Star of the Sea", p. 309.
76. Anderson, "Star of the Sea", p. 323.
77. Anderson, "Star of the Sea", p. 331.
78. Anderson, "Beringia" in The Shield of Time, p. 178.
79. Anderson, "Star of the Sea", p. 301.
80. Larry Niven, A World out of Time (New York: Ballantine Books, 1976).
81. Mary Shelley, Frankenstein or The Modern Prometheus (New York: Airmont Books, 1963).
82. Anderson, There Will Be Time, p. 98.
83. Poul Anderson, The Boat of a Million Years (London: Orbit Books, 1991), pp. 439-445.
84. Poul Anderson, The Golden Slave (New York: Kensington Publishing Corp., 1960, 1980).
85. Poul and Karen Anderson, The King of Ys (London: Grafton Books, Volume 1, 1988; Vol 2, 1988; Vol 3, 1989; Vol 4, 1989).
86. Poul Anderson, The Last Viking Trilogy (New York: Kensington Publishing Corp., 1980).
87. Poul Anderson, Rogue Sword (New York: Kensington Publishing Corp., 1960).
88. Poul Anderson, Hrolf Kraki’s Saga (New York: Ballantine Books, 1973).
89. Poul Anderson, The Merman’s Children (London: Sphere Books, 1981).
90. Poul Anderson, The High Crusade (New York: MacFadden Books, June 1964; September 1968).
91. Anderson, The Boat of a Million Years.
92. Poul Anderson, "The House of Sorrows" in P. Anderson, All One Universe (New York: TOR Books, My 1997), pp. 69-98.
93. Poul Anderson, "Eutopia" in Past Times, pp. 112-141.
94. Poul Anderson, Three Hearts and Three Lions (London: Sphere Books, 1973, 1977).
95. Poul Anderson, A Midsummer Tempest (London: Futura Publications Ltd, 1975).
96. Poul Anderson, "Losers' Night" in All One Universe, pp. 105-123.
97. Robert Heinlein, Time Enough For Love (New York: G. P. Putman’s Sons, 1973), pp. 450-589.
98. Isaac Asimov, The End Of Eternity (New York: Doubleday, 1955) and (London: Panther Books, January 1959; April 1964; November 1965).

Email address: paulshackley@gmail.com

The Logic of Time Travel: Part I

  (Note added 8 Oct 2020: Diagrams have disappeared from this post and I do not know how to restore them.) (10 May 2024: The diagrams are back, more or less.)

The basic concept of a time traveller is of someone who disappears and either reappears later or has appeared earlier. Disappearance is departure from the present. Later reappearance is arrival in the future. Earlier appearance would be arrival in the past. For story purposes, disappearance and (re)appearance are preferable to visible, tangible continuity between departure and arrival because they enable the traveller to arrive unexpectedly in the future or in the present when returning from the past. He also arrives either unaged and, in his own experience, instantaneously, or undergoes significantly reduced aging and duration. Otherwise he would merely endure time, like everyone else, not ‘travel through’ it.

Merely futurewards time travel would be merely invisible time dilation or temporal stasis. Therefore, time travel as such requires at least the possibility of pastwards travel, which includes returning to the present from the future. Anyone who arrives in the future but cannot return to the present is not a time traveller. Supernatural or magical time travel is fantasy whereas scientific or technological time travel is science fiction (sf).


Travel into the past reverses the order of cause and effect and therefore creates the causality paradoxes. When appearance or arrival precedes disappearance or departure, then an effect precedes its cause and might therefore cause it (this would be the circular causality paradox) or even prevent it (this would be the causality violation paradox). Causality violation is also called ‘changing the past’ because the cause can be prevented only by altering the course of preceding events. Altering events here means making them different from what they would have been if they had produced the cause and also different from the time traveller’s memories of them. In this sense, violating causality involves changing the past. Similarly, any changing of the past involves violating causality. For example, a time traveller who:

arrives in the 1930’s;
remembers having departed from the 1990’s with the intention of preventing the outbreak of World War II
and succeeds in preventing World War II;
thereby prevents his younger self from departing the 1990’s with the intention of preventing World War II.

In fact, in this case, he changes the course of so many lives that he almost certainly also prevents his own birth. Thus, his arrival in the 1930’s, and even his very existence, are, apparently, uncaused.


A paradox is either an actually or only an apparently self-contradictory proposition. Causality violation is on the face of it, actually contradictory: the cause occurs, because it produces its effect, and does not occur, because it is prevented by its effect.


There are several possible solutions, which are discussed further below. One involves divergent timelines. A timeline is a single, chronologically linear, sequence of events like familiar twentieth century history in which particular events occur on particular dates and only on those dates:


1914 beginning of World War I
1917 Russian Revolution
1918 end of World War I
1939 beginning of World War II
1945 end of World War II
1963 assassination of Kennedy
1969 first moon-landing
1991 end of Cold War, etc.


A sequence of events in which World War I occurred earlier or later or not at all or had a different outcome etc would be a different timeline. A sequence of events that was identical with our history until 1939 but then differed because, e.g., World War II did not occur could be described, from 1939 onwards, as a diverging timeline. Divergent timelines enter time travel fiction as a possible solution to the causality violation paradox because it can be argued that the cause occurs in an original timeline but is prevented in a divergent one. However, alternative and multiple timelines can also be considered in their own right simply as alternative versions of historical periods or of fictitious characters’ lives. (Stories simply about alternative timelines usually involve sideways travel between parallel universes in present time with no backwards or forwards time travel and thus are not time travel stories.) The questions of whether there can be more than one timeline and, if so, how they are related are considered further below while trying to solve the causality violation paradox.

Possible Solutions to the Causality Violation Paradox

1. The Logical Impossibility of Causality Violation in a Single Timeline
 Hitler was not killed or kidnapped in childhood. Therefore, either no-one tried to do this or someone tried and failed, even if we do not know the precise reason for failure: accident, apprehension, arrest etc and even if the person who tried had foreknowledge of Hitler’s career and had wanted to prevent it. This remains true even if the source of foreknowledge was time travel from Hitler’s future. In fact, introducing time travel increases the number of possible reasons for failure. It would not be easy for a time traveller from a later period to find the best time and place at which to intervene in order to interrupt Hitler’s early life, which might even be protected by pro-Nazi time travellers. Therefore, causality violation is logically impossible because it cannot be true that something happened when it is already known that it did not happen. More specifically, it cannot be true that someone performed a task when it is already known that it was not performed, thus that, if anyone tried, they failed.


Unfortunately, if time travel became common, then the number of adverse circumstances and accidents necessary to prevent causality violation would become statistically impossible. Therefore, there has to be some reason either why time travel does not occur (e.g. because it is impossible or because it is so difficult that no-one ever finds out how to do it in the whole history of the universe) or why its use is extremely limited (e.g. because it requires too much energy or because the theory is valid which stipulates that any time journey would also have to be a long space journey, thus precluding effortless disappearance and reappearance on Earth’s surface in recent historical periods).


(Added on 21 April 2006: Thanks to Colin G. Mackay for pointing out that there is no such thing as a "statistical impossibility". My argument should be re-phrased: Something must happen to prevent a logical contradiction but, by definition, it is extremely improbable that an extremely improbable event will always occur just in time to prevent any logical contradiction.)

2. The Immutability of Past Events

Past events, even when visited from the present, remain past and therefore cannot be affected by anyone from the present. Therefore, pastwards time travellers remain invisible, intangible and inaudible when in the past, not just when travelling to or through it. They are like ghosts from the future. Unfortunately, this is time-viewing, not time travel.

3. A Discontinuous Timeline

If causality is violated, then the prevented cause, e.g. the time traveller’s departure, simply does not occur, although it is ‘remembered’ by the appearing time traveller, and the effect, i.e. his appearance, is uncaused, therefore not really an effect. Events are discontinuous, as in quantum mechanics. This seems to be logically possible but is extremely paradoxical in the sense of apparently contradictory:

a traveller appearing now could be from the past, from the future or from no time;

 anyone leaving the present to kidnap Hitler will still fail because Hitler was not kidnapped but travellers from possible futures could prevent those futures;

some past events could have been caused by arbitrarily appearing time travellers that we do not know about;

 .c\xvvvba traveller arriving in what he regards as the past does not know whether he remembers a real future or a prevented one;

in fact, before leaving the present, a traveller knows that his memories are valid but, on arriving in the past, he does not know this!

Time travellers in this discontinuous timeline would almost certainly think in terms of divergent or multiple timelines even if time travel theory denied them.


The causal principle can be partly salvaged with the following diagram:

A

C

(B)

ABC is the usual causal order but here B is the time traveller’s departure and C is his arrival which prevents B. A is an earlier event, like the invention of a time machine, which would have caused B. Thus, A occurs and would have caused B which would have caused C which, however, occurs and prevents B. Thus, indirectly, A causes C. C would not have occurred without A although, to a casual observer, AC looks like a causality violation.

4. Divergent Timelines

If causality is violated, then a second timeline diverges from the original one:

Horizontal line =original timeline
Slanting line =divergent timeline
D =time traveller’s departure from his present
A =his arrival in the past
Questions:
Do the timelines diverge at the moment of the time traveller’s arrival?:

Or at some later moment when he definitely violates causality, e.g., by kidnapping Hitler?:

Sometimes, A would equal V:

e.g., if the time traveller appeared in front of the TV news reader the previous evening but, if he appeared unobserved in an otherwise empty room, would this change the past? Is the answer arbitrary: i.e., it would only change the past if we somehow knew that, in the original timeline, the room had remained empty? But why should the past not sometimes change without our knowledge?


In the scenario where the traveller can exist for a while in the past before violating causality:

two strange things happen at point V. First, to observers in the original timeline, the traveller simply disappears. Secondly, he creates around himself an entire new universe identical to the first except for any changes made by himself. How can this happen?

Having initiated and entered the new timeline, does he remain in it or revert to a moment just after point D on the original timeline? The latter does not seem to be logically necessary although I have seen it argued that the original timeline is the only one that he can properly exist in. What happens might partly depend on the method of time travel used. If the method involves sending a traveller into the past for a finite period, then recalling him to the present, the recalling to the present might or might not work between timelines, although I suspect that it would not.


If V does not prevent D from recurring on the divergent timeline, then what happens?

5. Disappearing Timelines

It is sometimes imagined that, if causality is violated, then the original timeline somehow ceases to exist but there are different possible meanings of this idea. One possible meaning is that in the original timeline, the entire universe ceases to exist at point D:

In this case, we would need to worry about anyone who intended to travel into the past in order to change history! However, although cessation of cosmic existence is logically possible at any time, it is not logically necessary in the case of causality violation, because the above discussion of diverging but co-existing timelines was not self-contradictory, and it is physically impossible according to empirically discerned conservation laws.


Further, if the entire universe were to cease to exist as a result of a causality violation, then it would more logically cease to exist at the moment of the violation, e.g., when Hitler is killed or kidnapped, not at any later moment, because, arguably, there would be no later moment in this timeline:

But then it makes more sense to say not that the original timeline has been caused to go out of existence but that it has been prevented from coming into existence. This implies a scenario not of two timelines, one of them ceasing to exist, but of one discontinuous timeline:

which was 3. above: A C (B)

6. Parallel Timelines

If we want more than one timeline but do not want time travellers creating universes, then we can say that the many universes simply co-exist but there are different ways to develop this idea. If the idea is used to explain causality violation by a time traveller, then its simplest version is as follows:

many initially identical timelines exist in parallel with each other;
whenever a time traveller moves either forwards or backwards in time, he also moves sideways into the next timeline;
therefore, when he seems to enter the past or future of his own timeline, he in fact enters the past or future of a second timeline identical to the first except for any changes that he makes in it, the first such change being his arrival:

(V may or may not prevent D in the second timeline but cannot affect D in the first timeline.)
There are three problems with this scenario. First, the time traveller does not get into the past of his own timeline, which is surely the purpose of time travel.

Secondly, because he does not get into his own past, there cannot be circular causality in this scenario: A in the second timeline cannot cause D in the first timeline. Of the two causality paradoxes, sf writers usually assume either that only circular causality is possible (because there is only one timeline) or that both are possible (presumably because timelines diverge when causality is violated but not otherwise) but not that only causality violation is possible (because they do not consistently assume the simplest parallel timelines theory). Thirdly, the time traveller never returns to his ‘present’ or starting point in the original timeline:

D2=departure from past for return to present
R=return to present, not until the third timeline

SF writers usually assume that, unless a time traveller changes the past, he can re-join his acquaintances in the original timeline, not just their duplicates in another timeline. The theory needs to be progressively complicated in order to allow for the kind of stories that get written.

First complication:

Futurewards time travel is along the same timeline and only pastwards travel is into a second timeline:

Second complication:

Only pastwards travel that leads to overt causality violation, like kidnapping Hitler, would take the time traveller into a second timeline. Thus, a mere reconnaissance into the past would look like this:

These two complications allow the time traveller to re-join his original acquaintances and allow A to cause D but also allow for causality violation.

A third complication would be that, after violating causality, the time traveller returns to the original timeline:

However, this complication is unnecessary because it is usually assumed that a traveller who changes history and returns to the present returns to an altered present which, in this scenario, is the present of the second timeline:

In this case, the traveller simply disappears from the point of view of his original acquaintances. Either they develop a parallel timelines theory or they conclude that attempted time travel is elaborate suicide.

Fourth possible complication:

Do two travellers from the same original timeline arrive in the same second timeline:

or not? Could there be a different series of timelines for each traveller, the second series at right angles to the page and others in further dimensions? This is another unnecessary complication which would prevent travellers from interacting with each other among the same historical events and would move the idea away from time travel towards parallel universes as such. Thus, parallel timelines with two out of four possible complications seems to be the simplest way to account for causality violation by time travellers.

7. Successive Timelines

There is a fifth possible complication of parallel timelines which allows for a third possible meaning of the idea that causality violation causes a timeline to cease to exist. In the parallel timelines theory as developed above, (i) the causality-violating time traveller enters each new timeline successively and (ii) the contents of each new timeline are determined by the contents of the previous timeline as modified by the time traveller. This suggests that there is a causal relationship between a timeline with a time traveller disappearing from it and the next timeline with the time traveller appearing in it. The phrases ‘new’, ‘successively’, ‘determined by’, ‘previous’, ‘causal relationship’ and ‘next’ suggest a temporal relationship between timelines.
We can imagine them not as paralleling each other in a fourth spatial dimension:

But as succeeding each other in a second temporal dimension:

If this is the case, then, from the point of view of a time traveller who has entered a new timeline:

his original timeline is now past in two senses. First, it is in his memory and thus in his subjective or personal remembered past. But in the discontinuous timeline scenario (3, above), this was compatible with the remembered timeline simply having no existence whatsoever. In the successive timelines scenario, the original timeline is in the objective past of the axis T2 and anything which is in an objective past can be said to have ceased to exist. In fact, the timeline will have ceased to exist from the point of view of a hypothetical four-dimensional observer for whom our T1 and T2 equal his S4 and T1, respectively.


This partly agrees with the way the issue is discussed in some sf stories. It is sometimes imagined that, if causality is violated, then the entire original timeline, from beginning to end, somehow ceases to exist. This is different from saying either that the universe ceases to exist at a particular moment of the original timeline or that the timeline has never existed. First, it existed. Now it does not. The terms ‘first’ and ‘now’ do not apply to any moments within the timeline itself. Subjectively, they apply to a temporal sequence in the time traveller’s memory.

Having ‘changed the past’, he experiences the ‘changed’ sequence of events while remembering an ‘original’ one. Objectively, the terms ‘first’ and ‘now’ apply either to nothing, as in the discontinuous timeline scenario, or (the only possible alternative) to a second temporal axis (T2) at right angles to the familiar direction of time (T1).


However, it is usually added in sf stories that cessation of a timeline means annihilation for its inhabitants so that, in their own self-interest, they should try to prevent D, the time traveller’s departure on his causality-violating mission. In fact, they would do better to prevent time travel or, failing this, to regulate it with a Time Patrol. Despite this assumption, it should be clear from the diagram that:

(i) T1 is the only arrow of time experienced by the inhabitants of the original timeline;
(ii) in T1, they exist until D and then continue to exist after it;
(iii) for them, the only result of D is that the time traveller disappears permanently from their universe;
(iv) in their experience, there is no moment at which their timeline ceases to exist;
(v) the time dimension in which the time traveller says that they have ceased to exist is not the one that their world lines extend along but is at right angles to it;
(vi) thus, all that they lose is some potential temporal width that they are not even aware of;
(vii) in the original timeline, an observer who knows about successive timelines can remark that this timeline is or will be (a new tense is needed) regarded as having ceased to exist by a time traveller;
(viii) however, this time traveller will himself have disappeared, or apparently ceased to exist, from the point of view of anyone who remains within the timeline.

Therefore, the inhabitants of the original timeline need not try to prevent D.

8. A Mutable Timeline

I have heard it argued that we can imagine one changeable timeline instead of many successive timelines. However, change involves a relationship between a state changed from and a state changed to. (If there were only one state, then there would be no change.) This relationship between changing states is a temporal relationship of ‘before’ and ‘after’.


Therefore, everything argued above about temporal relationships between timelines applies equally to temporal relationships between states of a timeline (if we want to use this changed terminology):


 

Inhabitants of an original state of the timeline need not be concerned that their state is or will be regarded as having ceased to exist by a time traveller who arrives in another state which has no existence on their T1 axis. The difference between successive timelines and a mutable timeline is merely terminological. Poul Anderson’s Time Patrollers sometimes speak as if they inhabit a single, discontinuous timeline, sometimes as if they inhabit a series of successive timelines and sometimes as if they inhabit a single mutable timeline. (1) (Although I have just argued that these last two options are identical, I here allow for characters and authors who seem to think otherwise.)


If they inhabit a single, discontinuous timeline, then some passages of some stories describe not events which did occur but events which would have occurred if they had not been prevented later in the story. We have to finish the story in order to be certain which events did occur. The passages set in ‘deleted timelines’ describe not experiences which the characters had but only experiences which they remember having had. The Patrollers definitely believe that any deleted timeline is utterly non-existent, even from its own point of view, i.e. that it does not have a point of view. This is consistent with the single, discontinuous timeline scenario.


On the other hand, even when they are in a particular timeline, Time Patrollers believe that it is possible that that timeline will be deleted and thus will turn out never to have existed in any sense whatsoever. I do not think that it is possible to make sense of this idea. First, if we assume a single, discontinuous timeline, then the deleted or prevented timeline should have no existence in the first place. Therefore the very fact that the Patrolmen are in the timeline while discussing it is already sufficient proof that it is not/has not been/will not be deleted.


Secondly, if we assume the parallel timelines/mutable timelines scenario(s), then yes, after causality has been violated, the original (state of the) timeline will have ceased to exist in the temporal dimension, T2, but it will still be true to say that it did exist in the past of T2. There will then be a subsequent (state of the) timeline. Within this timeline (I will now stop adding the qualifying phrase, ‘state of the…’), it will be true to say that the events of the original timeline have never occurred but this statement is true in T1, not in T2. It is not absolutely true. Patrolmen seem to be confusing questions of existence along two different temporal axes and their superhuman superiors, the Danellians, clearly have some reason to encourage this confusion.


However, the Patrolmen’s belief motivates them to preserve their timeline in T2 just as belief in God and the Devil motivated medieval monks to preserve Biblical and Classical literatures. Just as we can read about the Middle Ages without believing in the supernatural, so we can read about Time Patrol members without sharing their belief in the consequences of causality violation. A Time Patroller who is in his past has to prevent causality violations in order to be able to return to his version of the present and drama is added by his belief that that present would be not only inaccessible to him but also completely non-existent. The Time Patrol series more than compensates for any underlying incoherence by its colourful historical and even mythological reconstructions, its characterisation and its increasingly complicated time travel plots. Logical consistency is a major factor but not the only factor in the success or failure of time travel fiction.

9. Unwinding A Timeline

The Directors of Jack Finney’s top-secret government Project propose to send a time traveller into the past in order to alter Cuban history in the interests of U.S. foreign policy and they promise to be grateful to the traveller for completing this mission, clearly without considering that they and their memories will be affected by the alteration so that they will be unaware that it has happened or that they have benefited from it. (2) This may be an inconsistency, although a major one, in the minds of the characters rather than in the text of the novel. However, in the second novel, when history has been altered, a handful of people do have partial memories of other or ‘earlier’ timelines and even find physical evidence for them, like a ‘campaign button’ from John Kennedy’s second election campaign. (3) It is as if the causality violating time traveller had literally rewound a period of time and let it run forward again with an altered content but with some traces of the original content. This is presented as if it were an expectable, and not a highly questionable, consequence of causality violation. Four early chapters of the second Time novel serve only to move the action awkwardly, with the help of implausible sideways memories, from one timeline to another. (4) The characters could have learned that they were living in a subsequent timeline simply by reading the time traveller’s published account of the causality violation in which he prevented a meeting between the couple who would otherwise have been the parents of the founder of the Project. Then, without needing to have any memory of the Project, these characters could have set about retrieving it, as they do, by preventing the prevention of the meeting. However, Finney’s nostalgic accounts of past times and even of a lost timeline in which the Titanic did not sink and World War I was prevented more than compensate for any weakness in his time travel logic just as The Time Machine remains a classic despite the incoherence of its opening discussion of time first as a direction of static material extension and only of immaterial mental motion, then as a direction of variable material motion. (5) (In the latter case, there is nothing other than the entire universe for that universe to change its position in relation to, and the accelerating time machine should simply leave the rest of the universe behind it!)


To summarize theories of time travel:

1. The Logical Impossibility of Causality Violation in a Single Continuous Timeline
2. The Immutability of Past Events
3. A Single Discontinuous Timeline
4. Divergent Timelines
5. Disappearing Timelines
6. Parallel Timelines
7. Successive Timelines
8. A Mutable Timeline
9. Unwinding a Timeline

This is not necessarily a comprehensive list. A test of any story’s internal consistency could be: 

does it conform to any of these theories? If not, does it manage to be internally coherent by any other criteria? In practice, any given story will probably, if it is not completely incoherent, imply one of the above theories but not be fully consistent with it.

See Part II.

End-notes

1. Poul Anderson, The Time Patrol (New York: TOR Books, October 1991).
2. Jack Finney, Time and Again (New York: Simon & Schuster Inc, 1970; London: Arrow Books Ltd, 1990), pp. 386-387.
3. Jack Finney, From Time to Time (New York: Simon & Schuster Inc, 1996), pp. 20-21.
4. Finney, From Time to Time, pp. 42-65.
5. H. G. Wells, The Time Machine (Heinemann, 1895; London: Pan Books Ltd, 1953, 1973), pp. 7-12.