The Theory and Practice of Time Travel

For my own convenience, and for the entertainment of any reader who wishes to revisit the issue, I here republish that part of my review of a time travel movie, where the theory and practice of time travel is discussed in detail. 

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Time travel, in order to be time travel properly so called, must allow the time traveler to change the past and to visit hence foreknow the future.

The whole appeal, the whole point, of time travel is wish fulfillment. The one thing we humans cannot do, and can never do, is change the past and know the future. Such stories are speculations, or logic puzzles, about what might happen if we could.

In human experience, the past is fixed and the future is in flux. Because the future is in flux, we cannot foreknow it, not with certainty. We can guess or glimpse the future, perhaps, through human reason or divine revelation, or see shadows of what may be: but this does not fulfill the wish to escape the constraints of the human condition, and to have tomorrow’s winning lottery numbers today. One needs no fancy time-travel machine to visit a future that might be possible: one can dream up possibilities, as many as might be wished, sitting in an armchair.

Time travel, in sum, is the wish to change what cannot be changed, and to know what cannot be known.

The paradox is that if the past can be changed, then so can the present and the future, since these are determined by the past; therefore everything can be changed; whereas if the future is fixed, so must be the present and past, since these determine the future; therefore nothing can be changed.

Hence the problem is that time travel stories can never carry out their speculation to a logical conclusion, because the logic leads to no conclusion. No logic means no story.

And from the point of view of storytelling, if the hero has a time machine, he can visit any past scene and undo the consequences, and revisit any future scene, to learn firsthand the results of his attempt, and see any unexpected surprises beforehand. He can consult his future self to learn what he will eventually decide.

Editing the past gives him infinite power, and visiting the future gives him infinite knowledge. But no one can write a drama starring an infinite hero who faces no challenges.

A story, to be a drama properly so called, and not simply a descriptive vignette, must have a character whose acts select between the options the plot offers him, and the story must have a plot that limits the character’s options of action. Even if the viewpoint character is a passive observer, a Professor Aronnax or a Dr. Watson, he must be observing the active protagonist, a Captain Nemo or a Sherlock Holmes, whose decisions drive the plot.

Philosophically speaking, a drama must be both determinist and indeterminist: determinist, because the plot must follow the constraint of the law of cause and effect; and indeterminist, because the character arc must follow from the liberty of the law of free will.

But if changing the past and foreknowing the future are open options in the story, drama is impossible: for no action has consequences if the hero can revisit any scene in his past and, armed with perfect foreknowledge, alter the outcome to suit himself.

In which case the story can have no plot, for a plot consists of the consequences following the character’s decisions and actions.

Likewise, foreknowing the future renders free will meaningless. In which case, once again, the story can have no characters, for a puppet whose act are predetermined is not a character, that is, not a character in a drama.

So, whatever the reality of determinism and indeterminism as a philosopher might debate the issue, as a matter of the practical craft of storytelling, a story must have a character whose indeterminate hence free choice has determinate consequences from which he is not free. Otherwise, there is no drama, no tale.

For these reasons, time travel stories are always a cheat.

Such a story can be a clever cheat, and an entertaining one, but it is still a cheat: because the promise of time travel has to be limited or hobbled to make the character lack infinite power and infinite foreknowledge.

The time traveler has to be allowed to change some past events, and not others, and foresee some future events, and not others, in order for him to be human enough and limited enough to face a challenge from which alone drama springs.

In other words, in order to tell a story about time travel, the hero has to be allowed to have a time machine, but not be allowed to use it.

So, time travel stories, by their nature, threaten to be as unworkable and unsatisfying as one that ends with the main character walking up back in the black-and-white world of Kansas and discovering that all was a dream.

Unless one learns some life changing moral lesson in the dream — such as that there is no place like home — having it all be a dream is a cheat. It means that all the dangers faced were no threat, and any victories were unreal. And Mrs. Gulch is still sending the sheriff tomorrow to have your dog destroyed.

I use this example on purpose, because it is a counter-example: WIZARD OF OZ is rightly one of the best beloved films of all time, and it has what should be an unworkable and unsatisfying ending, namely, that Oz does not exist and nothing done there was real. But the final plot twist in the final scene is, as we all recall, that the farmhands, friends, and helpful wandering magicians were there in Oz, merely disguised as fantasy. The point of the story was not to liquidate the witch, but to learn not to run away from home, which was, after all, the grave and selfish error at the beginning of the film that sent the protagonist into dreamland.

Thematically, is it not a cheat at all. The film has a theme that G. K. Chesterton could have penned: the point of the drama was not to gain brains, heart and courage for your friends, but to discover that, like they did, what Dorothy falsely thought she was lacking was with her always, found in her own backyard. In a sense, Dorothy is right when she says it is a really real place: Oz is Kansas, if seen rightly.

In other words, by brilliant craftsmanship, the filmmakers of Oz made an element of storytelling that normally, by rights, should and often does drain all drama from the work, work to advantage.

As with dream-dramas, the art of telling a time travel tale is the art of turning the element that, by rights, should drain all drama from the work, into an element that works to the story’s advantage.

Let us turn now to the theory and practice of time travel storytelling:

In terms of the theory, time travel is impossible as a matter of logic.

A simple thought experiment can show why: imagine the time traveler throwing the switch to turn on his time window which is focused one moment back in his past. If he sees his own past hand reaching to flip the switch, he reaches through the window and grabs the wrist, preventing the time window from opening, which prevents him from reaching back and grabbing his past wrist. If he sees his past wrist being grabbed, he reaches past the arm wrestling and flips the switch.

If the thought experiment involving arm wrestling is too messy, the same thought experiment can be set up with a spring-loaded power switch leading to the time machine, whose wires run one second backward in time. Place the time machine on top as a deadman switch. If the power is on, the time machine launches and disappears, releasing the spring and opening the switch, which cuts the power one second in the past, which prevents the time machine from launching. If the time machine does not launch, however, then the switch is closed and the power is on, and the time machine launches.

Or, if the idea needs to be simpler and more murderous, ask what happens if the Time Traveler’s first act is not to visit the far future year of AD 803702, but to go back before his father’s birth and kill his grandfather. This thought experiment is commonplace enough (at least, commonplace among those of us whose free time allows us to ponder and debate such folderol) that it has a standard name: The Grandfather Paradox.

Now, no doubt the alert reader has already thought of the three basic ways to build a world to avoid or elude the Grandfather Paradox, which would preserve the appearances: a fatalistic universe, where free will is illusion; a chaotic universe where cause and effect is illusion; and myriad branching universe where both are illusion.

A prime example of the first is ‘By His Bootstraps’ by Robert Heinlein; of the second is ‘The Men Who Murdered Mohammed’ by Alfred Bester; of the third is ‘The Man Who Folded Himself’ by David Gerrold.

Any number of stories utilize uncounted halfway explanations drifting somewhere between fatalism, acausal chaos, or multiversal myriadism, but doing so inconsistently. The number of inconsistent explanations is limited only by the imagination of the science fiction writer, an ilk justly famed for their imaginative powers.

And, of course, the most common option of all is to explain nothing, by not bringing the topic up.

This, then, is the practice of how to write a story that, by rights, should be impossible to write. Let us discuss each in turn.

First, fatalism:

In OEDIPUS REX, any attempt to deflect or change what is known to be inevitable turns out, due to a crucial truth hidden from the hero, to be the exact act which brings it about.

Here we see a flaw with my theory of time paradox drama: Oedipus lives in a fatalistic universe, but he performs the actions, most notably, killing a stranger in a quarrel and marrying a widowed queen, which bring about the curse on Thebes and encompasses his own downfall. But no one in his right mind could claim, as I have apparently done, that a play taking place in a fatalistic universe lacks drama.

This is the first and the oldest way to preserve drama even in a situation where fatalism mocks free will: namely, by keeping the characters ignorant of crucial information, such as who is whom.

In such stories, the drama comes from the protagonist slowly unwinding the riddle, and discovering, to his desolation, that he is not who he thought, and that the very act he took to flee fate condemned him to it.

In ‘By His Bootstraps’, mentioned above, Mr. Heinlein attempts awkwardly to do what Sophocles does masterfully.

Let us dwell on this fatalistic short story in detail, because it is the best of its kind, and therefore shows the flaws of its kind most clearly.

In part, the awkwardness of Heinlein’s attempt is because of an awkwardness that cannot be avoided. The Oracle of Delphi speaks in riddles, and so what part of fate the oracle hides from Oedipus is in the hands of the gods.  Whereas a time travel story, in order to create a similar effect, some blind but convenient coincidence must hide from the time traveler the crucial facts of a scene he himself walks through more than once.

In the case of ‘By His Bootstraps’, our protagonist is conveniently drunk as a skunk when he is visited by a time traveler, his future self, and so he does not recognize himself, and this blurs his memory just enough that he does not notice, or does not mind, saying the exact same lines, word for word, he heard his later self say earlier, last time around.

Number Two tries to convince Number One into stepping into the Time Gate, when Number Three, from yonder down the timestream, arrives to prevent it. Number Three and Two get into a fistfight, while Number Four calls on the phone, and Number One is conveniently kayoed and knocked into the Time Gate, and hilarity ensues.

Please note that neither the name, nor anything whatsoever, about the protagonist is memorable, or needs to be remembered, for this story. The appeal of the story is not character development. The appeal is to see the cleverness by which the writer forces the protagonist to replay scenes word for word, without making it seem forced.

Here, the protagonist in the first scene conveniently does not recognize his day-older self because strong drink blurs his vision, and then he is conveniently knocked out so that he conveniently does not see what happens, and so on. In the next scene, he conveniently does not recognize his decade-older self because he is wearing a beard. Lois Lane, call your office.

At one point, the older time traveler conveniently fails to tell the middle time traveler that the man he is meant to invite through the time gate is their mutual younger self. The reason given for his silence is, and I kid you not: ‘you would not believe me‘ — when quite obviously he would and must, or otherwise the whole exercise has no point.

The surprise twist at the end is that the original time traveler who sent the time traveler to teach the time traveler how to be a time traveler was the time traveler himself.

Heinlein works a similar conceit in ‘All You Zombies,’ except by having the hero become a heroine halfway through the plot twist, to marry himself and give birth to himself.

Again, no names needed. The characters are forgettable because their actions ultimately make not a bit of difference. They are puppets in the puppet show of fate: everything happens in an eternal cycle for no other reason than that so it happened last cycle, which is to say, for no reason at all.

When the character is struggling in vain against the inevitable, the drama is not in the outcome, which is foreordained, but in the act of discovering that the struggle is in vain.

The appeal of fatalistic stories, then, rests in watching the author execute a revelation.

The drama, if the writer seeks a tragedy like Sophocles, comes when the helpless hero has revealed to him that he himself is the accursed villain, incest and parricide, hence the source of the curse on his city he was trying to cure, and he blinds himself, rather than gaze again at the horrific truth — enacting, perhaps, a punishment on himself so dire that his city is spared.

The cleverness, if the writer seeks a happier ending, is when the hero has revealed to him that the villain he has been trying to outwit was himself, not a villain at all, but a benefactor, who merely was maneuvering him into position to restart the cycle of events leading to his happy fate.

It so happens that in ‘By His Bootstraps’ the forgettable protagonist conceives for no clear reason the ambition to usurp the position of the time traveler, once he learns time travel is possible, but then has no ambitions beyond that, so the story begins precisely where it ends.

Which I suppose is the point, but the drama is missing. The drama has been replaced by a crossword-puzzle cleverness, where one letter serves two words, or, in this case, one event seen by two different viewpoints by the same man at two different hours has two different meanings.

But the protagonist himself is remarkably lacking in any needs or desires that would drive his efforts. There is nothing he needs to do. Which is perhaps proper for a puppet of unchangeable fate.

Indeed, one of the points made and repeated in the final scene of ‘By His Bootstraps’ is that the posthuman humans of the far future as docile and tame as the eloi of H.G. Wells, so that a man of the twentieth century can make himself king merely by impressing the natives, who apparently neither toil nor spin, nor marry nor are given in marriage, nor study war no more.

Apparently all the man of the Twentieth Century needs are copies of books by Machiavelli, Hitler, and Dale Carnegie. Whether or not Heinlein is being tongue in cheek here, is an open question. That is he is being deeply cynical, is not.

Heinlein wisely sprints past the whole question of kingly conquest and administration of the A.D. 30,000. Less than a page is spent on this topic: more paragraphs are spent in a sophomorically shallow discussion of free will.

A.D. 30,000 is every sailor’s daydream of a tropical paradise where comely island girls in grass skirts bearing fruit bowls atop the head will serve your every pleasure, with no cannibalism or human sacrifice to mar the mood. It is the garden of Eden, but with no cherubim with flaming sword to bar the way, and becoming king requires no riddle-games with a deadly sphinx, nor even the effort of prying a magic sword out of a stone.

None of the natives whose land the protagonist conquers without the slightest effort even has a name, except the toothsome slavegirl for whom the protagonist conceives an instant and nonchalant lust. And even her name is bestowed by him.

She herself has less personality that an Playboy centerfold, who at least might have the dignity of being named after her month. And, of course, conveniently, he does not recognize her ten years later when she ripens to the age of nubile maidenhood as she was he first met her, despite giving her the same name.

She does not have a father to impress, nor any rival suitor. There is not even a rival bidder at any slave auction, nor a mention of running through the forest to capture her with a lasso. This is a fantasy slave girl, who wants to be owned by a guy with no particular distinct virtues, talents, or likeable traits.

In other words, there is no drama whatsoever, no love triangles, no struggle to capture the slaves, nor to free them, no fight with lusty barbarians, no plague victims to save with modern medicine, no dinosaurs to shoot with a modern elephant gun.

Again, it is all forced, strained, and absurdly convenient. None of the rigmarole of the plot was necessary for the plot.

The rational time traveler would have frankly and simply explained the foreordained future to his younger self when first they meet, for he has no reason to object nor to resist. The only reason why the time traveler does not act this frank and simple way, is because the author has him act as he acted the first time around, coy and mysterious, solely in order for there to be a mystery at all for the younger self to discover.

Ouroboros eats his tail because he ate his tail because he will eat his tail — and he will eat it because he did and shall. Round and round. It is an unsatisfying meal.

When Oedipus hears from an oracle of his evil fate, he departs from Corinth, where the couple who raised him reside, so that he cannot possibly murder one or wed the other. The drama is not when he quarrels with a stranger and kills him – in the Sophocles play, this event is not even on stage – the drama is when he discovers, years later, that this slain stranger was his true father – one who, years ago, upon hearing the selfsame oracle, had abandoned Oedipus as a baby to die in the wilderness, only to be later found and raised by the couple who are not, as it turns out, his parents.

Likewise, in a time travel story set in a fatalistic world, there is no real time travel. As said above, time travel means knowing the future and changing the past. But in such stories, the knowledge gained from visiting the future will always be a deception, because the hero’s ignorance drives the drama. Likewise, in a fatalistic world, the past, after all is said and done, cannot be changed, because nothing can be changed: all attempts to do so merely bring about the fate already foreknown.

To read a well-crafted and dramatic story about this same theme, that is, to see what being trapped in a cyclic life of eternal return would really mean to a human, and not merely as a clever logic puzzle, read ‘Fifth Head of Cerberus’ by Gene Wolfe. It is not a time travel story, but the theme concerns a man trapped in an ouroboros cycle of his own making.

Second, acausality, also called chaos:

Few stories opt for the chaotic, non-causal option, where time travel makes free will absolute, but time itself is discovered to have no continuity.

In the Bester story mentioned above, ‘The Men who Murdered Mohammed’ an enraged scientist sees his wife in the arms of another man, and in retaliation, invents time travel, returns to the past, and undoes the betrayal before the fact by preventing their births — only to return and find the current time unchanged.

Frustrated, he changes more and more of history in ever widening ripples, but can change nothing in the present. In that universe, no one’s past can be changed by time travel save the time traveler’s own, and, if he erases himself, he becomes a phantom. As a phantom, he can go anywhere in time, and do anything he likes, but it changes nothing.

And again, the lack of drama inherent to time travel is cured by the same means, namely, that the hero slowly discover, to his sardonic dismay, that he can do anything, but at the price of nothing meaning anything. As before, the drama springs from the act of lifting the veil of ignorance.

Finally, myriadism, also called multiversity:

Very commonplace is the myriad worlds option, where changes in the past create differing branches, and any future visited is merely one branch of many.

This solves the time travel paradox by making time travel be not really time travel.

Real time travel, as said above, is the ability to change the past and foreknow the future. In a branching multiverse, changing the past creates a new branch but leaves the old intact, so the change is not really a change, and visiting the future visits only one of many, so any foreknowledge is merely a likelihood.

This bears further comment. Logically, in a myriad branching universe, either the time traveler can return to his native branch, or cannot. Either the change changes him, or it does not.

Let us suppose Captain Ahab, time traveler, returns to warn his younger self not to seek the white whale, so that he never loses his leg to the jaws of the monster, hence never becomes obsessed with hunting Moby Dick.

Rejoicing, he returns to the current day, where there are two of him, him and his twin. His twin has two legs, while he is still a cripple. Nothing changes as far as his memory is concerned, even if the rest of the world changes.

His twin has a memory of a forewarning he heeded from a man who looked like his one-legged version of his father, and his long career as a vine-dresser and wine-bottler (or whatever career Ahab would seek if he never went to sea). But his twin is not him.

In effect, a tale where you visit a world not yours is much the same as a tale which ends by waking from a dream: in Frank Kapra’s IT’S A WONDERFUL LIFE, George Bailey discovers what the world would be like had he never been born, and the moral lesson is all that he keeps from that visit. Like Dorothy Gale waking up back in Kansas, the moral lesson is all that is needed. George does not recover the money purloined by Potter the evil banker, any more than Dorothy saves her dog from the coming sheriff.

Or if the laws of time travel allow Ahab to be changed by his own changes, there is only one of him, and he forgets his life as a cripple, and his trip as a time traveler, and, again, nothing is changed, as least so far as he remembers. As far as he is concerned, time travel can change nothing. This is the same, ultimately, as the fatalistic option.

Or if the laws of time travel allow Ahab to return to the original, unedited branch from which he came, again, nothing is changed, neither for him, nor anyone, so the Pequod is still sunk, and poor Pip is still dead. This is the same, ultimately, as the chaos option. He is free to do anything he wants, but it has no consequences.

And, in any case, if a myriad worlds story takes the conceit to a logical extreme, as it does, for example in David Gerrold’s MAN WHO FOLDED HIMSELF, every single decision, not just the convenient ones, would be open to be revisited, and a new branch formed, including the decision to make a branch, or to not make a branch.

No matter what you do, the opposite also happens, if you wanted it to, or ever will want it to.

As might be expected, the time traveler in Gerrold’s tale is recruited by his future self, given a time-traveling belt which otherwise has no origin and no explanation. The time traveler creates myriad multiple twins of himself by branching away into other timelines.

The story has no plot, as it is just an examination of how degraded the time traveler can make himself once he realizes the world has no point because no events have any consequences.

He does not actually have anything he needs to do with the timebelt, no problems to solve, neither his own nor those of anyone else. He has no family to share the secret with, as his parents and children are himself, as is his Uncle Jim, and he has no friends.

The time traveler edits history as an idle pastime, killing Hitler or Christ or saving John F. Kennedy, but the side effects of the changes displease him, so he changes them back.

In other words, he has godlike powers, but they are never used to feed the hungry, cure the sick, raise the dead, tell the truth.

It is grotesque to see such a wish fulfillment fantasy carried out to such an extent, described in novel-length detail, where the protagonist is so shallow and selfish that he has no wishes, aside from an interest in his own personal comfort and sexual adventures.

(But since the time traveler in Heinlein’s ‘By His Bootstraps’ which inspired this novel, himself, is motivated by an urge to abandon his shrewish fiancée to cuddle with a pulchritudinous pleasure-slave, Gerrold is following in his master’s footsteps.)

One of the time traveler’s myriad twins is from a timeline where he was born a girl, who becomes his lover, and she is the time traveler’s own mother, but who is driven away by visits from his older self, who is a creepy old greybeard trying vainly to recapture his lost first love. And another twin, more decadent, becomes his own homosexual lover, and yet another twin tries to prevent him from doing so. And so on.

The tale ends where it begins, with the time traveler giving his younger self not only the belt, but the manuscript which contains the journal which is, itself, the book the reader is reading.

It is really a rather dismal book. Read at your own risk.

A myriad worlds story, as has been said, is the worst of both worlds. As in a chaotic world, since a time traveler able to spawn and visit infinite timelines, he can visit none that make any difference; and as with a fatalist universe, nothing he does changes anything, since the main branch, unchanged, must always exist since the branch where no branches branched off is always an option.

So much for myriad worlds. We turn now to the halfway explanations.

The halfway explanations I have encountered are usually set in a myriad worlds background, but one where the number of time branches, or the available decision points (called ‘Jonbar hinges’ by older fans) are sharply limited. The reason for the limit differs from author to author.

A clever way out of the problem of too many branches, and one I prefer, is seen in ‘Timesweepers’ by Keith Laumer, later expanded into the novel DINOSAUR BEACH.

The time travelers in this tale spend their efforts, as any author of time travel stories could have predicted (but only Laumer did) in a futile labor attempting to undo the paradoxes, side-effects, and temporal litter left by previous generations of time travelers, to prevent the degradation to the timestream. Except no time traveler dare undo the accidents and paradoxes which led to his past, so care is needed.

In the Timesweeper universe, the laws of nature allow the time traveler to create branches of the time stream, but not all branches are created equal: the improbable ones, created by too much meddling in the time stream, simply dissipate, slaying the whole universe within it. Meanwhile, the act of making a new branch weakens the main stem by diverting hence dividing the energy of its current. Time travel, in other words, has a dire cost.

The drama here is that the whole river-delta structure of time itself is unsteady, doomed to ruin, but the time travelers cannot use more time travel to fix the problem, because time travel is the problem. Preventing a paradox by retroactively unmaking it before it begins is itself a paradox: patching the hole makes more holes.

I call this a halfway explanation, because cause and effect sort of exists in a river delta universe, as a spectrum running from certain to probable to improbable to impossible; and free will sort of exists, but the universe itself is an active entity, one where fate will try by coincidence to force the time traveler onto the path that creates the least damage.

As one might expect, the only way to save the universe in such a universe is to eliminate the time travelers by eliminating the invention of time travel, which requires the self-sacrifice of self-erasure.

Please note that Mr. Larry Niven, in his essay on the theory and practice of time travel, was adroit enough to coin this as a law of time travel, namely, if time travel can change the past, time travelers will eventually and inevitably erase themselves from existence.

If not done deliberately, as in ‘Timesweepers’, then accidentally, as nothing less will prevent unwise meddling in the past from leading to more unwise meddling, eventually snowballing to self-elimination.

I call the explanation of paradox in ‘Timesweepers’ and those in stories like them “halfway” because the writer uses part of the explanation to explain paradoxes away, but not consistently.

There is a little bit of fatalism, a little bit of coincidences happening for no reason, and a little bit of myriad time branches, but always the number of branches is restricted for one reason or another.

Another example of such halfway worldbuilding, and one that is much more stark, is the malicious universe of ‘Try to Change the Past’ by Fritz Leiber.

In that story, soldiers in the Change War between two rival factions of time travelers are recruited at the moment of their death. The protagonist, going AWOL, attempts to undo his murder before it happens, by pre-retroactively unloading or removing the murder weapon, and finds all his attempts are in vain. The same death wound reappears regardless, caused by a different cause. In the Change War universe, cause and effect proactively alter events to create coincidences to bring about the foreordained result, no matter how unlikely the coincidence needs to be.

In effect, the timestream in the Change War universe does form branches, but the universe prunes them immediately, using any excuse, no matter how farfetched, to do so.

A Change War universe, simply put, is ruled by a particularly iron force of fate. It is a halfway explanation, because fate uses coincidences to make events happen for no reason. Fate puts her thumb on the scales, so that it looks as if you can change the past and know the future, but, upon examination, no, not really.

Such a biased universe, however, is one that allows the writer to display his cleverness more clearly, particularly if Fate is more forgiving than she is in the Change War universe.

For purposes of drama, if the act of time travel itself has a side effect, or produces a pollution, such as encouraging the breakdown of all cause and effect, the time traveler has a time machine, but the cost of running it may make certain visits to the past or future, certain eras or events (the ones the writer wishes not to revisit with paradoxes) simply off limits.

This is a halfway explanation simply because it runs the risk of seeming arbitrary: the writer still needs to invent what would happen if the time traveler tried to kill his young grandfather, and mention it in the text, lest he endanger the reader’s willing suspension of disbelief.

In other words, if there is a cost to time travel itself, something has to happen when the bill comes due: and this will be one of the three options discussed earlier, namely, fatalism, or acausality, or myriadism, but with the added variation that someone or something, malign fate or the Time Police or your own grandson trying to keep his own past intact, will show up to hinder the time traveler.

A well-regarded “halfway” explanation is to have the time traveler be allowed to make all the changes he wishes, but Fate arranges coincidences to bring the new timeline back into parallel with the original, as if destiny had inertia, so that the time traveler can change the events, but the final result is largely the same.

So, for example, Fate in such a universe allows you to kill your grandfather in the year before he met your grandmother,  but then when she shows up, the only person able to fill the hole in time left open is you, who may well look and act much the same as grandpa, so Fate will trick or force you into that role.

So Fate will urge or demand you to commit incest you in with grandma, whom you apparently will not recognize until it is too late, and you will be your own grandpa, as in the famous song.

This is a nice and neat explanation, except supposing you are not a Robert Heinlein character, and do not want to marry your young and pretty grandma, there is no explanation of what happens if you shoot yourself, or her, or double back on your own timeline to interrupt the wedding.

Or, rather, the ingenuity of the author must again be invoked to come up with clever ways to undo everything the time traveler does, without making it obvious that he can do nothing of consequence.

I confess I rather like this explanation, because it reminds me of a Bible story: Esther is told that she must risk her life to save her people, because this is the task heaven gave her. But if she does not, heaven will find another way to accomplish the same end, and save the people by other means, but woe unto her, for she will not be spared.

In time travel story terms, this like a version of the Change War idea which is moralistic rather than nihilistic. Esther is being granted a role to play in the future events, which are foreordained, but her part in them is not foreordained, but, rather, depend on her willingness. Her actions still have moral consequences, which rebound on her.

However, again, this explanation of a biased fate, or fate with inertia, work better for prophecy than for time travel, because the question revisiting and redoing a failed outcome does not come up.

The biased fate explanation will work, provided it is not examined too closely.

If Fate is strict and malicious, as she is in Leiber’s story, any attempt to change the past is thwarted by a coincidence. The chain of cause and effect leading to the coincidence merely appears as needed, out of nowhere, for no reason. So the explanation preventing the grandfather paradox is another paradox: fatalism is saved by acausal chaos, that is, events arising from nothing.

But using a paradox to explain a paradox explains nothing. The Change War cannot take place in the Change War Universe, if it were consistent, because no soldiers on either side of the war can change the past. There is nothing to fight about and no way to fight.

It is an explanation that does not actually explain.

In any case, if Fate is granted the power to create events out of nowhere, deus ex machina, merely for the purpose of getting fate back on track, then she need only create one single event, and save herself all the bother of doing more:

By Niven’s law, far easier than creating a coincidence of astronomical proportions to recreate the exact murder wound without the murder weapon, the universe, if followed the path of least resistance, would merely kill the first inventor of time travel, and prune any branch where time travel is possible the moment it forms.

Again, in the far more forgiving universe of the film trilogy BACK TO THE FUTURE, when the time traveler is about to be erased due to events his changing events leading to his own birth, he is warned by seeing photos or fingers fade, and can take quick action to do deliberately what Fritz Leiber’s universe does automatically.

Likewise, the explanation does not explain. If McFly fades from existence in the middle of a Chuck Berry song, then, logically, he also fades, or will fade, or had faded, or will have had faded, from existence before he hopped into the time-traveling DeLorean, hence before he, by mishap, preventing his parents from meeting, and thus it never happened — or had never happened; or will have had never happened.

If consistent, self-cancelling paradoxes in the Back-to-the-Future universe would fade from existence and cancel themselves before they happened, and time travel would be perfectly safe. In which case, if McFly cannot change the past, he never did, and there is no movie.

Moreover, and if Doc Brown can zap McFly’s girlfriend Jennifer in 2015 with a flashy thing to stun her, he could have done this selfsame thing to older Biff from the future two minutes before Biff buys the almanac and joyrides in the time-car to give it to his younger self — whereupon older Biff, who has no memory of such a visit, should fade from existence, then or earlier, and not have had done it. Why can Biff change his own past, but not McFly?

And, again, applying Niven’s Law, we see that, in the timeline formed by Biff of 2015 using the time machine, not only would Doc Brown fade because he died before inventing the time machine, so would the time machine … in which case, time travel is never invented, and all is safe.

Until all the events of the first movie then fade, and the opening of the second, including the scene where Biff of 2015 uses the time machine. This is self cancelling interference of a type that cancels its own cancellation.

For better or worse, fun as they are, halfway explanations routinely break the rules of their own make-believe.

The ending of the third Back-to-the-Future film is a perfect case in point. Doc Brown reappears at the moment the time-traveling DeLorean is destroyed in a time-traveling steam locomotive. Doc shows that he is married his true love and has fathered cute children. Doc Brown cheerfully laughs and says “Your future has not been written yet — no one’s has. Your future is whatever you make it. So make it a good one!” The music swells. The locomotive flies off into the sunset. Applause.

It is a great scene. It is a wondrous scene. It is a rightly well-beloved scene.

It also makes no sense at all.

If the future is whatever you make it, then what was the point of the second movie, which was spent trying to undo a time paradox, so that the folly of time travel could be undone, and the whole point of why Doc Brown wanted the time machine destroyed before it destroyed the universe?

Why destroy a time traveling motorcar in order to build a time traveling locomotive? Nothing in the film indicates any event to bring about Doc Brown’s change of heart, nor is the audience told that the rules of time travel do not threaten universe-destroying paradoxes after all.

That wonderful end line was not earned by any of the events seen in the trilogy of films coming before it.

The real theme which really should have come from the story told was that of Doctor Faust or Frankenstein, namely, that there are mysteries into which science should not peer, lest you accidently prevent your parents from meeting, and erase yourself.

And even a harmless visit to 2015 can result in an unexpected catastrophe, so you can never return to your own year, but instead you return in the rain to see your own gravestone. But no one wants that grim ending and that grim theme to cap a lighthearted adventure comedy.

So, the writers spun up the Animaniac’s Wheel of Morality, and picked the theme and moral from another story, and bumperstickered it hastily on this one. Which is as it should be, because it preserved the mood of the work.

It must be said that while there are three consistent ways around the Grandfather Paradox, and countless inconsistent ways, there is also the fourth way around the paradox: which is, of course, is merely never to bring it up at all.

That is, perhaps, the best way.

For this reason, my objection to time travel stories does not extend to that great majority of them, which, to be frank, do not address any ramifications of time travel at all.

In THE TIME MACHINE of H. G. Wells, the titular time machine serves the same use as the twister in THE WONDERFUL WIZARD OF OZ by Baum, the spooky Apache cave in A PRINCESS OF MARS by Burroughs, or the titular wardrobe made from magic lumber sitting in the spare room in THE LION THE WITCH AND THE WARDROBE by C. S. Lewis.

Namely, the time machine in this story, and those inspired by it, serves is merely as the stage machinery meant to carry the protagonist to the scene of the action.

The machine needs to be stolen by Morlocks shortly upon landing. Or, it needs to be out of fuel, such that it must be hit by a lightning bolt before the flux capacitor has enough power to work again.

This is because the machine is not needed for the adventure to play out, and, indeed, if it were in working order, it would solve too many problems, perhaps all of them.

Otherwise, the time traveler can go backward to the hour before Weena is kidnapped by Morlocks, and escort her neatly out of harm’s way before they arrive; or he can visit himself a moment before he falls in front of his grandfather’s car, and prevent his young version from preventing his parents from meeting.

It should come as no surprise that, the most famous time traveler of all time, the nameless Doctor of DOCTOR WHO, rides in a broken time machine.

His blue police-callbox-shaped time travel machine is broken hence unreliable precisely so that once the Doctor and his companions are landed in the Court of Kublai Khan or beneath the Pyramids of Mars, no one can use the machine to go back a minute nor an hour nor a day, and warn of the ambush of Cybermen nor the treason of the Master in disguise.

Likewise, the whole point of Irwin Allen’s television show THE TIME TUNNEL was that, in every episode, Doug and Tony the time travelers had no control over in what year they landed, nor could they leave at will, much less go back a day and replay the scene. As with the TARDIS, the Time Tunnel was unreliable.

But the one nagging question that always hangs over such stories is precisely why, once the machine is fixed and working properly, the time travelers of that day and year do not return with a toolkit in hand to repair the current machine, or, better yet, give the current time travelers a lift backward and forward by an hour or a day, to time-travel them out of their messes.

Even asking such a question answers it: it would spoil the fun. Only a killjoy, some grumpy curmudgeon who hated time travel stories would even ask such a question. Such time machines are fated never to be repaired until the day the castaways from Gilligan’s Island are rescued.

Time travel adventure stories are meant to allow your Connecticut hero to outwit Merlin the Magician with an almanac predicting the next solar eclipse, or to defeat armored knights with a modern firearms and electrified barbed wire, while mocking English monarchy.

They are meant to allow your hero to visit Nostradamus, or Robin Hood, or Napoleon, or rescue a lovely cavegirl from a dinosaur, and so on.

Or to chase Jack the Ripper to modern day San Francisco.

Or to see how a township from West Virginia would fare if flung into the midst of the Thirty Years War of 1632.

Or to visit the remote far future and see the Darwinian results of the English class system carried to an absurd and hideous extreme.

In sum, the point of time travel adventure stories is to visit the landscape and people not within our reach. That the adventure is set in the Stone Age or the Court of King Arthur is no more barrier to the imagination than if set on Mars, or a world circling Arcturus.

No explanation of what would happen if the time traveler doubled back on himself and tried to change the past is offered in tales of this kind, simply because the question never comes up. Neither does the idea of using the time machine itself to solve the problem come up, depending on how quickly the thing is stolen by Morlocks, or is otherwise whisked offstage.

But whether a halfway explanation is offered, or no explanation is offered, overthinking the matter inevitably diminishes suspension of disbelief.

The explanation for this is not hard to find.

Time travel is as impossible as an Escher drawing. The staircase going up and going down cannot be the same staircase, nor can the mill wheel raise the water to fill the sluice to create the waterfall to turn the mill wheel. Escher drawings are cunning visual jokes that appeal the mathematically minded.

Despite disliking time paradox stories, there is one thing I like about time travel stories: It is the same thing I like about Escher drawings.

Given the logical constraint that the thing is starkly, simply impossible, what tricks of perspective and aspect can the artist employ to create the plausible illusion of the impossible being possible?

Certain themes and settings make the illusion of the impossible being possible more convincing. Establishing or, more to the point, not establishing the rules of time travel are part of the worldbuilding that can aid or hinder the illusion.