The Cleverest Kind Of Time Travel Story

The cleverest kind of time travel story is one in which the past seems to have been changed but then turns out not to have been. Poul Anderson's The Dancer From Atlantis (London, 1977) is one such. Erissa remembers that, before the catastrophe, she and Duncan were together on Crete where he became the father of her first child.

However, the catastrophe occurs while Duncan is still en route to Crete. So why does Erissa remember events that cannot have occurred and who was the father? By the time Duncan arrives, Theseus has raped the teenage Erissa but the adult Erissa, accompanying Duncan, hypnotises her younger self with spurious memories. Thus, the causal circle is completed.

When the older Erissa and Duncan meet - the second time for her but the first for him - they resume a physical relationship that they had not in fact had when she was younger. Parting from the hypnotised younger Erissa, Duncan says, " 'Know that in the end I'll call you back to me.' " (p. 166)

We last see Theseus swaggering away - not a usual last scene for the villain of an action-adventure novel. However, by rescuing young Erissa, Duncan and the mature Erissa prevent Theseus from using her to consolidate his power through the Goddess religion. To that extent, they thwart his plans.

The high priestess called "the Ariadne," conspiring with the invader Theseus, had ordered that a thread of lamps be lit to guide his men through the main halls of the Minos' palace, called "the Labyrinth." Thus, very understatedly, Anderson presents a possible origin for the myth of Minos' daughter Ariadne helping Theseus to escape from the Labyrinth that had housed the Minotaur.

Duncan and Oleg, stranded in 1400 BC but knowing that futurian time travelers will probably observe the imminent Atlantean eruption, build anachronistic ships to sail nearby in the hope that the futurians, seeing the ships, will deduce the presence of stranded time travelers and rescue them. It works:

"A shining shape descended from the clouds."(p. 167)

- while the anachronisms burned in battle.

Erissa's Cretans celebrate the death and resurrection of the god, Asterion. While it is true that Paganism included such myths, I wonder if Anderson had to invent some of the details for novelistic purposes? Lastly, I would like to read a time travel story in which the hero is told of a happy summer that he spent in Crete and ends the story by beginning that summer exactly as it had been related to him!

Doctor Who

Doctor Who will celebrate its fiftieth anniversary in November, 2013. Like Superman, Star Trek and Flash Gordon, it is a story that needs to be retold from the beginning, getting it right this time. The Time Lords should be:

not aliens but our future;
not extraterrestrials but extra-temporals;
our evolutionary successors, like Poul Anderson's Danellians.

The originals of the Doctor and his companions fighting the Daleks are the Time Traveller and Weena against the Morlocks.

On a Doctor Who fan's shelves, I saw:

a boxed set of CD's of the first three Doctor Who stories - the beginning;
a boxed set of DVD's of the two feature films starring Peter Cushing - an alternative beginning;
a "Doctor Who: Lost in Time" CD collection of episodes of early stories that no longer exist in their entirety - truly "lost in time";
the DVD of the television film starring Paul McCann in his single appearance as the Doctor;
CD's of various stories featuring different Doctors;
thus, television and cinema history.

The TV series is Doctor Who but the first feature film is Doctor Who And The Daleks and the second is Daleks Invasion Earth 2150 AD so the Daleks take over the titles. In the TV series, the second story is "The Daleks". Curiously, the poster for the second film prominently features not a Dalek but a roboman with Daleks and other figures in the background.

I have stopped watching the TV series which cleverly presented the circular causality paradox in the first "Weeping Angels" story but mishandled causality violation when Rose tried to prevent her father's death. I advise Whovians to read The Time Machine, The Time Patrol and The Time Traveller's Wife.

Time Travel Memoirs Fragments

Sitting in the window of the cafe, I saw her turn into the street and walk along it towards me. As she got nearer, I realised that she was looking directly into the cafe and even making eye contact. I looked away. She came in, sat opposite and addressed me by name. I looked at her but didn't know her. Simultaneously, she started speaking and stopped making sense.

"We haven't met yet so it is alright for me to talk to you. I want you to deliver a letter for me."

She put it on the table, a sealed envelope with a name, an apartment number and an address in another part of the city.

"There are only two of them here. They are both watching his building. They don't know I am here and they won't recognise you. Go inside, press his buzzer, say you have a message from..." - she told me her name. "He will let you in. They won't know which apartment you have gone into."

I thought: Alright. She's crazy but I don't mind delivering a letter. This is sufficiently intriguing. Maybe the guy will explain more.

I asked, "How did you know to find me here?"

"You will tell me."

We spoke briefly but I could not keep track of the conversation. She said we would be friends, smiled and left.

He let me in but kept me away from the window. He read the letter, nodded and said, "Congratulations. You are the only person from this period to see inside this apartment while I am here. Only her name got you in. She must have wanted you to know something."

I was beginning to notice details like a very strange wall map of Europe with a "Wiser Federation" occupying the British Isles.

I asked, "When are you from?"

He did not tell me a date but said, "I'm from Solway City, there," pointing to the North West of England on the map. "She's from the High Ones period much later. She had some difficulty getting into the twentieth century..."

Seeing that I did not know what to ask next, he continued, "Several groups are operating in your period which is a crucial turning point but every period is, of course. They can't change definitely known events but they try to influence longer term tendencies. Our friends outside..." - he nodded towards the window - "...would prefer if the Irish were less willing participants in the Federation because that could affect events later, but they won't succeed."

"Federation?"

"...of Welsh, Irish, Scottish and English Republics. That is probably as much as I should say."

I thought he looked Irish but with something different. Neither North nor South but something else.

"If she said you will be friends, you will. Leave as you came."

I did. All I know now is that some day she and I will meet.

Historical Dramas II

(Because this post is about time travel, I have copied it from the Poul Anderson Appreciation blog.)

The child who would have grown up to be Cyrus the Great was killed as an infant. Later, a Time Patrolman, Keith Denison, was forced to play the role of the adult Cyrus returned from hiding to claim his throne. After sixteen years in this role and thirteen years before he is due to fall in battle, Denison is found by a colleague, Everard, and they discuss how to get him out of there. They are in a scenario where history can be changed, hence the need for a Patrol.

If they simply proceed to the twentieth century, then they will arrive in a timeline where Cyrus, who is necessary for post-Exilic Judaism and thus also for Christianity, instead of dying in battle in the twenty ninth year of his reign, disappeared in the sixteenth or seventeenth year of his reign. That is acceptable neither to them nor to the Patrol.

Denison comments:

"Suppose I had not showed up? Mightn't Harpagus have found a different psuedo-Cyrus? The exact identity of the King doesn't matter. Another Cyrus would have acted differently from me in a million day-to-day details...But...if he was...reasonably able and decent...then his career would have been the same as mine in all the important ways, the ways that got into the history books." (1)

Here again is the idea of a historical role that could have been played by a different actor. Everard replies that it was Denison's mysterious appearance that gave Harpagus the idea of passing him off as a returned Cyrus.

The eventual solution is for the Patrolmen to prevent the murder of the infant Cyrus who then grows up to play his historical role. To check that history is still on track, they attend a winter solstice festival where they see Cyrus ride past with his courtiers, including Harpagus.

Denison comments:

"He's younger than I was...And a little smaller...different face entirely, isn't it?...but he'll do." (2)

Like seeing a remake of the film with a different actor but these guys are doing it with real history! He'll do? He's the right one!

Now something very strange happens to a legendary story. When Everard had arrived in 542 BC, Croesus had told him that Harpagus, ordered to kill the infant Cyrus, instead:

"...exchanged the prince for the stillborn child of a herdsman..." (3)

so that:

"...our lord Cyrus grew up as a herdsman." (3)

This story, the one told by Herodotus, is "...a typical hero myth...", told of Moses, Romulus, Sigurd etc, yet is sworn to by eyewitnesses. (4) Everard senses a mystery. Of course, he learns that the infant Cyrus was indeed killed and that the story of his growing up as a herdsman was devised in order to pass off the imposter, Denison, as the returned prince.

Later, he suggests to Denison:

"...all the scientific historians in the future are convinced that the story of Cyrus' childhood as told by Herodotus and the Persians is pure fable. Well, maybe they were right all along. Maybe you experiences here have been only one of those little quirks in space-time which the Patrol tries to eliminate." (5)

So a story that was a lie in the deleted timeline becomes a legend in the revised timeline. The scientific historians do not know what underlies the events that they study.

(1) Anderson, Poul, The Guardians Of Time, New York, 1981, p. 102.
(2) ibid., p. 123.
(3) ibid., p. 83.
(4) ibid., p. 84.

Historical Dramas I

It is commonplace that an actor can play the role of a fictitious character, like Hamlet, or of a historical character, like Julius Caesar, on stage or screen and that anyone who confuses the actor with the character misunderstands the dramatic process.

The idea of time travel raises the intriguing possibility of playing a historical role on the stage of real history. In a Superman comic, a time traveller investigating the identity of a twentieth century superheroine wound up playing the role of the superheroine, using futuristic technology to generate the superpowers of flight etc. (See "Getting Superman Right" on my Comics Appreciation Blog.)

In Michael Moorcock's Behold The Man, Karl Glogauer plays the role of Jesus in first century Palestine. The stage is set but no one else comes forward to play the role so a time traveller does. He tells parables when he remembers them. The idea is not that he is completing a causal circle but that he is making Christianity as real as possible. Without him, the New Testament would have been entirely mythological but the subsequent history would not have differed. First century Palestine is a stage on which he can do what he wants but he opts to play the role.

Time travellers would be able to get away with a lot. According to Leon Trotsky's History Of The Russian Revolution, Vladimir Lenin, on the run, attended a crucial Bolshevik committee meeting in disguise, wearing a wig and with beard shaved off (what a sight)! I cannot help imagining that that disguised Lenin was really a time traveller playing a historical role: kidnap Lenin; take his place at the meeting; hypnotise him with spurious memories as of having attended the meeting; put him in his bed to wake up the following morning. Simple. A group of time travellers could do this to the entire committee.

In Last Men In London, Olaf Stapledon envisaged a different kind of time travel intervention in the life of Lenin, one that connects with our earlier reference to Jesus. The Last Men time travel mentally, not physically. One of them performs a psychological experiment by inducing a mystical experience in Lenin who, after an hour of ecstasy, whispers, "I come not to bring peace, but a sword," and continues writing...

This line of thought started with reflections on two works by Poul Anderson which I will discuss in "Historical Dramas II" on www.poulandersonappreciation.blogspot.co.uk.

Circular Causality I

We appreciate the ingenuity of circular causality paradoxes in time travel fiction but usually forget the details. Here are some examples.

"The Chronic Argonauts" (1888) by HG Wells

The Chronic Argonaut, an earlier version of the Time Traveller, moves into an empty and shunned house, moves backwards in time while remaining in the house, is attacked as an intruder by the house's then occupants, a man and his two sons, defends himself and flees back to his present, leaving the father dead. With no evidence of an intruder, the sons are convicted of their father's murder which is why their house is later empty and shunned.

"Stitch in Time" (1961) by John Wyndham

A young man approaching a country house in order to propose to a young woman is pulled fifty years into the future by scientists who are then experimenting with time in the house. The woman, now in a wheelchair, still lives in the house and greets the young man who, after his initial shock, recognises her and, when returned to his present, does not propose to her. Not having received the expected proposal, she marries someone else and has a son whose organisation buys the house, letting her stay in it, and conducts the time experiments.

"Chronoclasm" (1953) by Wyndham

A man has an affair with a woman from his future. When she has returned to the future for the last time, he writes letters to her which will be found and will cause her to travel into her past to find him.

"By His Bootstraps" (1941) by Robert Heinlein

In 1952, Bob Wilson, now drunk, had locked himself in his apartment all day to finish his thesis. A strangely familiar man exiting a disc of nothing threw Wilson's only hat into this "Time Gate" and urged Wilson to follow because an older man would offer a deal enabling all three to run the country. A similar man exiting the Gate argued against. They were interrupted by a nuisance phone call, then by a call from a woman, Genevieve, claiming that Wilson had left his hat in her apartment that afternoon. In a three-sided fight, Wilson was punched unconscious through the Gate. Waking, he was greeted by an older, bearded man, Diktor, who gave him a drugged drink to help him sleep off fatigue, drunkenness and shock. Waking again, he received from Diktor a list of reference books and other resources to bring from 1952. Diktor, benevolent despot of a peaceful society, needed Wilson's help to maintain his position. A young woman, Arma, served food and drink.

Diktor sent Wilson to 1952 to persuade someone, his younger self, to come through the Gate. An older Wilson arrived to argue against. Wilson followed his punched self through the Gate where Diktor showed him his sleeping form. Wilson questioned Diktor's motives and went through the Gate to dissuade his younger self from going through. Having failed, he was rung again by Genevieve who thought that they had become engaged that afternoon. Later, hearing footsteps stop outside his door, he went through the Gate to see Diktor and himself receding down a corridor. Retrieving his hat and appropriating Diktor's hand-written English-future dictionary, he made a Gate appear in 1952 outside his apartment and went to buy the items on Diktor's list. Because the outside Gate disappeared in his absence, he had to use the one in his apartment after his younger selves had left. Meanwhile, he visited Genevieve and made a nuisance phone call.

Ten years before his encounter with Diktor, he impressed the tribespeople with modern music played on a mechanical recorder, then used his tabu status and sociological knowledge to control society. He occupied Diktor's apartments, grew a Diktor-style beard, named a servant Arma, copied from the old dictionary into a new one when the old one wore out and was called "Diktor" which meant "chief." Watching his apartment through a Gate, he saw a hat and an unconscious body exit the Gate. He placed the hat and dictionary where Wilson would find them, started to plan a list for Wilson, then spoke to him.

The Time Travel Archives

a publishing idea

Defining statement: the Time Travel Archives would be uniform editions of good time travel stories and novels covering the basic concept, the causality paradoxes and divergent timelines but excluding stories or novels whose internal logic of time travel was regarded by the Editor as unacceptably inconsistent.

However, works by Wells, Anderson and Finney that compensate for questionable logic in other ways would be included. A rigorous application of the defining statement would rule out The Time Machine, The Time Patrol and Time and Again.

The Basic Concept, Causality Violation and Divergent Timelines

"Missing One's Coach" Anonymous
A Connecticut Yankee In King Arthur's Court by Mark Twain
The Time Machine by HG Wells
Lest Darkness Fall by L Sprague de Camp
Bring The Jubilee by Ward Moore
Past Times (a revised edition) by Poul Anderson
The Time Patrol by Poul Anderson
The Shield of Time by Poul Anderson
Jack Finney's time travel short stories collected
Time And Again by Jack Finney
From Time To Time by Jack Finney

Circular Causality

"The Chronic Argonauts" by HG Wells
"A Stitch in Time" and "Chronoclasm" by John Wyndham
"By His Bootstraps" and "'- All You Zombies-'" by Robert Heinlein
The Door Into Summer by Robert Heinlein
The Technicolor Time Machine by Harry Harrison
Beyond The Barrier by Damon Knight
The Corridors Of Time by Poul Anderson
The Dancer From Atlantis by Poul Anderson
There Will Be Time by Poul Anderson
Bid Time Return by Richard Matheson
The Time Traveller's Wife by Audrey Niffenegger

The Curious Sub-Sub-Genre of Juvenile Historical Fantasy Time Travel Novels by English Women Writers
 
A Traveller In Time by Alison Uttley
The Moon Dial by Helen Cresswell
Tom's Midnight Garden by Philippa Pearce
The Story Of The Amulet by E Nesbit
etc

Not to be included 

The End Of Eternity by Isaac Asimov
The Legion Of Time by Jack Williamson
The Big Time by Fritz Leiber
Up The Line by Robert Silverberg
The Man Who Folded Himself by David Gerrold
11/22/63 by Stephen King

For discussion of some of these works, see here and here. An extra volume of the Archives would be an Editorial explanation of inclusions and exclusions. Apart from the circular causality paradox and the curious sub-sub-genre, I found it impossible to list the categories separately. Causality violation is possibly attempted in A Connecticut Yankee and subtly implied in The Time Machine before it occurs in Lest Darkness Fall, thus initiating a divergent timeline.

"Missing One's Coach" and the first five items in the "Circular Causality" list could form a collection.

Time and Eternity

Introduction

Indian "sutras," telegrammic statements recited or written in lists, conveniently summarize philosophical propositions and arguments. "Time and Eternity" is a suitable subject for sutras because Indian philosophy addresses eternity although less can be said about it than about time. Time, a consequence of motion, is commonly but mistakenly regarded as also a direction of motion. HG Wells' Time Traveler propagates this error before introducing the machine that supposedly accelerates along time. Thus, discussion of time includes discussion of alleged motion along time and thus also discussion of fictitious time travel.

The sutras are grouped in fours for convenience but the beginning of a line of thought does not always correspond to the beginning of a new quatrain. This experimental format has limits. 
 

Time

Space is relationships between material objects.
Motion is change of spatial relationships.
Time is the relationship between relationships changed from and relationships changed to.
A static reality would be atemporal.

Three dimensions of space and one of time account for experience.
We perceive space but presuppose time.
Perceiving one three dimensional state, we remember others.
To remember a material state is to recognize it as previously perceived.

Recognition presupposes temporal endurance.
The temporal presupposition transforms immediate sensations into coherent experiences.
Objects extend and move through space but endure through time.
Extension is spatial, endurance is temporal and motion is spatiotemporal.

Motion involves both distance covered and time taken.
Endurance resembles extension, not motion.
A human body extends but does not move from its feet to its head.
It endures but does not move from its birth to its death.

To occupy adjacent points in space simultaneously is to extend between them.
To occupy adjacent points successively is to move between them.
To occupy successive moments is to endure between them.
To occupy adjacent moments successively would be to move between them.

Motion Through Time?

But we do not move through time.
However, our experience of motion and time makes us speak of motion through time.
I have heard it said that we move into the future at the rate of sixty minutes per hour.
This proposition reflects our experience but is conceptually confused.

Comparing endurance to extension entails imagining successive moments as cross-sections of the body.
Instead, we imagine succession as motion, like an insect crawling up the body.
To move is to occupy different places successively.
To endure is to occupy different moments successively.

We think that successive occupation is motion.
However, we successively occupy places because we move from place to place.
But we successively occupy moments because they succeed each other.
We exist in each successive moment without moving between them.

We do not move from one o'clock to two o'clock but exist at and between them.
There is no two o'clock before we reach it or one o'clock after we have left it.
But there is a Paris before we reach it and a London after we have left it.
"When I reach 65..." and "When I reach Paris..." have different meanings.

The age of 65 is not now waiting somewhere else until I move to it.
I can reach it by existing for another five years.
To exist is also to move but through space, not time.
We do not experience the Earth's motion through space.

Thus, we can reach 65 without apparently moving even through space.
Places co-exist in space while we move between them.
Moments succeed each other in time while we exist in them.
We experience moment 1 when it occurs, then 2 when it occurs.

We do not experience 1 while it co-exists with 2, then 2 while it co-exists with 1.
Moments do not co-exist in one spatial dimension while we successively experience them in a separate temporal dimension.
Physicists postulate extra dimensions but not to explain motion along time.
At 60 mph, 60 miles is distance covered and one hour is time taken.

At 60 minutes per hour, 60 minutes is not distance covered.
60 minutes and one hour are both time taken.
If there is no distinction between distance covered and time taken, then there is no motion.
60 miles alone are distance.

60 minutes or one hour alone is duration.
Only 60 miles in 60 minutes is motion.
Thus, 60 minutes in one hour is a period of duration, not a rate of motion.
We can reduce speed to 50 mph but not time to 50 minutes per hour.

We feel that we move through time but not around the Sun or the galactic center.
Therefore, feeling is an inadequate criterion of motion.
Four co-ordinates describe our occupation of a place at a time.
Changing the temporal co-ordinate changes the time at which we are in that place.

One co-ordinate describes a moment.
Changing the temporal co-ordinate merely changes the moment.
There is no sense in which it can change the time at which we are in the moment.
Every moment in which there is consciousness is seen as the present moment in that moment but only in that moment.

However, no single moment has any unique status.
Subjectively, the present divides time into past and future.
Objectively, there are only temporal relations of before and after.
Objective time is divisible into natural or artificial periods.

A day in 3000 CE is unchangeably later than us and earlier than our successors in 4000 CE.
Accumulating memory generates the illusion that the present moves along time so that the past grows while the future shrinks.
However, perception of a subjective present occurs in every moment of consciousness.
Therefore, it does not move from each moment to its successor.

A conscious subject and present objects are interdependent.
"Present", whether contrasted with "past and future" or with "absent," means "presented to consciousness."
Therefore, there is no moment of consciousness that the present has vacated or not yet reached.
That past moments are not now conscious might suggest that consciousness has moved from them to the present.

However, past moments were conscious.
And they do not now exist unconsciously elsewhere.
The present tense refers to the subjective present, to current periods or to several times if a single proposition is equally applicable.
Multiple applications of the present tense neither negate succession nor entail simultaneity.

Real simultaneity between apparently successive events would eliminate succession.
But alleged motion of consciousness between simultaneous events re-introduces succession.
Experience is successive.
Therefore, accounts of experience need not eliminate succession, especially not to re-introduce it.


That consciousness is present in only one moment at a time is tautologous or false.
That it is present in each moment only in that moment is tautologous.
That all moments co-exist with consciousness occupying them one at a time is false.
The latter implies that past events are still occurring now but without consciousness.

HG Wells' Time Traveler identifies endurance with extension in a fourth dimension.
He adds that immaterial subjects of consciousness successively observe three dimensional cross-sections.
However, consciousness is an organism-environment interaction.
It is not an immaterial entity intersecting otherwise unconscious four dimensional objects.

The Time Traveler identifies the fourth dimension with time.
In fact, it would be a fourth dimension of space.
Motion along it would take time.
JW Dunne argued for an infinite regress of temporal dimensions which would surely be static at infinity.

The Time Traveler also inconsistently identifies endurance with motion of objects along the fourth dimension.
The totality of objects has no external spatial relationship.
Therefore, it cannot move.
Even if it could, its motion would take time.

Therefore, the dimension moved through would not be time.
Also, such motion would not explain changes occurring within the universe.
But it is these changes that generate time.
Therefore, time is an internal feature of the universe.

Time Travel

It is not an external dimension.
A time traveler who did somehow move faster than a moving universe would leave it behind.
Therefore, he would not find it waiting for him on arrival.
He would have to wait for it to catch up.

But the Time Machine moves only through space like everything else.
It remains stationary on the Earth's surface.
It endures indefinitely with decelerated internal processes.
Returning, it reverses the direction of its endurance or duration.

Its increased speed is supposed to explain its invisibility and intangibility in transit.
But it has no speed.
Also, a bullet or propeller accelerated to invisibility remains lethally tangible.
The Time Traveller's physical and mental processes are decelerated in relation to his environment.

He sees the sun moving so fast that it becomes an arch across the sky.

A new drug does accelerate another Wells character's physical and mental processes.
Consequently, he sees his environment as immobile.
The Time Traveler in transit should be visible and tangible but apparently static.

This condition is "stasis" or time dilation, not time travel.

His sensation of speed is unaccounted for.
Dr Who's TARDIS disappears at one place and time and appears at another.
Its occupants experience reduced duration in another realm.

Poul Anderson's Time Patrolmen and Audrey Niffenegger's time travelers merely disappear and re-appear.

Twain's Connecticut Yankee finds himself in the Arthurian period and returns to the nineteenth century by suspended animation.
Preceding Wells, he uses the phrase "transposition of epochs."
Anderson's mutant time travelers sense motion but merely endure in an attenuated form.

None of these "time travelers" moves through time.

However, language has incorporated Wellsian "time traveling" terminology.
Anderson's Wardens and Rangers walk or drive along corridors that have been rotated onto the temporal axis.
Thus, they do move along time.

The height, width and length of a "time corridor" are spatial.

However, its length has been made to extend in the direction along which Earth endures.
People arriving in a corridor from different times should be separated on arrival only by spatial distances.
Therefore, they should arrive simultaneously in the corridor's temporal dimension.

But for story purposes, travelers along corridors rarely meet.

So Anderson explains merely that duration occurs in the corridors but on a different plane.
Rangers entering a corridor from a time several seconds after the central character also arrive inside it several seconds later.
This contradicts the premise that temporal intervals outside the corridor correspond only to spatial intervals within it.

The direction of endurance within a time corridor may be a fifth dimension.
Or it may correspond to an external spatial dimension.
In the latter case, a spatial corridor could give access to different periods of the corridor's internal history.
Like Twain and Wells, Anderson presents a story, not a treatise.

The speed of light is 186,000 miles per second.
Accordingly, the relativistic space time equivalence is 186,000 miles to one second.
However, the corridor equivalence is about a foot to a month.
Thus, a 6000 year time corridor need not extend for 6000 light years before it is rotated onto the temporal axis.

Paradoxes

The time travel paradoxes are circular causality and causality violation.
An effect causes or prevents its cause.
A timeline is a single sequence of events occurring along one temporal dimension, T1.
A single timeline can incorporate circular causality.

At least four alternative scenarios incorporate causality violation.
One scenario is branching timelines.
However, this involves creating universes ex nihilo.
A time traveller "changing the past" disappears into a new universe branching away from the original universe.

This is logically possible but physically questionable.
Parenthetically, timelines might diverge without time travel.
Divergence would occur at every moment of indeterminacy, not at arbitrary moments of extratemporal interference.
If divergence does occur, there is no contact between divergent universes.

Such contact does occur in fiction.
But parallel universe travel is not time travel.
Men Like Gods is not The Time Machine. 
Piper's Paratime Police regulates traffic between parallel Earths with alternative histories.

Anderson's Time Patrol regulates traffic and prevents causality violations along a single timeline.
Asimov's Eternals violate causality to increase human happiness.
Heinlein's temporal bureau closes causal circles.
One temporal agent is her own parents.

Some fictitious organizations fight wars by changing history.
But most such narratives are logically inconsistent.
Premises are ambiguous or events contradict them.
Asimov's Eternals need to complete a causal circle in a no longer existent timeline (see below).

Wardens and Rangers fight a war by manipulating the history of a single timeline.
They hope to influence their future from which they are barred by their successors.
Anderson's mutant time travelers divide into warring factions in a single timeline.
One group, the Eyrie, recruits Jack Havig, Boris and others on the original Good Friday.

When Havig organizes against the Eyrie, he sends Boris to infiltrate the Eyrie on Good Friday.
Thus, Boris must have recognized his fellow Eyrie recruit as his future anti-Eyrie organizer.
The Eyrie has evidence of its future success.
Therefore, Havig's group fakes the evidence.

Thus, not events but their significance is changed.
In the second causality violation scenario, timelines succeed each other along a second temporal dimension, T2.
Like spatial dimensions, temporal dimensions are at right angles to each other.
To travel backwards or forwards in T1 is always to travel forwards in T2.

Thus, a time traveler never returns to his original timeline.
He enters the past or future of a subsequent timeline.
He cannot complete a causal circle in his original timeline.
Timelines are identical except for changes made by a time traveler.

The first such change is his arrival.
Asimov's "Realities" are timelines and his "physiotime" is T2.
His Eternals have altered many successive timelines.
Their original timeline is far in the past of T2.

In the present of T2, it no longer exists.
Thus, return to it is impossible.
Yet such a return is necessary for the existence of the Eternals.
Of course, a text may bear other interpretations.

Attempted clarification of The End of Eternity suggests internal inconsistencies.
This problem applies to several works about frequent causality violations.
In one unproblematic text, a single time traveler deliberately prevents the Dark Ages.
Thus, a timeline of uninterrupted civilization succeeds the Dark Ages timeline.

In another novel, a fictitious time traveler accidentally prevents Confederate victory at Gettysburg.
Thus, the American Civil War timeline succeeds a "War of Southron Independence" timeline.
In the third causality violation scenario, a single timeline contains discontinuous events.
Thus, a time traveler with counterfactual memories appears ex nihilo at Gettysburg.

Such macroscopic quantum events are possible because time travel is.
Confederates should have occupied a position ensuring victory at Gettysburg.
But their advance was halted by the arrival of a time traveller from the independent Confederate States.
Thus, a potential future generates a time traveller who prevents that future.

In this scenario, if we perceive or remember an event, then we know that it has not been prevented from occurring.
But, if a time traveler arriving in his past remembers an event, then he does not know that it has not been prevented from occurring.
Thus, the meaning of "memory" is widened.
This scenario is counterintuitive though not logically contradictory.

Scenarios without time travel preclude any potential futures from which time travelers could arrive.
Thus, without time travel, the Confederates would have won at Gettysburg.
In the first causality violation scenario, Southron Independence occurred in the original timeline.
In the second, it occurred in the past of T2.

In the third, it never occurred.
The fourth causality violation scenario is specific to the Time Patrol series.
This scenario may not be distinct or coherent.
Patrolmen speak of one mutable timeline, not of many branching timelines.

They do not believe that a temporal round trip takes them between timelines.
They speak as if their current timeline can be prevented from existing.
Therefore, none of the first three scenarios applies.
T1/T2 terminology is not used but is implicit.

T1 is a single timeline.
T2 is the relationship between successive timelines.
Or T2 is the relationship between successive states of a single mutable timeline.
The difference is terminological.

We can say either that one timeline succeeds another or that the single timeline changes its content.
In either case, there is a relationship between a state changed from and a state changed to.
This summary uses the term "timeline", not the phrase "state of a timeline".
In the Time Patrol scenario, most time travel is along a single timeline.

Thus, it occurs within a single moment of T2.
The timeline incorporates circular causality.
Most time travelers are already a part of the past that they visit.
However, they can change past events.

 

Usually, other events change to counteract a change made by a time traveler.
However, at nexus points, a small change can alter the entire future.
Occasionally in T2, there is a quantum change to another timeline.
Quantum changes result either from time travelers or from random fluctuations in space-time-energy.

Small changes in the life of one medieval knight affect the outcome of the European church-state conflict.
Therefore, the knight is a personal causal nexus to be eliminated by the Patrol.
Unpredictable fluctuations remain when organizations of time criminals have been apprehended.
By guarding the timeline, the Patrol preserves cosmic order.

Cyrus the Great is killed in infancy.
Later, a captured Time Patrolmen is forced to play the role of the adult Cyrus.
Sixteen years into his reign, he is rescued by a fellow Patrolman.
They travel further back in time to prevent the murder of the infant Cyrus.

Thus, despite changing events, they preserve Cyrus's pivotal role in the Patrol-protected timeline.
But they leave behind in T2 a timeline in which Cyrus disappeared sixteen years into his reign.
Patrolmen claim that that timeline simply does not exist.
Therefore, they claim to inhabit the third scenario.

This entails that some passages in the story describe events that did not occur.
Even while experiencing an event, Patrolmen claim that the event may be prevented from occurring.
However, if the event is being experienced, then it is occurring.
If it is occurring, then it has not been prevented from occurring.

A timeline can be prevented from currently existing in T2.
Then that the events of that timeline never occurred is true in T1.
In Finney's second Time novel, characters who have not time traveled partially remember prevented events.
Former timelines are remembered sporadically and indistinctly, like dreams.

Finney's fifth scenario seems to be that events unwind and rewind in an altered sequence but with indistinct memories of the original sequence.
But preventing events should prevent any memory of them.
Usually in fiction, former timelines are remembered only by time travelers from them.
Causality violation entails unintended self-duplication of time travelers.

Thus, at time t1, a time traveler experiences event A.
Disliking A, he travels from t2 to t1 in order to change A to A~.
Thus, at t1, his younger self experiences A~.
The time traveler who changed A to A~ returns to t2.

At t2, the time traveler who had experienced A~ has no reason to travel back to t1.
Thus, at t2 the traveler who experienced A~ coexists with the traveler who changed A to A~.
The problems of self-duplication are probably greater than the problem of leaving A unchanged.
In the timeline guarded by the Patrol, Roger II of Sicily died in 1154 (Timeline 1).

Volstrup was a Patrolman based in Palermo during Roger's reign.
In an aberrant timeline, Roger died in battle in 1137 (Timeline 2).
The appalled Volstrup informed an earlier Patrol base.
He visited that base for consultation.

After consultation, Patrol intervention saved Roger's life at the battle in 1137 (Timeline 3).
Ostensibly, the preferred timeline was restored.
However, Timelines 1 and 3 differ in detail.
Unlike Timeline 1, Timeline 3 has Volstrup visiting an earlier base and Patrolmen intervening at the battle in 1137.

Like Timeline 1, Timeline 3 has Volstrup in Palermo not hearing about Roger's death.
That Volstrup neither informs nor visits an earlier base.
But the Volstrup who did visit an earlier base and who does remember Roger's death returns to Palermo in the 1137 of Timeline 4.
At that point, Volstrup is duplicated.

Yet Anderson writes as if there is only one Volstrup in Palermo after the battle in 1137, the one who remembers Roger's death.
Volstrup thinks that at any moment he and his environment can cease ever having been.
But that is not a consequence of causality violation.
There are potential timelines in which Volstrup was not born and therefore does not exist in adulthood.

But there is no timeline in which he was not born and therefore ceases to exist in adulthood.
Nothing can exist until a particular moment, then, at that moment, cease having existed until that moment.
An entire timeline can cease to exist in T2.
But that affects no one's experience in T1.

A time traveler departing on a successful mission to end our timeline will think that we have ceased to exist.
But he will think this in a temporal dimension at right angles to ours.
There is no cessation of existence at any moment in our T1.
It is difficult to make the Time Patrol series fully coherent.

The Literature

The time travel writers whose works are worthiest of analysis are Wells and Anderson.
The Time Machine addresses the nature of time and the future of mankind.
Anderson's six volumes address time travel paradoxes, history and the future.
The Time Patrol is the only sustained high quality time travel series.

Thus, Anderson supersedes his immediate predecessor, Robert Heinlein.
Heinlein perfected the circular causality paradox in three works with futuristic American settings.
Wells' "The Chronic Argonauts" and two short stories by John Wyndham developed this paradox in contemporary British settings.
Harry Harrison linked circular causality to history and humor.

Richard Matheson and Audrey Niffenegger each wrote one novel integrating a love story into circular causality.
Jack Finney perfected nostalgic time travel fiction focused on the late nineteenth and early twentieth centuries.
L Sprague de Camp and Ward Moore each wrote one definitive historical causality violation novel.
There have been two or three good time travel films but several atrocious ones.

Eternity

By attending fully to the objects of immediate awareness, we enter "the eternal present".
Approaching death, we may also approach "the light of eternity", the perception of life as complete.
Hypothetical beings perceiving all human life as eternal would necessarily endure in a second temporal dimension.
Duration-less perception, beginning and ending simultaneously, would be unconsciousness.

"Eternal" means endless, changeless, timeless/atemporal or transtemporal.
Thus, energy is endless, the past is changeless, mathematics is timeless and the eternal present transcends time.
Thus also, several meanings of "eternal" would remain applicable even if the future were known to be finite.
We live in time and eternity.

A temporal process is a sequence of incomplete syntheses between change and resistance to it.
Thus, masses are more resistant then gasses, organisms more dynamic then inanimate objects etc.
A complete synthesis would be changeless but ever-new.
This might correspond to some mystical accounts of eternity.

Mundaka Upanishad: "Even as a spider sends forth and draws in its thread, even as plants arise from the earth and hairs from the body of a man, even so the whole creation arises from the Eternal."
The Dhammapada: "All things indeed pass away, but the Buddhas are for ever in Eternity."
Blake: "To see a world in a grain of sand and heaven in a wild flower, hold infinity in the palm of your hand and eternity in an hour."
Engels: "Nothing is eternal but eternally moving, eternally changing matter and the laws by which it eternally moves and changes."

Whatever Happened To The Time Traveller?

Let us assume, as a fictional premise, that Wells’ true account, The Time Machine, was mistaken for fiction because the Time Traveller never returned. It follows that a nineteenth century inventor built a time machine in his laboratory whereas the collective approach of twentieth century science somehow prevented scientists from realizing that time travel was possible. What has been discovered once can be discovered again. 

Future time travellers, realising that Wells’ Time Traveller was a real person, will want to meet him. Perhaps the silent dinner guest was a time traveller who did not speak because he did not want to risk changing the conversation recorded by Wells? Future time travellers would show Wells’ account to the original Time Traveller before his final departure. Knowing that he would not return to the nineteenth century, he would accompany them to their period. They could: 

build bigger time machines;
explore past and future history;
rescue Weena;
round up Eloi into a reservation;
exterminate the Morlocks;
rescue people from disasters throughout history and
repopulate the paradisal Earth with undevolved humanity. 

Wells wrote no sequel but the Time Traveller might visit a twenty first century writer. Since I am not a fiction writer, it is unlikely to be me.

Yossi, the Time Traveller

After the High Ones left, some of us searched their Palace for humanly usable (or even just comprehensible) technologies. I was looking after my two-year old granddaughter, Yossi, on the lawn outside the Palace while members of our group searched different Chambers inside. Yossi kept running behind bushes, then went up the ramp into the Building.

Knowing that her father, Rick, was checking computers in the first Chamber, I took my time about following her. When I got inside, however, Rick was alone, absorbed in machinery. Yossi must have gone further in. Passing Rick, I found myself in a corridor with openings to several Chambers. Becoming alarmed, I hurried forward and suddenly noticed a young woman who momentarily reminded me of my daughter, Aileen. Smiling, she pointed to a particular opening. Thanking her, I dashed inside and immediately saw Yossi on a dangerously high platform without a railing: typical High Ones architecture, not designed for human use. Someone had erected a humanly usable ladder to the platform but Yossi could not possibly have climbed it, especially not in the time she had been out of my sight.

More startling was another Yossi, sitting on the floor at the foot of the platform. Standing back, I could see two Yossis simultaneously. This was not quite as surprising as you might think. I knew that, theoretically at least, time travel was possible inside the Palace. After all, it was believed that the High Ones had gone away into Time, not into Space, and there was evidence to support this belief. First, there was the “Hall of the Time Gate” which no one knew how to operate and some thought was merely ceremonial. Secondly, there was the peculiar architecture of the Palace: not just unprotected platforms but openings into thin air high in the impossibly curving walls of uninhabitable, floorless Chambers, as if the Builders could fly or as if we perceived only a cross-section of a multi-dimensional structure. Extra dimensions could be used for space warps or time warps. If Yossi had travelled even a minute into the past, then she would obviously coexist with her younger self until the younger self disappeared at the end of that minute. And, if she had stumbled onto time travel, then she had definitely made the breakthrough that we were all looking for. Was the platform a time machine?

My first thought was that the Yossis had better be prevented from seeing each other in case they became alarmed but, before I could move, the Yossi on the platform stood, pointed and addressed me with:

“Oh, look, there’s Yossi!”

She spoke! She spoke in a sentence! She pronounced her name correctly: “Yossi”, not “Sossi”! She was older than the one I had been looking after who, I hoped, was the one on the floor. I didn’t want three of them. I needed to get to the older Yossi. She was in greater danger because she was on the platform and (I reasoned) the younger Yossi could be presumed to be safe because it was known that she would survive to become the older Yossi. I raced up the ladder and, addressing Yossi from the top of it while pointing to the figure on the floor, asked:

“Is she your…”

(Could I say “your younger self”?)

“…your little Yossi?”

“Yes. I played with her.”

When? There had not been time.

I continued: “When there are two of you, you must tell me which is older because that is the one I must look after.”

“I know.”

She knew! She already knew something about the intricacies of time travel. I climbed up and picked her up. Then, as we looked down at the younger Yossi, that one disappeared while the one I was holding said:

“Bye bye, Yossi.”

The disappearance was more like the fading of a memory than the departure of an object. Had I really seen her there a moment ago or had it been ages ago?

I asked:

“Do you remember being her?”

“Yes.”

“Where did she…you go?”

“To a lady.”

She was not going to say any more and I was already starting to realise that I should not enquire too closely. The events were already safely past for Yossi. 

My first hypothesis had been that this older Yossi had come back to us from our future. Now it seemed that the younger Yossi had travelled perhaps a month into the past and had grown into the Yossi I was now holding. During that month, she had been looked after by someone, a “lady,” had improved her speech, learned something about time travel and possibly played with her younger self without our knowledge. I was learning to reinterpret chronokinetic events continually. Yossi could have travelled backwards or forwards in time in a more complicated sequence than the one I had just hypothesised but it was at least the simplest explanation for what I had seen so far. 

As I carried her through the Palace to rejoin Aileen and her friends, I wondered if Yossi and I could just cover up what had happened but this was impossible. I began to notice that she was bigger and was dressed differently and, as soon as she saw her mother, she ran towards her, speaking in a way that showed she had changed.

That was how it began. Since then, we have had to become used to Yossi’s time travelling. Sometimes she just disappears. When that happens, we know that she has gone forward for a day or two. Then, we just have to wait for her to reappear, unaged, unchanged, sometimes even in mid-sentence. Sometimes, she is seen in the distance, talking to her older or younger selves. When that happens, we do not interact with them unless one of them invites us to. 

Events which the oldest Yossi present regards as past must be allowed to unfold as she remembers them. If she remembers that she did not tell us something that we wanted to know, then so be it. We can probably be told it later but obviously did not need to know it at the time.

Once, Aileen glimpsed a taller figure that could have been the “lady” who interacts with Yossis. Sometimes, events are chaotic. Once, Sheila answered the door late at night to be confronted with a twelve year old Yossi who handed over her younger, sleeping self and immediately disappeared. We wanted to tell her that this was unacceptable but had to wait until we saw a Yossi who was old enough to say it to. We had a day of looking after a very small baby again and now suspect that she had been taken from her pram at a very early age and returned to it before our younger selves were able to notice that she had gone.

On another occasion, several Yossis walked past my half open door, saying, “Hello, Granda” in descending order of height. The smallest could only manage, “Lo, Wawa.” I was astounded that they had managed to involve one so young but I knew that she was perfectly safe so I resisted the temptation to run down the corridor after them. I expect that we will be seeing younger Yossis as guests at the older ones’ birthday parties for years to come but I will have to live through each twelve month period consecutively in order to find out.

As you know, Anderson and others have significantly advanced time travel theory by studying Yossi’s reported comings and goings: having disappeared, she either reappears (travels into the future) or turns out to have appeared earlier (travelled into the past). In the latter case, an effect (appearance/arrival) precedes its cause (disappearance/departure) and sometimes causes it (which is the paradox of circular causality) but apparently never prevents it (which would be the paradox of causality violation). When Yossi knows that her older self has been seen to appear or to act in a particular way at an earlier time, she simply accepts that, at some point in her personal future, she will have reason to appear or to act as her older self is already known to have done. She usually forgets about her reported actions until she finds herself performing them although, occasionally, she sets out to do what she is known to have done in order to complete a particular series of events. Thus, there is both circular causality and deliberate avoidance of causality violation.

She seems to have learned at an early age that, if she attempts the opposite, deliberate causality violation, even just by appearing at a time and a place at which she did not appear, then events outside her control, possibly including a fatal accident, will prevent her, so she doesn’t try.

We do not know whether she subjectively experiences anything between objectively disappearing and (re)appearing or how she controls her time of arrival. In fact, I think she is still learning how to control it. Certainly her earliest temporal journeys were controlled by her older selves or by hypothetical time travelling colleagues, not by the travelling self. The theoreticians’ first mistake was to think that the Yossi whom I followed into the Palace had accidentally discovered time travel. Rather, one of her older selves had carefully arranged for that Yossi to travel back a month or so in order to get the process started. Even an early trip to the past that seems to us completely pointless is, to her, a remembered part of her learning experience and therefore must be allowed or even caused to have occurred. The platform is not a time machine but she was put on it in order to get me away from the younger Yossi who was scheduled to disappear soon after I entered the Chamber.

Yossi is obviously protecting and instructing her younger selves while preparing her present self for longer journeys and a greater purpose. She has visited periods before and during High Ones rule which is a bit nervous-making but:

she only embarks on such a journey after she has seen herself return from it so she knows it is safe to go;
the periods visited are the safest in human or superhuman history;
she assures us that, while she is in such periods, she automatically disappears at the first sign of potential danger.

(I think that a time traveller can develop a sixth sense for danger and disappear before consciously realising what the threat was, especially if all that she is trying to avoid is detection. This would explain some reported apparitions in and around the Palace.)

A Yossi from several years in our future mentioned that, after she had extended her range, she provisionally decided to visit a particular pre-High Ones period called “the twentieth century” but took the usual precaution of waiting to see herself return before setting off. Since she did not return, she did not go and now avoids that period like the plague. I have subsequently confirmed that it was a lethal period of social conflicts that have since been resolved and transcended. As for our future, Yossi’s knowledge is, of course, limited and she is understandably reluctant to share it except to say that we have some surprises ahead of us.

She could, if she wanted, spend several consecutive years in a chosen historical period and return to us at the moment from which she left but then we would miss those years of her development so she will not let this happen yet. If she does eventually settle down in an earlier period, then she could already be one of her own ancestors (bequeathing a gene for time travel?). Scientists and historians want information on the High Ones, who concealed themselves during the latter years of their “reign” when they did not rule human beings directly but did alter our environment and influence our culture, fundamentally for the better, but Yossi has a different agenda which I have only recently begun to understand.

I had been confident from the start that the younger Yossi was safe because I knew that she would survive to become the older one. I now have reason to believe that the older Yossi was safe for a similar reason. The clue was the identity of the woman who pointed to the Chamber of the Platform. None of the other exploratory groups in the Palace had a young female member who remotely resembled Aileen and, when I thought about it, the woman had literally appeared in front of me. The corridor had been empty when I entered it but I had been too concerned about Yossi, then grateful for the woman’s help, to think about where she had come from. So who was she? Not a disguised High One, I hope. The one man who thought he saw a High One, by ingeniously but only temporarily adopting the Time Gate as a chronoscope, has since been too overawed to talk about it. Besides, I now know that we have an advantage even over the High Ones. I have listened carefully to Yossi’s remarks about her temporal journeys and have even overheard some of her conversations with her teenage selves.

Thus, I have pieced together an understanding of what my mysterious woman, Yossi’s “lady,” will do after she has made her last remembered visit to her younger selves. She, the adult Yossi, will travel into the far past and lead the human beings who became the High Ones and built the Palace. 

Acknowledgements

This story:

was inspired by a dream about my granddaughter, Yossi Clark;
incorporates her parents and grandparents as supporting characters;
derives its terminology and crucial aspects of its setting from Robert Heinlein’s classic science fiction story, “By His Bootstraps”;
pays tribute to another master of time travel literature, Poul Anderson,
and incorporates part of a phrase from Action Comics, no. 591, cover-dated August 1987, where, in an episode entitled “Past Imperfect,” scripted and drawn by John Byrne, Superman describes the return of a time machine into the time stream as “…more like the fading of a memory than the movement of a vessel!” (This memorable phrase stayed in my mind until February 1996, when I wrote about Yossi.)

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