Yes, Time Travel is not very Atomic Rocket. The main reason this page is here is because if you start messing around with faster than light travel, you are going to be ambushed by time travel sooner or later. The second reason is that some theories about time travel grow out the fourth dimension. Thirdly one of my favorite atomic rocket science fiction authors when I was growing up was Lady Andre Norton, and she has written some pretty sharp time travel novels.

This is actually such a huge topic that it deserves its own website. But somebody else can do that.

Suggested reading:

Physicists hold exceedingly tight on to the concept of Causality. Probably because without it the entire structure of physics crumbles into flaming ruin and you can't get any work done. Causality mandates that causes must precede effects, or it becomes impossible to make predictions. That is why it is called Causality. Science in general and physics in particular is all about making predictions. Step two of the Scientific Method, in fact.

So physicist hate time travel with every existential fiber of their being because time travel can kill causality like a sledge hammer splattering a roach.

You see, time travel raises the possibility of creating a dreaded temporal paradox. And all it takes is one of those bad boys to blow causality right out of the freaking water. And remember that it is not just travel that can cause a temporal paradox, any transfer of mere information can be just as bad. This includes time-communicators (allowing one to talk to a person in the past or future) or a time-viewer (allowing one to see events in the past or future). So don't try to use a chronoscanner to see which horse will win at the racetrack or the time-police will show up and beat the snot out of you.

The only thing that makes physicists more angry is the fact that there appears to be nothing in physics that forbids time travel. And believe me, they have been looking real hard. More technically: the distinction between cause and effect is not made at the most fundamental level within the field of physics.

World-class physicist Dr. Stephen Hawking was so annoyed at this he formulated his Chronology protection conjecture, which holds that the laws of physics are such as to prevent time travel on all but submicroscopic scales. Why? Because time travel really cheeses him off, that's why.

As previously mentioned, this is also the reason physicists are so hostile to faster-than-light travel. As it turns out, Einstein's relativity shows that FTL travel and Time travel are two different terms for the exact same thing.

If a present-day scientist were confronted with a real time machine, he would certainly say that the machine had to be run by the rules of magic. His argument would go like this: Science is based on logic. Anything that produces logical paradoxes is not science. Time machines produce logical paradoxes. Therefore, if time machines exist, they must use magic, not science.

From INDISTINGUISHABLE FROM MAGIC by Robert Forward (1995)

Time Travel Theory

To travel back in time, you need to follow what is called a Closed Timelike Curve. This is a world-line that is a loop, instead the more conventional wavy vertical line. Ordinary world line start at a given point in space and time, then moves into the future like all normal things do. A closed timelike curve eventually doubles back, traveling into the past until it connects with itself.

In his The Theory and Practice of Time Travel, Larry Niven notes that the basic motivation for inventing time travel is the same motivation of a child fervently wishing that something had never happened. If they could only change history so that they had hit the baseball to the left instead of the right it wouldn't have broken Old Man Smith's picture window. It is the desire for a "do-over" in other words.

So the primary motive for time travel is to change the past. Unsurprisingly that is also its prime danger.

All of the current "real" time machines visualized as thought-experiments by real physicists share a common flaw: you cannot use a time machine to travel to a point in time prior to the construction of that particular time machine.

Notice that this time machine, like all the other time machines that are allowed by the Einstein Theory of Gravity, can only take you backward in time to the moment that the machine is turned on, and forward in time to the moment that the machine is turned off. Einstein's laws do not allow a future time-machine maker to go back into time to tell himself how to make the machine. Thus, at least one of the possible time-machine paradoxes is avoided.

From INDISTINGUISHABLE FROM MAGIC by Robert Forward (1995)

Larry Niven also makes a good case that in a given universe where time travel is possible, and using it to change the past is possible, the end result is that time travel will not be invented in that universe. Why? As he puts it:


GIVEN: That the universe of discourse permits both time travel and the changing of the past.
THEN: A time machine will not be invented in that universe.

For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past.

If we assume that there is no historical inertia, no Conservation of Events, then each change makes a whole new universe. Every trip into the past means that all the dice have to be thrown over again. Every least change changes all the history books, until by chance and endless change we reach a universe where there is no time machine invented, ever, by any species.

Then that universe would not change.

(ed note: in other words, the universe will keep changing until it reaches a stable state, and that state is one where no time machines are invented)

Now assume that there is an inertia to history; that the past tends to remain unchanged; that probabilities change to protect the fabric of events. What is the simplest change in history that will protect the past from interference?

Right. No time machines!



Mr. Niven also points out that time travel violates other laws of physics besides causality.

If your TARDIS travels from 3000 CE to 2000 CE, as far as the 2000 CE dwellers are concerned the TARDIS appears out of nowhere. Which is a drastic violation of the law of conservation of mass. The laws of the universe will be sternly unimpressed if you feebly protest that an equivalent tonnage of matter disappears a thousand years later. The laws will point out that for ten centuries there was an extra TARDIS around.

Sending a signal back in time is just as bad, only this time you are violating the law of conservation of energy. Which is to be expected since matter and energy are pretty much two aspects of the same thing.

And to top it off, physical time travel violates any law of motion, since motion always relates to time. Which has a ripple effect on conservation of momentum, statements about kinetic energy, plus any and all laws of gravity.

Admittedly these violated laws are of lesser importance than causality, but they are still nothing to sneeze at.

Time Travel Models

There are arguably five main theoretical models of time travel. There is no particular evidence for any of them, they are all theoretical.

No Timeline
AKA Presentism, Nowism. There is no past and future. The only thing that exists is the present movement. Obviously in this model time travel is impossible, thus it is immune to temporal paradox.
Fixed Timeline
AKA Fixed History, Determinism Eternalism, Block Time, Block Universe, Static interpretation of time, Four-dimensionalism. History cannot be changed by time travel (either past or future). Time travel is possible but all journeys have always been part of the timeline. There is no free will. Obviously immune to temporal paradox.
Self-Healing Timeline
AKA Elastic, Historical Inertia, Rubber-Band History, Plastic Time With High Resistance. This is in between Fixed Timeline and Dynamic Timeline. History can be changed by time travel, but due to inertia it tries to get back on its original track. Changes tend to be smoothed out. If a time traveler assassinated Hitler, some other similar leader would arise and World War 2 would happen anyway. This is sometimes called the "Law of Conservation of Reality." Danger of temporal paradox. The Novikov self-consistency principle may or may not apply (former is paradox-proof self-healing timeline).
Dynamic Timeline
AKA Overriding, Malleable, Plastic Time. History can be easily changed by time travel. This ranges on a spectrum between Temporal Balancing Act (where it is possible to make controlled changes and un-changes) and Temporal Chaos Theory {Chaotic Time} (where a traveler's mere presence causes zillions of uncontrolled changes per second, see Butterfly Effect). Danger of temporal paradox. The Novikov self-consistency principle may or may not apply (former is paradox-proof dynamic timeline).
Multiple Timelines
AKA Multiverse, Alternate Time lines, Parallel Universes, Cross-time, Branching Timelines. In the physics version, each time a time traveler changes history, the time line branches into the changed history line and the unchanged history line. In the literary version, each time any decision is made by any creature in the entire universe, the time line branches into one branch per possible outcome. Immune to temporal paradox

TV Tropes covers this at some length in its Temporal Mutability entry (aka The Sliding Scale Of How Easy It Is For Time Travelers To Change The Past, And Why).

My dear wife was confused when she saw The Time Traveler's Wife because she was a big fan of the Outlander series. In Outlander's version of time travel it is impossible for a person to travel in time and meet themself at a difference age. My wife was confused when this turned out not to be the case in The Time Traveler's Wife, it was breaking the rules. She didn't know that in fiction, the main ground-rules are one of the above five theoretical models. Other rules (like meeting yourself) are not as important, and are added at the whim of the author.

Actually, now that I think about it, the "impossible to meet yourself" rule was also in the movie Time Cop. Only it was not impossible, it would merely result in both version of yourself merging into a fleshy ball before imploding into a singularity, since two objects cannot occupy the same space in the same time. This is also a non-scientific rule added at the whim of the author.

No Timeline

AKA Presentism. There is no past and future. The only thing that exists is the present movement. Obviously in this model time travel is impossible, thus it is immune to temporal paradox.

How dull. In Clifford Simak's classic fix-up novel City, there is no room in the past or future for any past or future. That area is filled with parallel dimension worlds (the Cobbly worlds).

One world and then another, running like a chain. One world treading on the heels of another world that plodded just ahead. One world's tomorrow; another world's today. And yesterday is tomorrow and tomorrow is the past.

Except, there wasn't any past. No past, that was, except the figment of remembrance that flitted like a night-winged thing in the shadow of ones mind. No past that one could reach. No pictures painted on the wall of time. No film that one could run backward and see what once had been.

Joshua got up and shook himself, sat down and scratched a flea. Ichabod sat stiffly at the table, metal fingers tapping.

"It checks," the robot said. "There's nothing we can do about it. The factors check. We can't travel in the past."

"No," said Joshua.

"But," said Ichabod, "we know where the cobblies are."

"Yes," said Joshua, "we know where the cobblies are. And maybe we can reach them. Now we know the road to take."

One road was open, but another road was closed. Not closed, of course, for it had never been. For there wasn't any past, there never had been any, there wasn't room for one. Where there should have been a past there was another world.

Like two dogs walking in one another's tracks. One dog steps out and another dog steps in. Like along, endless row of ball-bearings running down a groove, almost touching, but not quite. Like the links of an endless chain running on a wheel with a billion billion sprockets.

From City by Clifford Simak (1952)

Fixed Timeline

AKA Fixed History, Determinism Eternalism, Block Time, Block Universe, Static interpretation of time, Four-dimensionalism. History cannot be changed by time travel (either past or future). Time travel is possible but all journeys have always been part of the timeline. There is no free will. Obviously immune to temporal paradox.

An easy fix to save Causality from an assassination attempt by a temporal paradox is to assert that it is impossible to use time travel to change history (aka the Post-selected model). Which is a big help, but no fun at all for science fiction authors (for one it kills off the do-over wish fulfilment aspect). And it also means all events are predetermined and you do not have any free will. Awkward, that.

If taken to extremes, it makes time travel very dangerous for the traveler. If they traveled into the past, raindrops would tear through their body like bullets because the traveler (and their body) could not alter the history of the raindrop's passage to the ground.

A fixed timeline is related to the concept of Block Time or the Block Universe. If the block extends from the past up to but stopping at the present, this is the Growing block universe. If the block extend from the past to the present and on to the future, this Eternalism.

What is Block Time? Here is an explanation from The Fourth Dimension: A Guided Tour of the Higher Universes by Rudy Rucker. Imagine that the fourth dimension is time (not the fourth spacial dimension). Since trying to visualize the fourth dimension will melt your brain, as an analogy people visualize two dimensional people living in a two dimensional space ("Flatland") and the third dimension is the flatlander's time dimension. A two-dimensional plane is the flatland universe, and the people are squares, triangles, and other geometric shapes sliding around.

Say Mr. Square is waiting somewhere, and around noon his father the triangle comes close for a second, then moves away. The block universe would look like Figure 1. The lower plane is 11:59, the middle plane is 12:00 and the upper plane is 12:01. Since Mr. Square is not moving his track through the time dimension (the third dimension in Flatland's case) is vertical. His father's track curves at angles because he is moving. In a way, it is like somebody made an overhead movie of flatland, took the film (ask your parents if you never hear of movie film), cut in into frames, and stacked them sequentially, as in Figure 2.

A larger view of Flatland's block universe would look like figure 3.

Note that from the view of a higher dimensional being (such as three-dimensional us), the block universe is static. This is why it works so well with the fixed timeline concept.

There is a temptation to visualize animating the block universe diagram, by illuminating successive slices to represent "now" (Figures 4 and 5). But this is incorrect, Dr. David Park calls this the "fallacy of the animated Minkowski diagram".

Author Richard Garfinkle wrote a science fiction novel All of an Instant, which is actually set around a block universe. It is just that entities who live outside of the block universe can ... alter it.

Once a time machine exists, then decisions made at one point on the time-line of an individual can affect not only the future of that individual, but also the past of that individual. Time machines will also raise philosophical questions. If you in the future had sent a message back in time to yourself in the past, then does the "you" in the future have any free will? For you know that you must and will send the message at the proper time in the future.

Yet, "free will" has always been limited by the laws of nature. For instance, if your past self has made the decision to jump off a bridge, your future self is bound by that decision. When time machines exist, your future decisions, in the same way, can bind your past self to the consequences of that "yet-to-be-made" decision.

From INDISTINGUISHABLE FROM MAGIC by Robert Forward (1995)

The oldest of all, going back to Greek times. Philosophers call it fatalism or determinism. A fatalist believes that everything that happens is predetermined to the end of time; that any attempt to change the predetermined future is fated, is a part of the predetermined future itself.

To a fatalist, the future looks exactly like the traditional picture of the past. Both are rigid, inflexible. The introduction of time travel would not alter the picture at all, for any attempt on the part of a time traveler to change the past has already been made, and is a part of the past.

Fatalism has been the basis for many a tale of a frantic time traveler caught in a web of circumstance such that every move he makes acts to bring about just the calamity he is trying to avert. The standard plot sketch is reminiscent of Oedipus Rex; when well done it has the same flavor of man heroically battling Fate and losing.

Notice how fatalism solves the Grandfather Paradox.

You can't kill your grandfather, because you didn't. You'll kill the wrong man if you try it; or your gun won't fire.

Fatalism ruins the wish-fulfillment aspect of time travel. Anything that averts the Grandfather Paradox will do that. The Grandfather Paradox is the wish-fulfillment aspect. Make it didn't happen.


(ed note: In the story, the United States has used a time machine to set up an operation one hundred and one million, three hundred twenty-seven thousand year in the past. About the middle of the Cretaceous period, Tyrannosaurus rex and all. The purpose is to ship petroleum through time into the the "present."

In the "present," the global political situation can be charitably described as "going to hell in a hand-basket", with global thermonuclear war looming. In the Cretaceous, Herries and Father Gonzales discuss the situation.)

      “Who’s denying us the chance?” asked Herries. “Just ourselves, H. Sapiens. Therefore I wonder if we really are able to do good.”
     “Don’t confuse sinfulness with damnation,” said the priest. “We have perhaps been unfortunate in our successes. And yet even our most menacing accomplishments have a kind of sublimity. The time projector, for example. If the minds able to shape such a thing in metal were only turned toward human problems, what could we not hope to do?”
     “But that’s my point,” said Herries. “We don’t do the high things. We do what’s trivial and evil so consistently that I wonder if it isn’t in our nature. Even this time travel business…more and more I’m coming to think there’s something fundamentally unhealthy about it. As if it’s an invention which only an ingrown mind would have made first.”

     Herries looked up into the steaming sky. A foul wind met his face. “There are stars above those clouds,” he said, “and most stars must have planets. I’ve not been told how the time projector works, but elementary differential calculus will show that travel into the past is equivalent to attaining, momentarily, an infinite velocity. In other words, the basic natural law which the projector uses is one which somehow goes beyond relativity theory. If a time projector is possible, so is a spaceship which can reach the stars in a matter of days, maybe of minutes or seconds. If we were sane, padre, we wouldn’t have been so anxious for a little organic grease (petroleum) and the little military advantage involved, that the first thing we did was go back into the dead past after it. No, we’d have invented that spaceship first, and gone out to the stars where there’s room to be free and to grow. The time projector would have come afterward, as a scientific research tool.”

     “Of course,” said the priest, “the problem is basically philosophical. Don’t laugh. You too were indulging in philosophy, and doubtless you think of yourself as an ordinary, unimaginative man. Your wildcatters may not have heard of Aristotle, but they are also thinking men in their way. My personal belief is that this heresy of a fixed, rigid time line lies at the root of their growing sorrowfulness, whether they know it or not.
     “Heresy?” The engineer lifted thick sandy brows. “It’s been proved. It’s the basis of the theory which showed how to build a projector: that much I do know. How could we be here at all, if the Mesozoic were not just as real as the Cenozoic? But if all time is coexistent, then all time must be fixed—unalterable—because every instant is the unchanging past of some other instant.”
     Father Gonzales said “But we are mortal men. And we have free will. The fixed-time concept need not, logically, produce fatalism; after all, Herries, man’s will is itself one of the links in the causal chain. I suspect that this irrational fatalism is an important reason why twentieth-century civilization is approaching suicide. If we think we know our future is unchangeable, if our every action is foreordained, if we are doomed already, what’s the use of trying? Why go through all the pain of thought, of seeking an answer and struggling to make others accept it? But if we really believed in ourselves, we would look for a solution, and find one.”

From WILDCAT by Poul Anderson (1958)

Self-Healing Timeline

AKA Elastic, Historical Inertia, Rubber-Band History, Plastic Time With High Resistance. This is in between Fixed Timeline and Dynamic Timeline. History can be changed by time travel, but due to inertia it tries to get back on its original track. Changes tend to be smoothed out. If a time traveler assassinated Hitler, some other similar leader would arise and World War 2 would happen anyway. This is sometimes called the "Law of Conservation of Reality." Danger of temporal paradox. The Novikov self-consistency principle may or may not apply (former is paradox-proof self-healing timeline).

At one end of the spectrum we have the free-will fun kind of time travel (Dynamic Timeline), where history can be altered but causality is toast. At the other end we have the deterministic no-free-will no-fun kind of time travel (Fixed Timeline), where history is unalterable but causality is safe and secure.

In between is a sort of compromise, where historical events have "inertia" or "hysteresis". A history conservation law so to speak. So if you go back in time and kill Hitler, some other German charismatic malcontent will take his place and World War 2 will occur on schedule anyway. The time traveler will really have to work at it to overcome the inertia leading to the war, just killing Hitler is not enough.

Between the deterministic and free will modes of time travel lies a kind of compromise position:

We assume a kind of inertia, or hysteresis effect, or special conservation law for time travel. The past resists change. Breaks in time tend to heal. Kill Charlemagne and someone will take his place, conquer his empire, mate with his wives, breed sons very like his. Changes will be minor and local.

Fritz Leiber used Conservation of Events to good effect in the Change War stories. In TRY AND CHANGE THE PAST, his protagonist went to enormous lengths to prevent a bullet from smashing through a man's head.

He was sincere. It was his own head. In the end he succeeded—and watched a bullet-sized meteorite smash into his alter-self's forehead.

Probabilities change to protect history. This is the safest form of time travel in that respect. But one does have to remember that the odds have changed.

Try to save {insert major historical figure here} with a submachine gun, and the gun will positively jam.

But if you did succeed in killing your own six-year-old grandfather, you would stand a good chance of taking his place. Conservation of Events requires someone to take his place; and everyone else is busy filling his own role. Except you, an extraneous figure from another time. Now Conservation of Events acts to protect you in your new role!

Besides, you're already carrying the old man's genes.


Dynamic Timeline

AKA Overriding, Malleable, Plastic Time. History can be easily changed by time travel. This ranges on a spectrum between Temporal Balancing Act (where it is possible to make controlled changes and un-changes) and Temporal Chaos Theory {Chaotic Time} (where a traveler's mere presence causes zillions of uncontrolled changes per second, see Butterfly Effect). Danger of temporal paradox. The Novikov self-consistency principle may or may not apply (former is paradox-proof dynamic timeline).

The way to get the most fun out of time travel is to accept it for what it is. Give up relativity and the conservation laws. Allow changes in the past and present and future, reversals in the order of cause and effect, effects whose cause never happens...

Fatalistic time travel also allows these causative loops, but they are always simple, closed 1oops with no missing parts. The appearance of a time machine somewhere always implies its disappearance somewhere—and somewhen else. But with this new, free will kind of time travel…

We assume that there is only one reality, one past and one future; but that it can be changed at will via the time machine. Cause and effect may loop toward the past; and sometimes a loop is pinched off, to vanish from the time stream. The traveler who kills his six-year-old grandfather eliminates the cause of himself, but he and his time machine remain-until someone else changes the past even further back.

RocketCat sez

After Robert Heinlein died, Jack Williamson took the mantle of "The Dean of Science Fiction". They sure got that right! Actually, Jack's 1931 novel Birth of a New Republic probably inspired Heinlein to write The Moon is a Harsh Mistress.

Williamson's early stuff was pure space opera, but even there he got the science right (he invented the word "Seetee" for antimatter). Williamson was quite aware of Heisenberg's uncertainty principle and used it accurately in quite a few novels. I was impressed with how Heisenberg could be applied to ghostly astral monsters in his Darker Than You Think, but applying it to time travel was sheer genius.

Somebody in Hollywood is missing a bet. Movies based on Williamson's The Legion of Space, After World's End, or The Legion of Time would be box-office gold. The Three Musketeers paired with Shakespeare's Falstaff with ray-guns, spaceships, cosmic horror, and a goddesss-like leading lady armed with the ultimate weapon; what's not to like? It would certainly show Star Wars how real space opera is done!


(ed note: Jack Williamson's time travel theory is a unique combination of multiple timelines and fixed timelines joined with Heisenberg's uncertainty principle. With the fallacy of the animated Minkowski diagram thrown in, just for fun.

In the novel there are two contradictory future time-lines. Only one can exist, so they are battling each other for survival. Basically each is trying to increase the probability of their timeline at the expense of their opponent. There is the Utopian future of Jonbar, with their ethereal virginal princess Lethonee, and the hideous nightmare of Gyronchi, with the evil but savagely beautiful Sorainya quote "clad in a gleaming crimson tunic of woven mail that swelled with her womanly curves" unquote. Scarlet woman indeed.

Nice usage of C. S. Lewis' "male heart haunted by a terrestrial and an infernal Venus" motif from The Screwtape Letters. You can see this in the Tarot major arcana card "The Lovers" in some decks with Paris of Troy standing between Helen and the goddess Aphrodite. Or a man torn between the virgin and the temptress, hand-in-hand with the virgin but simultaneously clutching at the temptress' skirt. Maybe even the barest hint of the Madonna–whore complex. But I digress.

Anyway, our hero Dennis Lanning is the only one who can thwart the sinister designs of Sorainya. He is visited by both ladies and draw into the conflict. He also discovers that the time war started due to the meddling of his old college roommate Wilmont McLan. Wilmont creates a time-viewer, which unbeknown to him damages the probability of Jonbar's existence. Sorainya beguiles Wilmont into creating a time machine so he can travel to meet her. Whereupon she captures him, torturing him for ten years while her scientists study the time machine. Finally Lethonee manages to engineer Wilmont's escape. He then recruits Dennis and other soldiers from different times for the fight to defeat Sorainya)

      (Wilmont McLan wrote) "To an external observer, gifted with four-dimensional senses, our quadraxial universe must appear complete, fixed, and forever unchanging. The sweep of time is no more than the hand of a subjective watch; it is no more than the intangible ray of consciousness, illuminating human experience. In any absolute sense, the events of yesterday and tomorrow are alike eternal, immutable as the structure of space itself."

(ed note: but in his latter book Wilmont McLan realizes that Heisenberg's uncertainty principle makes the future quite changeable.)

     "I'll try to tell you, Denny." Her (Lethonee) face was illuminated, like a shrine, by the shimmer of the jewel in her hands. "The world is a long corridor, from the beginning of existence to the end. Events are groups in a sculptured frieze that runs endlessly along the walls. And time is a lantern carried steadily through the hall, to illuminate the groups one by one. It is the light of awareness, the subjective reality of consciousness.
     "Again and again the corridor branches, for it is the museum of all that is possible. The bearer of the lantern may take one turning, or another. And always, many halls that might have been illuminated with reality are left forever in the dark.
     "My world of Jonbar is one such possible way. It leads through splendid halls, bright vistas that have no limit Gyronchi is another. But it is a barren track, through narrowing, ugly passages, that comes to a dead and useless end."
     The wide solemn eye of Lethonee looked at him, over the slumberous flame of the jewel. Lanning tensed and caught his breath, as if a light cold hand, from nowhere, had touched his shoulder.
     "You, Denny Lanning," she went on, "are destined, for a little time, to carry the lantern. Yours is the choice of reality. Neither I nor Sorainya can come to you, bodily—unless perhaps at the moment of your death. But, through a partial mastery of time, we can each call to you, begging you to carry the lamp into our different halls. Denny—" The silver voice caught with emotion.
     "Denny, think well before you choose. For your choice will bring life to one possible world. And it will leave another in the darkness, never to be born."

     "Don't forget, Sorainya," he (Denny) muttered. "I saw the shark."
     She tossed back her head, and her hair fell like a yellow torrent across the colored cushions. And the lure of her smile set a pain to throbbing in his throat.
     "The shark would have killed you, Denny. But you should know that death alone can bring you to me—and to the strong new life the gyrane gives. For our lives were cast far apart in the stream of time. And not all the power of the gyrane can lift you out of the time-stream, living—for then the whole current must be deflected. But the stream has little grasp upon a few dead pounds of clay. I can carry that to Glarath, to be returned to life."

     (Wilmont told Denny) "Mere probability is all we have left. And my first actual invention was a geodesic tracer, designed for probability analysis. It was a semi-mathematical instrument, essentially a refinement of the old harmonic analyzer. Tracing the possible world lines of material particles through time, it opened a window to futurity."
     The hoarse whisper paused, and old Wil McLan limped to the side of the dome. His scarred trembling hands lifted a black velvet cover from a rectangular block of some clear crystal mounted on the top of a metal cabinet.
     "Here is the chronoscope," he said. "A sort of window into time. It creates special fields, that bend radiation into the time-axis. We get a stereoscopic image in the crystal screen—there's a selective fluorescence to the beat frequencies projected from below."
     The old man snapped a switch, manipulated dials at the end of the crystal block. It lit with a cloudy green. The green cleared, and a low cry escaped Lanning's lips. Within the crystal, microscopically clear, he saw a new world in miniature.

     "It happened," the hoarse voiceless gasp went on, "that Gyronchi was the first future world, out of those possible, that the chronoscope revealed. Happened that I found Sorainya, splendid in her armor, fencing with one of her human ants.
     "You can see that she is—well, attractive. At first the range of the instrument was limited to her youth, where scenes of such barbarity are less frequent. Remember, Denny, I was thirty years younger when I first saw her, back in 1945. Her glorious beauty, the military pomp of her empire—I was swept away.
     "Neglecting all the other possible worlds, I followed her, for months—years. I didn't know, then, all the harm the temporal searchbeam was doing." His white head bowed; for a moment he was speechless. "But no process whatever can reveal the state of an electron without changing that state. The quanta of my scanning ray were absorbed by the atoms that refracted them. The result was an increase in the probability factor of Gyronchi—that is the root of all the tragedy."
     The scarred face made a grimace of pain.
     "The blame is mine. For, before I was aware of it, the absorption had cut down the probability of all other possible worlds, so that Gyronchi was the only one the limited power of my instrument could reach. That blinded me to the crime that I was doing.

     (Wilmont said) There is a terrific resistance to the displacement of any body in time. For the geodesies are anchored in the future, as well as in the past. The removal of a living person, which might warp all futurity, is impossible. And even to dislodge inert matter requires tremendous power.
     "Nothing less than atomic energy, I soon perceived, could even begin to overcome that resistance. I set out, therefore, with the searching ray of the chronoscope, to study the atomic science of the future. But there I met a curious difficulty…
     "For the instrument, which, after all, can only trace out probabilities, sometimes queerly blurred the fine detail of script or printing. Los Alamos and the Kremlin were equally open to the searching beam. I studied the works of many future scientists—of John Barr and Ivor Gyros and many more. But essential words always faded.
     "There is a law of sequence and progression, I found at last, operating along a fifth rather than the temporal dimension, which imposes inexorable limits. It is that progression which actually creates reality out of possibility. And it is that higher law which prohibits all. the trite absurdities met with in the old speculation about travel in time, such as the adventurer in time who returns to kill himself. The familiar logic of cause and effect is not abolished, but simply advanced to a higher dimension.

     "I went alone, (foolishly using his time machine to travel to Sorainya)" Wil McLan looked back to him, with hollow, haunted eyes. "For the Chronion, with all her millions of horsepower, could not have drawn a crew of sound men from their places in time. Even alone, I had difficulty. An overloaded field coil burned out. The laboratory caught fire, and I was badly injured. The very accident, however, so weakened my future geodesies that the time-drive could pull me out. At the very instant the burning building collapsed, we broke free into the tune stream."

     (after ten years in Sorainya torture chambers, a projection of Lethonee helps Wilmont escape) "With seconds to spare, I got aboard the Chronion, started the converters, and escaped into time. I returned to the early twentieth century (that point in time where the time-lines branch into Jonbar and Gyronchi). And then at last, guided by Lethonee down the fainter geodesies of her possible world, I came to Jonbar."
     "Jonbar—" Lanning interrupted again, with a quick gesture at the crystal block of the chronoscope. "Can we see Jonbar, in that? And—Lethonee?"
     Very gravely, Wil McLan shook his white, haggard head.
     "Presently, we shall try," he whispered. "But the probability factor of Jonbar has become so small that I can reach it only with the utmost power of the scanning beam, and then the images are very poor. For Jonbar is at the brink of doom."
     "But there is still one chance." A stern light flashed in his hollowed eyes. "Jonbar hasn't given up. It was Lethonee's father, an archeologist digging in the Rockies where my laboratory used to be, who found there the charred books and age-rusted mechanisms from which he rediscovered the secret of time.
     "He made the time crystal. With it, Lethonee soon discovered the menace born of my unwitting tampering with probability. And she brought me to Jonbar to aid the defense. That is why I have been gathering up you and your men, Denny."
     Lanning was staring at him, frowning.
     "I don't understand," he muttered. "What can we do?"
     "These two possible worlds, each armed with the secret of time, are fighting for survival." A fierce glint burned in the old man's eyes. "Either Jonbar or Gyronchi—either Lethonee or Sorainya—may exist. But not both. The battle is on, all along the front of time. The outcome will be fixed by that higher progression, in the fifth dimension."
     "But you can see the future," broke in Lanning. "Can't you tell?"
     "The chronoscope reveals no certainties," said McLan. "Only probabilities—which it changes even as it reveals them." His white head shook. "I know, though, that the balance of probability is far in favor of Sorainya."

     "But we can help?" he demanded. "What is our part?"
     "No direct geodesies link Jonbar and Gyronchi," explained McLan. "Therefore they have no common reality. They are contradictory. They can explore each other's trains of probability. But there can be no physical contact, because the existence of each is a denial of the other. Their forces, therefore, can never come directly to grips.
     "Our contemporary world, however, joined by direct geodesies with all possible futurities, has a common existence with both Lethonee and Sorainya. That's how you get into the picture, Denny."
     "Huh?" Lanning leaned forward desperately. "They both talked of destiny. You can tell me what they meant?"
     "You are in the key position, Denny," breathed McLan. "Fate has made you the champion of Jonbar. Your triumph alone can save it. If you fail, it is lost."
     "And that's why they came to me?"
     "Sorainya has sought to cause your death." The old man nodded. "To carry you to Gyronchi, where your aid would insure her victory. And Lethonee took it upon herself to watch over you, until the moment we could pull you aboard the Chronion."
     "Death..." Lanning whispered the echo. "Then we are—dead?"
     "I came back to find you and a band of your contemporaries, to serve Jonbar. Since it is impossible to draw a sound, living man from his place in time—to do so might wrap the whole continuum—we had to wait until the moment when each of you was actually dead, to draw you aboard through the temporal field. Jonbar has provided a corps of surgeons, who were able to revive you immediately."

     Wil McLan had been collecting weapons. There were a dozen Mauser rifles, two dozen Luger pistols, four crated machine guns, several boxes of hand grenades, and a hundred thousand rounds of assorted ammunition, that all had come, along with a stock of food and a few medical supplies, from a sinking munitions ship.
     "The first precaution," McLan told him. "We located a torpedoed ship, when we first came back from Jonbar, to collect supplies and arms—and test our technique of recovery. Weapons from Jonbar, you see, wouldn't function against targets from Gyronchi."
     Since McLan's helpers from Jonbar would be unable to enter Gyronchi, Lanning detailed Clark, Barinin, and Willie Rand as a crew for the Chronion, and himself learned something of her navigation, as the time ship drove steadily down the geodesies of Jonbar. The hydrogen converter throbbed endlessly beneath the deck, but Wil McLan seemed disheartened with their progress.
     "The world we seek is now all but impossible," he rasped. "The full power of the field drives us forward very slowly. And at any instant the geodesies of Jonbar may break, for they are weak enough already, and leave us—nowhere!"

     "We are already doing all that can be done," he said. "The geodesies of Jonbar are like microscopic wires drawn out thinner and thinner by the attenuation of probability. If the tracer loses them, or if they snap, Jonbar is—lost!"

     (Our Heroes arrive at Jonbar and Lethonee) "Gyronchi has carried out some new attack," she (Lethonee) told him. "The dynon (the future race that springs from Jonbar) tried to bring a warning from the future, but they were cut off. Now the time crystal shows no future at all, beyond tonight. This is the last possible night for Jonbar. Unless—"
     Her haunted eyes clung desperately to Lanning's face.
     "Unless the tide of probability is changed."

     "How can you be—not real?" Lanning stood gazing at her quiet loveliness, framed against the terrace garden. "What's the difference between reality and—such a seeming as you are?"
     She hesitated, with a little frown of thought.
     "There is a flow from probability to certainty, along the fifth dimension," she explained. "Probabilities are infinite, but there is only one reality. Many conflicting futures are possible, but the past is simple and complete! The geodesies branch at each point of uncertainty, but the flow of realization must always take one branch and obliterate the rest. All the geodesies tend to absorb energy; all possible worlds strive for reality. But the energy of probability must always be withdrawn again from all those other worlds that might have been, to create the single one that can be. All the rest must vanish, as their probability fades to zero."
     "And Jonbar is—vanishing?"
     She nodded. "It—and I. We were given creation by the atomic power of the Chronion, bringing you down the geodesies. We are only an illusion of possibility, the reflection of what may be—a reflection that is doomed."

     (Our Heroes escape from Jonbar just as it vanishes from probability, and travel back to the present in order to save the day. Wilmont explains Gyronchi latest attack on Jonbar) "They went back into the past," said the voiceless-man. "Back to the turning point of probability. They found something there—it must have been a small material object, although we got no glimpse of it—which was the very foundation of Jonbar. Using gyrane power, they wrenched the thing, whatever it was, out of its place in time. The broken geodesies cut off the possibility of Jonbar."
     "What became of this object?"
     "They kept it concealed. And they carried it back to Gyronchi. It is guarded, there, in Sorainya's fortress."
     "Guarded?" Lanning echoed. His fingers twisted together in a sudden agony of hope, and his eyes rose to search McLan's wealed face. "Then if we took it—carried it back—would that help Jonbar?"
     "Yes." The bent white head moved to a tiny nod. "If we could recover the object, if we could discover where they found it, in space and time, if we could put it back there, if we could prevent Sorainya from disturbing it again until the turning point has passed in the fifth dimension—then Jonbar would again be possible."

     (Wilmont discovers the key change in time that will decide which future will occur, Jonbar or Gyronchi) "The time is an afternoon in August of the year 1921," whispered Wil McLan. "The broken geodesies of Jonbar had already given us a clue to that. Now I have found the place, with the search beam."
     Lanning gripped his arm. "Where?"
     "It's a little valley in the Ozarks of Arkansas. I'll show you the decisive scene."
     McLan limped to the metal cabinet of the geodesic analyzer. His broken fingers set its dials. A greenish luminescence filled the crystal block, and cleared. Lanning bent forward eagerly, looking into that strange window of probability. An eroded farm, folded in the low and ancient hills. A sagging paintless shack, a broken window gaping and the roof inadequately patched with rusty tin. A rocky cow pasture, its steep slopes scantily covered with useless brush. A small freckled boy in faded overalls and a big ragged straw hat, trudging slowly barefoot down the slope, accompanied by a gaunt yellow dog, driving two lean red-spotted cows home to the milking pen.
     "Watch him," whispered Wil McLan urgently.
     As Lanning watched, the boy stopped to encourage his dog digging furiously after a rabbit. He squatted to observe a colony of ants. He ran to catch a gaudy butterfly, and carefully dissected it with a broken pocket knife. He rose unwillingly to answer the calls of a slatternly woman from the house below, and ambled after the cows again. Wil McLan's gnarled fingers closed on Lanning's arm, urgently.
     The boy paused over something beside a sumac bush, and stopped to pick it up. The object blurred oddly in the crystal screen, so that Lanning could not distinguish it. The scene was erased, as Wil McLan snapped off the mechanism.
     "Well?" Lanning turned to him, in bewilderment. "What has that to do with Jonbar?"
     "That is John Barr," said the voiceless man. "For that metropolis of future possibility will be—or may be—named for him. He is twelve years old in 1921, barefoot son of a tenant farmer. You saw him at the turning point of his life—and the life of the world."
     "But I don't understand!"
     "The geodesies diverge from the thing he stoops to pick up," whispered Wil McLan. "It is either the magnet that we recovered from Sorainya's citadel—or else only an oddly colored pebble that lies beside it. That small choice—which Sorainya sought to decide by removing the magnet—determines which one of two possible John Barrs is to be ultimately established in reality."
     "Just a scrap of iron," Lanning said.
     "The seed of Jonbar," answered McLan. "If he picks up the discarded magnet, he'll discover the mysterious attraction it has for the blade of his knife, and the strange north-seeking power of its poles. He'll wonder, experiment, theorize. His curiosity will deepen. The scientist will be born in him.

     "But at last, in 1980, a tired but triumphant little man of seventy-one, he'll publish his great discovery. The dynatomic tensors—soon shortened to dynat. A totally new law of nature, linking life and mind to atomic probability. I had stumbled on one phase of it, with the hydrogen converter. But his tensors will open up a tremendous new technology for the direct release of atomic energy, under full control of the human will.

     "If we're unable to replace the magnet," McLan whispered again, "the boy John Barr will pick up the pebble instead, and the tide of probability will be turned—as, indeed, it is turned—toward Gyronchi. The boy will toss the pebble in his hand, and throw it in his sling to kill a singing bird. All his life thereafter will want a precious spark. It will remain curiously similar, yet significantly different.
     "John Barr, in this outcome also, will run away from his father's home, but now to become a shiftless migratory worker. He will marry the same woman, raise the same two children, and leave them at last. The same ingenuity, turned to the same basic problems of probability, will lead him to invent a new gambling device, on which he will make and lose a fortune. He will die, equally penniless, in the same year, and lie at last in the same graveyard.
     "The secret of mentally released atomic power will now be discovered nine years later, but with a control far less complete than John Barr would have attained. The discoverer will be one Ivor Gyros, an exiled engineer from Soviet Eurasia, working with a renegade Buddhist priest. Calling their half-mastered secret the gyrane, the two will guard it selfishly, use it to destroy their enemies and impress the superstitious. They'll establish a fanatical new religion, and a new despotic empire. That's the beginning of the cult of the gyrane, and Sorainya's dark dynasty. You have seen the end of them."

From THE LEGION OF TIME by Jack Williamson (1952)

“I was just getting up from the bench, to go back to the room and get my old automatic, when I—remembered!

“That’s the only word I know. Memory. It seems a little strange, though, to speak of remembering things that haven’t happened yet. That won’t happen, some of them, for a thousand years and more. But there’s no other word.

“I’ve talked to scientists about it, Doctor. A psychologist, first. A behaviorist. He laughed. It didn’t fit in, he said, with the concepts of behaviorism. A man, he said, is just a machine. Everything a man does is just mechanical reaction to stimulus.

“But, if that’s so, there are stimuli that the behaviorists have never found.

“There was another man who didn’t laugh. A physicist from Oxford, a lecturer on Einstein—relativity. He didn’t laugh. He seemed to believe what I told him. He asked questions about my— memories. But there wasn’t much I could tell him, then.

“What he told me helped to ease my mind—the thing had had me worried. I wanted to talk about it to you, Doctor. But we were just getting to be good chess companions, and I didn’t want you to think me too odd.

“Anyhow, this Oxford man told me that Space and Time aren’t real, apart. And they aren’t really different. They fade one into the other all around us. He spoke of the continuum and two-way time and a theory of the serial universe. I didn’t understand it all. But there’s no real reason, he said, why we shouldn’t remember the future—all of us. In theory, he said, our minds ought to be able to trace world-lines into the future, just as easily as into the past.

“Hunches and premonitions and dreams, he believed, are sometimes real memories of things yet to come. I didn’t understand all he said, but he did convince me that the thing wasn’t—well, insanity. I had been afraid, Doctor.

“He wanted to know more about what I—remembered. But that was years ago. It was just scattered impressions, then, most of them vague and confused. It’s a power, I think, that most people have to some degree—it simply happens to be better developed in me. I’ve always had hunches, some vague sense to warn me of danger— which is probably why I’m still alive. But the first clear memory of the future came that day in the park. And it was many months before I could call them up at will.

From THE LEGION OF SPACE by Jack Williamson (1934)

ON October 22 and 29, Mr. J. W. Dunne—in whose book, “An Experiment with Time”, published a few years ago, evidence of apparent prevision of future events was presented, with a suggested explanation in terms of the character of the time concept—gave two lectures before the Royal College of Science Mathematical and Physical Society on “The Serial Universe”.

Mr. Dunne described the nature of a ‘regress’, in which every term except the first is defined by its relation to the preceding and following terms and which therefore produces an infinite series. He showed that if, in the traditional manner of physics, we regard the scientific description of the world as being necessarily based on the exploration of an objective system with independently existing instruments of observation, we are compelled to employ a concept of time which is regressive, though it has not hitherto been so recognised. Such a concept is adapted to our reasoning powers because we are self-conscious beings, and self-consciousness itself is essentially regressive.

The difficulties of modern physics have arisen because attention has been concentrated on the first term only of the temporal regress, which lacks the vital double character of the succeeding terms. Mr. Dunne very acutely applied his ideas to the problems of relativity—attributing the appearance of ‘imaginary’ time in the Minkowski world to the rotation of the axis of second-term time though 90° into coincidence with that of first-term time—and to the quantum theory, in which the ‘uncertainty’ of Heisenberg’s principle was found to be regressive and located in the instruments of observation instead of the world observed, which remained determinate.

The substance of the lectures, considerably amplified, is to appear almost immediately in book form.

From THE SERIAL UNIVERSE Nature 134, 729-729 (10 November 1934)

Multiple Timelines

AKA Multiverse, Alternate Time lines, Parallel Universes, Cross-time, Branching Timelines. In the physics version, each time a time traveler changes history, the time line branches into the changed history line and the unchanged history line. In the literary version, each time any decision is made by any creature in the entire universe, the time line branches into one branch per possible outcome. Immune to temporal paradox

Also called "Parallel Universes" or "Cross-time". In his Discworld novels Terry Pratchett calls it the "trousers of time" theory.

These can be found in Andre Norton's Crosstime novels and "All The Myriad Ways" by Larry Niven.

In many science fiction novels, a new time line is spawned every time a decision is made. In many attempts by physicists to protect causality, a new time line is only spawned when some meddling time traveler alters history (the theory of parallel universes).

Known as the theory of multiple time tracks.

Let there be a myriad of realities, of universes. For every decision made by any form of life, let it be made both ways; or in all possible wars if there are more then two choices. Let universes be created with every choice.

Then conservation of matter and energy holds only for the universe of universes. One can move time machines from one universe to another.

You've got to admit it's flamboyant!

You still can't visit the past. But you can find a universe where things happened more slowly; where Napoleon is about to fight Waterloo, or Nero is about to ascend the throne. Or, instead of changing the past, you need only seek out the universe where the past you want is the one that happened. The universe you want unquestionably exists. (Though you may search a long, weary time before you find it.)


Temporal Paradoxes

A logical paradox is a statement which is both true and false at the same time. Example: "This statement is false". A temporal paradox is when that happens with time travel. Logical paradoxes strike at the heart of logic, while temporal paradoxes strike at the heart of causality and physics.

There are more or less three major classes of events that are commonly called Temporal Paradoxes.

Actually the Grandfather Paradox is the only full-blown causality-destroying paradox. The other two are not paradoxes, they are just terrifically strange.

There are a couple of ways to avoid temporal paradoxes.

In quite a few science fiction stories, dealing with a temporal paradox is pretty much the entire plot.

Grandfather Paradox

The classic Time-travel paradox is the so-called "Grandfather paradox" (though it actually should be called the "Grandmother paradox"). The technical term is "inconsistent causal loop." This is where a future event prevents an event in the past by its backward influence on the past event. Remember that bootstrap paradoxes are when a future event causes an event in the past.

Boris Badenov sneaks into Mr. Peabody's Wayback Machine (actually the WABAC machine, but who cares?) and travels back in time to when Boris' grandfather was a baby. Boris then gives his infant grandfather a lit stick of dynamite then cackles evilly as his grandfather is blown to bits. Bah-hah-hah!

But wait! Boris' grandfather is now smithereens, he'll never grow up, beget Boris' father, who will beget Boris. In other words, Boris will never exist.

But if Boris never exists, then he will never travel back in time to assassinate his grandfather. In which case grand-pop will beget Poppa Boris, who will beget Boris. Who will then proceed to assassinate his grandfather. Start back at the beginning and repeat.

Does Boris' grandfather get blown up? Both Yes and No! A paradox.

Boris says: "Natasha, next time I get fiendish plan, do me big favor? Sharrup my mouth!"

You can tighten this up a bit by having the time traveler going back in time to murder their infant self instead of their grandfather. The technical term is autoinfanticide.

The real key factor in the paradox is not the grandfather, it is the presence of something that eliminates the cause or means of traveling back in time.

Example of eliminating the cause: the Hitler's murder paradox. A common time-travel trope is traveling back in time to murder Hitler before he starts World War 2. The trouble is, if one succeeds, the time traveler will have grown up in a world where Hitler and World War 2 never existed. So the time traveler has no cause or reason to travel back in time to murder some guy he never heard of in order to stop a war that never happened. Which means Hitler was not assassinated, and the paradox is upon us.

Example of eliminating the means: The Self-Destructing Time Machine paradox. Your grandfather invented and build a time machine. You hate the old man, but if you kill him you'll wind up to be the prime suspect. So you steal his time machine and go back in time. If you kill him at a point in time before you were born, obviously the police will never suspect a person who hasn't been conceived yet. Unfortunately, since you are long on anger and short on brains, you neglected to take into account the fact that you will be killing grandpaw before he invents and builds the time machine (i.e, the means for traveling in time). The temporal paradox sanctions Causality with extreme prejudice as the universe tries to figure out where the heck the time machine came from.

The basic Grandfather Paradox is a type of "eliminating the means." It is just that the "means" is not the time machine, the "means" is the time-traveler.

Another thing to keep in mind is that paradoxes do not require somebody to travel back in time. You can make perfectly good causality-destroying paradoxes by just sending information back in time. Send a message to your future self asking it to send a message to your past self telling you not to send any messages in the present.

Bootstrap Paradox

A Causal Loop is where an event in the future causes an event in the past, through time travel. More simply it is where an effect is its own cause.

It is also known as a bootstrap paradox, ontological paradox or a closed time loop. It is very popular in science fiction stories.

This upsets physicists since Causality says it should be cause and effect NOT effect and cause.

The classic science fiction bootstrap stories are Heinlein's By His Bootstraps and All You Zombies.

It really becomes a full blown paradox when you take an object's beginning and ending then merging them so that the object's world-line becomes a world-circle. You wind up with an object that has no origin or ending. Though really this is not a paradox so much as it is just very very weird.

Example. The pocket-watch from the movie Somewhere In Time.

  1. In 1971 the young Richard Collier is approached by an elderly lady. The lady gives him a pocket watch and says "Come Back To Me."
  2. Richard keeps the watch, and is unaware that the lady dies that night.
  3. In 1979 Richard is a playwright, but has writers block. He goes to a charming Grand Hotel.
  4. At the hotel, in its museum, he falls in love with a photo of Elise McKenna.
  5. Doing some research, he is shocked to discover that Elise McKenna was the elderly lady who gave him the watch.
  6. Richard manages to travel back in time to 1912, where he and Miss McKenna fall in love.
  7. Richard gives Miss McKenna the watch

The paradox is: when was the watch constructed? It has no beginning and no end.

Another example is SkyNet from the Terminator movies. In the future, the evil SkyNet supercomputer creates Terminator robots to exterminate mankind. It sends a T-800 Terminator back in time to kill Sarah Connor. Even though the T-800 is stamped flat in a hydraulic press, enough of it is left for scientists to examine. They discover computer techniques that humans would never had figured out on their own.

And of course they use these techniques to create ... SkyNet. So where did these technique come from? The same place as Elise McKenna's pocket watch.

This works with ideas and theories as well. Example: Floyd studies Einstein's relativity in college, basically learning the theory from Einstein. Floyd then uses a time machine to go visit Einstein in 1890. Floyd then proceeds to teach the young Einstein the theory of relativity. Einstein taught Floyd who then taught Einstein. Who invented the theory in the first place?

TV Tropes calls this Stable Time Loop.

Predestination Paradox

This is not so much a paradox as it is an unfortunate feature of time travel in a universe run by Fixed Timeline or drastically Self-Healing Timelines.

It is vaguely related to the Bootstrap Paradox, and also to a self-fulfilling prophecy. It happens when a time traveler is motivated to travel into the past to stop an awful event from happening. But when they try, it turns out that the time traveler is the one that caused the event in the first place.

If they had not time traveled in the first place, there would not have been an awful event in the first place. Thus then there would not have been any motivation to time travel, because nobody would time travel to stop something that never happened.

Determinism is really depressing sometimes.

TV Tropes calls this You Can't Fight Fate.

Avoiding Paradoxes

The way to keep physicists happy is to postulate a law of science that somehow makes temporal paradoxes impossible.

Jason Hinson shows there are four ways of enforcing a "no-paradox" rule for time travel. Parallel Universes, Consistency Protection, Restricted Space-Time Areas, and Special Frames. In some ways Special Frames is the best, though it directly contradicts part of Relativity (the first postulate of special relativity is that there are no special frames, "no privileged inertial frames of reference"). Oh well. For details, you'd best read the Hinson article.

The latter three are examples of the Novikov self-consistency principle. It states that: "The only solutions to the laws of physics that can occur locally in the real universe are those which are globally self consistent." In other words if you try to use a time machine to make a paradox, the universe will fix things so you automatically fail. This is sometimes summarized as "you can't change recorded history." Note this implies that the Time Protection Corpstm will go out of their way to not record or observe unknown parts of history, in order to give the Corps some wiggle room on their missions.

It seems to me that a time machine ruled by the Novikov self-consistency principle would operate in a very strange and non-intuitive way. It might be that occasionally the chrononaut would set up a trip and the time machine would refuse to operate. Then the chrononaut would know that somehow someway the proposed trip would cause a paradox.

Or even worse, after a time trip, the chrononauts would discover that if they try certain actions the entire universe throws up random events preventing said actions to avoid creating a paradox. By the same token the entire universe might throw up random events forcing a chrononaut to perform some action, since the action is necessary to prevent the creation of a paradox.

Novikov Principle of Self Consistency

Amazingly enough, there are presently a series of papers appearing in the scientific literature that discuss in great mathematical detail the problem of paradoxes created by time machines. The first of these papers are those by Friedman et al., Echeverria et al., and Novikov to be found in Recommended Reading at the end of this chapter. To everyone's amazement (including the amazement of the scientists writing the papers), they find that instead of their mathematics showing that the existence of logical paradoxes proves that time machines cannot exist, their mathematics indicates that the logical paradoxes cannot exist! For every paradox that they can dream up (for example, someone deciding to go back into time to shoot himself), a detailed mathematical analysis of the way that nature will behave according to the known laws of physics shows that nature will automatically adjust itself so that the paradox will not occur!

These results are now embodied in the Novikov Principle of Self Consistency, which states that: "The only solutions to the laws of physics that can occur locally in the real universe are those which are globally self consistent."

In the example of the person deciding to got back into time and shoot himself, either he changes his mind, or the gun doesn't work, or the bullet misses, or he kills someone who looks just like him, or something else happens to prevent the paradox. Admittedly, the event that prevents the paradox may have a low probability of happening, but once a time machine exists, then possible physical events around that time machine are constrained so that the Principle of Self Consistency is observed, and low probability events become high probability events.

Once you have made a decision to jump off a bridge, nature will severely limit your possible future courses of action. In the same way, once you have made a decision to turn on a time machine, nature will severely limit your future courses of action to prevent you from creating any paradoxes.

From Indistinguishable From Magic by Robert Forward (1995)

Recently physicists Daniel Greenberger and Karl Svozil have shown that the laws of quantum mechanics enforces Consistency Protection (paper here). Translated into English, they maintain that time travellers going back into the past cannot alter the past (i.e., the past is deterministic). This is because quantum objects can act sometimes as a wave. When they go back in time, the various probabilities interfere destructively, thus preventing anything from happening differently from that which has already taken place.

In Matt Visser's The quantum physics of chronology protection he notes that physicists see time travel as problematic, if not outright repugnant. They hate paradoxes, and they hate the fact that relativity contains a plethora of paradox-creating closed timelike curves (AKA time machines). Visser says "General Relativity is in fact infested with peculiar geometries that seem to produce time machines."

Visser say that physicists tend to have one of four reaction to this unpleasant state of affairs:

  1. Go whole-hog: embrace time travel and try to control the paradoxes. The least radical ones favor the Multiple Timelines solution to paradoxes. More radical ones favor exotic stuff like non-Hausdorff manifolds and multiple coexisting versions of the "present".
  2. Grudgingly allow the existence of time travel but forbid changing history. They want a Fixed Timeline and/or a strong Novikov self-consistency principle. They are not too happy about the implied elimination of free will, but you can't have everything.
  3. Beg quantum physics to come up with reason that time travel is impossible. This is Hawking's Chronology protection conjecture.
  4. Ignore it and hope it goes away. They decide to not think about such repugnant things until the experimental evidence becomes too huge to sweep under the rug. This is sometimes called the "Boring Physics" conjecture. They favor things like global hyperbolicity (no, I do not understand it) and cosmic censorship.

Time Machines

Time travel into the future is relatively easy. After all, we are all moving into the future at the rate of one second per second (local). It is possible to low the rate for yourself (relative to, say, Terra) by moving at relativistic velocities or loitering near a strong source of gravity. For instance, if you and your starship is moving relativistically at 99% the speed of light relative to Terra, then if you do a round trip that takes one day your time (proper time), upon arrival everybody on Terra will say a week has passed.

Time travel into the past is the difficult part.

Current physics admits to two possible methods of time travel into the past:

  • Traveling faster than the speed of light
  • Using one of the weird spacetime predicted in general relativity that can contain closed timelike curves (time machines)

Under Einstein's Relativity theory, all faster-than-light starships are time machines. Actually relativity states that FTL travel and Time Travel are two different terms for the exact same thing. Most physicists categorically reject the possibility of FTL travel out of hand, mainly because they hate time machines so much.

Dr. Marc Rayman tells us that in all time travel theories that seem to be allowed by real science, there is no way a traveler can go back in time to before the time machine was built. This tells me that one should have an army or two around a time machine as it finishes construction, in order to deal with the invasion force(s) from the future that come charging out.

Dr. Matt Visser noted "General Relativity is in fact infested with peculiar geometries that seem to produce time machines." Examples include:

Traversable wormholes
A Traversable wormhole is one where the hole is held open with exotic matter (negative cosmic strings or something like that). It can be converted into a time machine by accelerating one of the wormhole mouths relativistically relative to the other mouth, then bringing them back together. You can find more details here. Stephen Baxter used this in his science fiction novel Timelike Infinity.
Alcubierre metric
The Alcubierre metric is used in the Alcubierre Drive. It is a clever way to use general relativity as a faster than light drive. Objects cannot move faster than light, but Einstein didn't say anything forbidding a piece of spacetime from moving faster than light. It is a time machine because all FTL drives are. The Alcubierre drive has some major difficulties.
BTZ black hole
A BTZ black hole is a black hole solution for (2+1)-dimensional gravity with a negative cosmological constant. Since our universe has 3+1 dimensional gravity with a positive cosmological constant, we cannot use them.
Gödel metric

The Gödel metric was invented by Kurt Gödel specifically to prove that Einstein's equations of spacetime contained a severe flaw. Among other weird things it predicts that every single event inside such a metric will have several closed timelike curves passing through it (a time machine at every corner, so to speak).

Einstein seemed to be of the opinion that given the choice between his equations of spacetime being wrong and the weirdness of the Gödel metric being true, he'd chose the latter. Take that, Gödel!

The Gödel metric only applies to our universe if the value of the cosmological constant precisely matches the density of matter.

van Stockum dust
van Stockum dust is similar to a Gödel metric, but the universe's density has to increase with distance from the rotation point. It allows closed timelike curves, but it does not seem to apply to our universe.
Kerr metric

The Kerr metric describes the warped space around black holes with mass, spin, but no charge (i.e., practically all naturally occurring black holes). There are three other kinds of black holes, but we don't care about them at the moment.

Anyway apparently there is an inner region inside the black hole that contains closed timelike curve. The Cosmic censorship hypothesis suggests that if you actually try to use the closed timelike curves as a time machine, the inner region will collapse. Much like the weakly god-like AI called the Eschaton in Charles Stross's novel Singularity Sky, who commands: "Thou shalt not violate causality within my historic light cone. Or else."

Krasnikov tube
A Krasnikov tube is a speculative mechanism for space travel involving the warping of spacetime into permanent superluminal tunnels. Much the same as using traversable wormholes as time machines, you displace the ends of the tubes in time as well as space.
Misner space
Misner space is an abstract mathematical spacetime that acts like the world in such video games as Asteroids (if you exit off the left edge, you will instantly re-enter from the right). Our universe does not act like that so this is worthless to us.
Tipler cylinder
A Tipler cylinder is a dense massive cylinder of infinite length rotating near the speed of light. Due to dragging of the metric, it creates a region containing close timelike curves. A spacecraft passing through the proper curve can travel back in time. Stephen Hawking pointed out that if the cylinder's length is less than infinity, it will require exotic matter with negative energy (which may not even exist, and in any event nobody knows where to obtain any).

Causal Weapons

If you have a time-travel system where history can be changed (no fixed or self-healing timelines, see Stuart Armstrong's system below), about thirty seconds after someone invents a time machine somebody will have the bright idea of weaponizing it. If you have always been at war with Eastasia, you could pop back in time to a critical juncture and alter things so Eastasia vanishes from the pages of history.

Such time meddling is ill-advised, since there are always law of unintended consequences. And don't even bother trying to assassinate Hitler, that trick never works.

This is why it is so dangerous to use your star fleet to attack the planet Gallifrey, homeworld of the Time Lords. If you do not instantly wipe them out, one will pop into a TARDIS and instantly you and your home planet will have never existed.


And God said: Lines Aleph Zero to Aleph One — Delete

And the Universe ceased to exist.

Then She pondered for several aeons, and sighed.

Cancel Programme GENESIS She ordered.

It never had existed.

From siseneG by Arthur C. Clarke (1984)

Time travel can also be used for short term gain. A good example is Cordwainer Smith's short story The Crime and the Glory of Commander Suzdal. The commander is duped into traveling to the planet Arachosia, which is a trap. The natives capture his ship, and start burning their way into the airlock. Oh Noes! Whatever will the commander do?

As it turns out the commander is a tricky son-of-a-spacer. Agent ships of the Instrumentality have all sort of top-secret equipment. One of them is the chronopathic device, a time distorter that can send a eighty thousand ton starship one second into the past in order to escape destruction. Another was a life-bank containing fertilized ova of a selection animals. Suzdal noticed that the moon of Arachosia was habitable.

Suzdal took ferilized ova of eight pairs of cats. He genetically encoded them to [a] serve man, [b] evolve into an intelligence species and [c] invent high technology. He then adjusts the time distorter so instead of sending eighty thousand tons one second into the past, it instead sends four kilograms two million years into the past. He projects the cat ova in artificial wombs to the moon.

Instantly the evil Arachosians are attacked by a huge Cat starfleet that appears out of nowhere. The cats scream their allegiance to to commander Suzdal, and proceed to shoot the living snot out of the Arachosians. Suzdal escapes in the confusion.


Ilya forced himself to uncurl his fingers from the arms of his chair. "You're saying that the capsule we're about to retrieve is a bomb?…"

[Rachel] grinned, humorlessly. "If you don't believe me, that's your problem. We've seen half a dozen incidents like this before — the UN Defense Intelligence Causal Weapons Analysis Committee, I mean — incidents where one or another secret attempt to assemble a causality-violation device came to grief. Not usually anything as crude as your closed timelike flight path and oracle hack, by the way; these were real CVDs. History editors, minimax censors, grandfather bombs, and a really nasty toy called a space ablator…"

From SINGULARITY SKY by Charles Stross (2003)

Causal weapons are a myth, an arms-dealer’s con trick. Their official mention in the Ley Accords was no more than a paranoid’s precaution, deliberate disinformation, or a warning to those who might become too attached to oracles and knight’s-move stratagems. The universe was block, that was known; no retroactive editing-out permitted. Their reality, therefore — that was on the same level as the Precursor fittlers, three-ended wormhole recipes, bootleg godseeds, pocket universes, negative-mass antimatter, treasure maps to the Amphigory fleet and volumetric claims to Sector Zero, the Absolute Center and Place of Creation, that any downport slash-trader stocked in abundance for the most gullible and rich marks.

So why, he wondered, did this benzene-sweating azayf look so terrified of the… half-melted-semi-helix-blob-thing on the table between them?

Change War

A nation with a monopoly on time travel technology is more or less invincible. Examples include L. E. Modesitt, Jr.'s The Fires of Paratime and Isaac Asimov's The End of Eternity. Invincible, except of course from sabotage caused by an internal traitor or internal disaffected agent.

But if others have the secret of time travel, you will suddenly have a Time War on your hands. Your time warriors travel back to eliminate the nation of Eastasia, only to be confronted by Eastasian time traveling defenders. Meanwhile Eastasian kill teams will be elsewhen trying to erase your nation from the history books.

The technical name is "Change War."

Examples of science fiction stories featuring time war include Poul Anderson's THE CORRIDORS OF TIME, Jack Williamson's THE LEGION OF TIME, Barrington Bayley's THE FALL OF CHRONOPOLIS, Simon Hawke's TIMEWAR series, Fritz Leiber's THE BIG TIME, Diana Wynne Jones's A TALE OF TIME CITY, David Wingrove's ROADS TO MOSCOW series and Andre Norton's TIME TRADER series. Not to metion the TERMINATOR franchise.

However, your team might be suddenly confronted by time warriors from 40,000 AD whose existence depends upon Eastasia and take a dim view of your short-sighted actions. This can quickly get out of hand. Your time warriors might suddenly be faced with several thousand other time-traveling groups from all sorts of historical eras. Some might even want to help you.

Even worse, the Time-Police might show up. This is an organization(s) existing in your far future who (for whatever reason) frown upon history alterations (especially changewars). They usually have the equipment, training, and backing of an organization huge enough to make your time warrior's lives really difficult. Examples include Keith Laumer's Dinosaur Beach and Poul Anderson's Time Patrol series.

Some time cops are like the coast guard, where commercial time travel is common. Such cops prevent civillians from smuggling contraband, leaving time-litter, or making egregoius changes in history. The worse they have to deal with is the occasional criminal mastermind.

Other time cops have a monopoly on time travel, and alter history in directions that are best for the human race, e.g., Isaac Asimov's THE END OF ETERNITY. Often there is some controversy as to what constitutes "best." Occasionally things slip and rival time cops can appear, with different ideas of what is "best." This usually degenerates into a changewar among the time cops.

In Dinosaur Beach, the initial wave of absent-minded scientists who invent time travel seriously damage the timeline. The time police are attempting to clean up the mess. The amusing bit is that the time police think they are virtuously preserving reality but are actually making things worse. That and the fact that they have been infiltrated by a second group of time police from even further in the future. The basic problem is the time police does not realize (or deliberately ignores) the fact that truly fixing the timeline will also erase the time police from ever existing. And they figure that is a bad thing.

Time police are always more powerful than your protagonists (outside of your protagonists being time police themselves) because otherwise there is no dramatic tension. If your warriors come from a mighty changewar army it does not make sense that you have much to fear from time-cops walking a beat. So the novel probably won't even have any. But if your changewar army consists of ten poorly equipped agents on individual missions up and down five thousand years, they will always be looking over their shoulders for temporal patrollers.

In the computer game The Journeyman Project after the invention of the worlds first time machine the government locks it down under tight security, and establishes the Temporal Security Annex (aka Time Police) to make sure nobody uses time travel technology to alter history. Their problem is, if anybody alters history, how the heck will they know? Everything will instantly change including them, and everybody knows there was never a person named Hitler nor was their ever something called World War 2.

The answer was a high density CD-ROM (the game came out in 1993, OK?) containing all of recorded history. It was placed back in Dinosaur times, presumably before the time of any evil time-travel history-changing. Presumably any evil history alteration can only change the timeline following the alteration, but will have no effect on the timeline prior to the alteration. This means even if history is changed, the CD-ROM will stay unchanged in the dinosaur age.

Every day the Temporal Security Annex dispatches an agent with a new CD-ROM (appended with last day's history) back in time to the Dinosaur cache. There the new CD-ROM will be added to the cache. But first, the agent runs a comparison between the new disc and the old disc. If there is a discrepancy, it is a red alert that history has been changed. The place where the two discs diverge will pin point the historical moment that the villains did their dirty work. The Temporal Security Annex will then spring into action to try and undo the history change.

Of course this system makes the assumption that the change is minor enough that the Temporal Security Annex still exists.

Things are simpler in THE FALL OF CHRONOPOLIS. The Achronal archives are protected from outside time changes by some kind of technobabble called quote "time barriers" unquote. These records contain details about people, cities, and civilizations that have vanished from the time stream due to the on-going change war.

And if your time warrior team is really flat out of luck, they will find themselves being attacked by some species of Clock Roaches. These are not time cops, they are more like the immune system for the entire struture of time itself. They are powerful like elemental forces of nature, and they do not like anything messing with causality.



I’ve been looking to develop a system of time travel in which it’s possible to actually have a proper time war. To make it consistent and interesting, I’ve listed some requirements here. I think I have a fun system that obeys them all.

Time travel/time war requirement:

  • It’s possible to change the past (and the future). These changes don’t just vanish.
  • It’s useful to travel both forwards and backwards in time.
  • You can’t win by just rushing back to the Big Bang, or to the end of the universe.
  • There’s no “orthogonal time” that time travellers follow; I can’t be leaving 2015 to go to 1502 “while” you’re leaving 3015 to arrive at the same place.
  • You can learn about the rules of time travel while doing it; however, time travel must be dangerous to the ignorant (and not just because machines could blow up, or locals could kill you).
  • No restrictions that make no physical sense, or that could be got round by a human or a robot with half a brain. Eg: “you can’t make a second time jump from the arrival point of a first.” However, a robot could build a second copy of a time machine and of itself, and that could then jump back; therefore that initial restriction doesn’t make any particular sense.
  • Similarly, no restrictions that are unphysical or purely narrative.
  • It must be useful to, for instance, leave arrays of computers calculating things for you then jumping to the end to get the answer.
  • Ideally, there would be only one timeline. If there are parallel universes, they must be simply describable, and allow time-travellers to interact with each other in ways they would care about.
  • A variety of different strategies must be possible for fighting the war.

Consistent time travel

Earlier, I listed some requirements for a system of time travel – mainly that it be both scientifically consistent and open to interesting conflicts that aren’t trivially one-sided. Here is my proposal for such a thing, within the general relativity format.

So, suppose you build a time machine, and want to go back in time to kill Hitler, as one does. Your time machine is a 10m diameter sphere, which exchanges place with a similarly-size sphere in 1930. What happens then? The graph here shows the time jump, and the “light-cones” for the departure (blue) and arrival (red) points; under the normal rules of causality, the blue point can only affect things in the grey cone, the red point can only affect things in the black cone.

The basic idea is that when you do a time jump like this, then you “fix” your points of departure and arrival. Hence the blue and red points cannot be changed, and the universe rearranges itself to ensure this. The big bang itself is also a fixed point.

All this “fixed point” idea is connected to entropy. Basically, we feel that time advances in one direction rather than the other. Many have argued that this is because entropy (roughly, disorder) increases in one direction, and that this direction points from the past to the future. Since most laws of physics are symmetric in the past and the future, I prefer to think of this as “every law of physics is time-symmetric, but the big bang is a fixed point of low entropy, hence the entropy increase as we go away from it.”

But here I’m introducing two other fixed points. What will that do?

Well, initially, not much. You go back into time, and kill Hitler, and the second world war doesn’t happen (or maybe there’s a big war of most of Europe against the USSR, see configuration 2 in “A Landscape Theory of Aggregation”). Yay! That’s because, close to the red point, causality works pretty much as you’d expect.

However, close to the blue point, things are different.

Here, the universe starts to rearrange things so that the blue point is unchanged. Causality isn’t exactly going backwards, but it is being funnelled in a particular direction. People who descended from others who “should have died” in WW2 start suddenly dying off. Memories shift; records change. By the time you’re very close to the blue point, the world is essentially identical to what it would have been had there been no time travelling.

Does this mean that you time jump made no difference? Not at all. The blue fixed point only constrains what happens in the light cone behind it (hence the red-to-blue rectangle in the picture). Things outside the rectangle are unconstrained – in particular, the future of that rectangle. Now, close to the blue point, the events are “blue” (ie similar to the standard history), so the future of those events are also pretty blue (similar to what would have been without the time jump) – see the blue arrows. At the edge of the rectangle, however, the events are pretty red (the alternative timeline), so the future is also pretty red (ie changed) – see the red arrows. If the influence of the red areas converges back in to the centre, the future will be radically different.

(some people might wonder why there aren’t “changing arrows” extending form the rectangle into the past as well as the future. There might be, but remember we have a fixed point at the big bang, which reduces the impact of these backward changes – and the red point is also fixed, exerting a strong stabilising influence for events in its own backwards light-cone).

So by time travelling, you can change the past, and you can change part of the future – but you can’t change the present.

But what would happen if you stayed alive from 1930, waiting and witnessing history up to the blue point again? This would be very dangerous; to illustrate, let’s change the scale, and assume we’ve only jumped a few minutes into the past.

Maybe there you meet your past self, have a conversation about how wise you are, try and have sex with yourself, or whatever time travellers do with past copies of themselves. But this is highly dangerous! Within a few minutes, all trace of future you’s presesence will be gone; you past self will have no memory of it, there will be no physical or mental evidence remaining.

Obviously this is very dangerous for you! The easiest way for there to remain no evidence of you, is for there to be no you. You might say “but what if I do this, or try and do that, or…” But all your plans will fail. You are fighting against causality itself. As you get closer to the blue dot, it’s as if time itself was running backwards, erasing your new timeline, to restore the old one. Cleverness can’t protect you against an inversion of causality.

Your only real chance of survival (unless you do a second time jump to get out of there) is to rush away from the red point at near light-speed, getting yourself to the edge of the rectangle and ejecting yourself from the past of the blue point.

Right, that’s the basic idea!

Multiple time travellers

Ok, the previous section looked at a single time traveller. What happens when there are several? Say two time travellers (blue and green) are both trying to get to the red point (or places close to it). Who gets there “first”?

Here is where I define the second important concept for time-travel, that of “priority”. Quite simply, a point with higher priority is fixed relative to the other. For instance, imagine that the blue and green time travellers appear in close proximity to each other:

This is a picture where the green time traveller has a higher priority than the blue one. The green arrival changes the timeline (the green cone) and the blue time traveller fits themselves into this new timeline.

If instead the blue traveller had higher priority, we get the following scenario:

Here the blue traveller arrives in the original (white) timeline, fixing their arrival point. The green time traveller arrives, and generates their own future – but this has to be put back into the first white timeline for the arrival of the blue time traveller.

Being close to a time traveller with a high priority is thus very dangerous! The green time traveller may get erased if they don’t flee-at-almost-light-speed.

Even arriving after a higher-priority time traveller is very dangerous – suppose that the green one has higher priority, and the blue one arrives after. Then suppose the green one realises they’re not exactly at the right place, and jump forwards a bit; then you get:

(there’s another reason arriving after a higher priority time traveller is dangerous, as we’ll see).

So how do we determine priority? The simplest seems time-space distance. You start with a priority of zero, and this priority goes down proportional to how far your jump goes.

What about doing a lot of short jumps? You don’t want to allow green to get higher priority by doing a series of jumps:

This picture suggests how to proceed. Your first jump brings you a priority of -70. Then the second adds a second penalty of -70, bringing the priority penalty to -140 (the yellow point is another time traveller, who will be relevant soon)

How can we formalise this? Well, a second jump is a time jump that would not happen if the first jump hadn’t. So for each arrival in a time jump, you can trace it back to the original jump-point. Then your priority score is the (negative) of the volume of the time-space cone determined by the arrival and original jump-point. Since this volume is the point where your influence is strongest, this makes sense (note for those who study special relativity: using this volume means that you can’t jump “left along a light-beam”, then “right along a light-beam” and arrive with a priority of 0, which you could do if we used distance travelled rather than volume).

Let’s look at that yellow time traveller again. If there was no other time traveller, they would jump from that place. But because of the arrival of the green traveller (at -70), the ripples cause them to leave from a different point in space time, the purple one (the red arrow shows that the arrival there prevents the green time jump, and cause the purple time jump):

So what happens? Well, the yellow time jump will still happen. It has a priority of 0 (it happened without any influence of any time traveller), so the green arrival at -70 priority can’t change this fixed point. The purple time jump will also happen, but it will happen with a lower priority of -30, since it was caused by time jumps that can ultimately be traced back to the green 0 point. (note: I’m unsure whether there’s any problem with allowing priority to rise as you get back closer to your point of origin; you might prefer to use the smallest cone that includes all jump points that affected you, so the purple point would have priority -70, just like the green point that brought it into existence).

What other differences could there be between the yellow and the purple version? Well, for one, the yellow has no time jumps in their subjective pasts, while the purple has one – the green -70. So as time travellers wiz around, they create (potential) duplicate copies of themselves and other time travellers – but those with the highest priority, and hence the highest power, are those who have no evidence that time jumps work, and do short jumps. As your knowledge of time travel goes up, and as you explore more, your priority sinks, and you become more vulnerable.

So it’s very dangerous even having a conversation with someone of higher priority than yourself! Suppose Mr X talks with Mrs Y, who has higher priority than him. Any decision that Y does subsequently has been affected by that conversation, so her priority sinks to X’s level (call her Y’). But now imagine that, if she wouldn’t have had that conversation, she would have done another time jump anyway. The Y who didn’t have the conversation is not affected by X, so retains her higher priority.

So, imagine that Y would have done another time jump a few minutes after arrival. X arrives and convinces her not to do so (maybe there’s a good reason for that). But the “time jump in an hour” will still happen, because the unaffected Y has higher priority, and X can’t change that. So if the X and Y’ talk or linger too long, they run the risk of getting erased as they get close to the “point where Y would have jumped if X hadn’t been there”. In graphical form, the blue-to-green square is the area in which X and Y’ can operate in, unless they can escape into the white bands:

So the greatest challenge for a low priority time-traveller is to use their knowledge to evade erasure by higher priority ones. They have a much better understanding of what’s going on, they may know where other time jumps likely end up at or start, they might have experience at “rushing at light speed to get out of cone of danger while preserving most of their personality and memories” (or technology that helps them do so), but they are ever vulnerable. They can kill or influence higher priority time-travellers, but this will only work “until” the point where they would have done a time jump otherwise (and the cone before that point).

From TIME TRAVEL SYSTEM by Stuart Armstrong (2015)

Conspiracy Theory History

A couple of science fiction authors figure that if there is an eon-long time war going on, us mortals might be able to detect it by taking a deep look at the history books. The idea is that since history is changed by changing societies, examining the broad historical trends of various societies might give clues as to the motives of the two sides of the war. Any trend that seems to repeat during various eras is suspect.

The two novels (that I am aware of) which include this concept are The Corridors of Time by Poul Anderson and Giant's Star by James Hogan. I would hazard a guess that this theme is rare in science fiction because it is a lot of work for an author to do for a crummy novel.


(ed note: the two nations engaged in the time war are the Rangers and the Wardens, from approximately the year 4,000 CE. Our hero Lockridge is a current day person recruited by Storm of the Wardens.)

      “Okay,” he said. “I seem to’ve enlisted in a war on your side. Do you mind tellin’ me what the shootin’s for? Who are your enemies?” He paused. “Who are you?”
     “Let me continue to use the name I chose in your century,” Storm (The Koriach of the Westmark) said. “I believe it was a lucky one.” She sat brooding a while. “I do not think you could really grasp the issue of my age. Too much history lies between you and us. Could a man from your past really feel what the basic difference is that divides East and West in your time?”
     “I reckon not,” Lockridge admitted. “In fact, quite a few of our own don’t seem to see it.”
     “At that,” Storm said, “the issue is the same. Because there has really only been one throughout man’s existence—distorted, confused, hidden behind a thousand lesser motivations, and yet always in some fashion the clash between two philosophies, two ways of thought and life—of being—the question is forever: What is the nature of man?”

     Lockridge waited. Storm brought her gaze back from the night, across the low fire to him, stabbingly intense.
     “Life as it is imagined to be against life as it is,” she said. “Plan against organic development. Control against freedom. Overriding rationalism against animal wholeness. The machine against the living flesh. If man and man’s fate can be planned, organised, made to conform to some vision of ultimate perfection, is not man’s duty to enforce the vision upon his fellow man, at whatever cost? That sounds familiar to you, no?
     “But your country’s great enemy is only one manifestation of a thing that was born before history: that spoke through the laws of Draco and Diocletian, the burning of the Confucian Willow Books, Torquemada, Calvin, Locke, Voltaire, Napoleon, Marx, Lenin, Arguellas, the Jovian Manifesto, and on and on. Oh, not clearly, not simply—there was no tyranny in the hearts of some who believed in supreme reason; and there was in others, like Nietzsche, who did not. To me, your industrial civilization, even in the countries that call themselves free, comes near to an ultimate horror; yet I use machines more powerful and subtle than you have dreamed. But in what spirit? There is the issue of battle!”
     Her voice dropped. She looked into the forest walling this meadow. “I often think,” she said slowly, “that the downward turn started in this very millennium (1827 BCE), when the earth gods and their Mother were swept aside by those who worshipped skyward.”
     She shook herself, as if to be rid of something, and continued in a level tone, “Well, Malcolm, accept for now that the Wardens are keepers of life—life in its wholeness, boundedness, splendour, and tragedy—while the Rangers would make the world over in the machine’s image. It is an oversimplification. I can perhaps explain better to you later on. But do you find my cause unworthy?”
     Lockridge regarded her, where she rested like a young wildcat, and said with a surge that drove out all terror, remorse, and aloneness: “No. I’ll go along. I already have.”
     “There are dangers,” she warned.
     “So what is the situation, anyway? What do we have to do?”
     “Let me begin at the first,” Storm said. “As I told you, the struggle between Rangers and Wardens cannot be fought in our own time on any major scale. Instead, it has moved largely into the past. Bases are established at strategic points and—no matter now. I know the Rangers have a stronghold in Harald Bluetooth’s reign. Though the Asa religion was already one of Sky Father, still, the introduction of Christianity was another advance for them, laying the foundation for centralised monarchy and the eventual rationalistic state. Thence came the men we met.”
     “Huh? Wait! You mean you people change the past?”
     “Oh, no. Never. That is inherently impossible. If one tried, he would find events always frustrated him. What has been, is. We time travellers are ourselves part of the fabric. But let us say that we discover aspects of it which are useful to our respective causes, we get recruits, build up strength for the final contest.

     “But now, here we are (in 1827 BCE). There is a Warden base in Crete, where the old faith is still strong. Unfortunately, I cannot simply call them to come fetch us. The Rangers are also active in this milieu—it is, as I said, a crucial one—and they might too likely intercept the message and find us before our friends can. But once we have reached Knossos, we can get an armed escort, from corridor to corridor until I have reached home. You will be dismissed in your own era.” She shrugged. “I left a good many dollars hidden in the United States. You may as well have them for your trouble.”
     “Skip that,” Lockridge said roughly. “How do we get to Crete?”
     “By sea. There has long been trade between these parts and the Mediterranean. The Limfjord is not far away, and a ship from Iberia, which is under the religion of the megalith builders, should call sometime this summer. From Iberia we can transship. It should take no longer, and is less hazardous, than following the amber route overland.”
     “M-m-m … okay, sounds reasonable. And I suppose we have enough metal on us to buy passage. Or do we?”
     Storm tossed her head. “If not,” she said haughtily, “they will not refuse to carry Her Whom they worship.”
     “What?” Lockridge’s mouth fell open. “You mean you can pose as—”
     “No,” she said. “I am the Goddess.”

     “Storm,” he asked slowly, “did you start the cult of the Goddess to get the idea of peace into men?”
     Her nostrils dilated and she spoke almost in scorn. “The Goddess is triune: Maiden, Mother, and Queen of Death.” Jarred, he heard the rest dimly. “Life has its terrible side. How well do you think those weak-tea-and-social-work clubs you call Protestant churches will survive what lies ahead for your age? In the bull dance of Crete, those who die are considered sacrifices to the Powers. The megalith builders of Denmark—not here, where the faith has entered a still older culture, but elsewhere—kill and eat a man each year.” She observed his shock, smiled, and patted his hand. “Don’t take it so hard, Malcolm. I had to use what human material there was. And war for abstractions like power, plunder, glory, that is alien to Her.

     “He (Ranger leader Brann) is here alone. But no more were needed. I think he must have come out of the tunnel under the dolmen earlier than we did, sought the Battle Axe people, and made himself their god. That would not be hard to do. This whole inwandering of the Indo-Europeans—Dyaush Pitar’s, Sky Father’s, the sun’s worshippers, herdsmen, weaponmakers, charioteers, warriors, the men of clever hands and limitless dreams, whose wives are underlings and whose children are property—this was engineered by the Rangers. Do you understand? The invaders are the destroyers of the old civilisations, the old faith; they are the ancestors of the machine people. The Yuthoaz belong to Brann. He need but appear among them, as I need but appear in Avildaro or Crete, and in their dim way they will know what he is and he will know how to control them.

     A stir and a growl came from the Yuthoaz in the shadows. Brann waved them back, though he kept a hand near the energy pistol at his broad coppery belt. “She does make a rather overwhelming impression, does she not?” he murmured. “No doubt she told you that her Wardens stand for absolute good and we Rangers for absolute evil. You would have no way of disproof. But think, man. When was such a thing ever true?”
     “Listen,” Brann said earnestly. “I do not, myself, maintain that we Rangers are models of virtue. This is as ruthless a war as was ever fought, a war between philosophies, whose two sides shape the very past that brought them into being. I ask you, though, to consider. Is the science that sends men beyond the moon, liberates them from toil and famine, saves a child from strangling with diphtheria—is it evil? Is the Constitution n the United States evil? Is it wrong for man to use his reason, the one thing that makes him more than an animal, and to harness the animal within him? Well, if not, where do these things come from? What view of life, what kind of life, must there be to create them?
     “Not the Wardens’ way! Do you seriously think this earthward-looking, magic-muttering, instinct-bound, orgiastic faith of the Goddess can ever rise above itself? Would you like to see it return in the future? It has done so, you know, in my age. And then, like the worm that bites its own tail, it has gone back to cozen and terrify men in this twilight past, until they crawl before Her. Oh, they can be happy, in a fashion; the influence is diluted. But wait until you see the horror of the Wardens’ real reign!
     “Think—one small archaeological item—the aborigines here bury their dead in communal graves. But the Battle Axe culture gives each his own. Does that suggest anything to you?”
     Lockridge had a fleeting odd recollection of his grandfather telling him about the Indian wars. He’d always sympathised with the Indians; and yet, if he could rewrite their history, would he?

     Man’s history was the history of religion.

     What Auri (a local Bronze-age girl the protagonist is protecting) had, who slept so peacefully here among thunders, and Auri’s people, and the Indians he had seen in Yucatan, and every primitive race he knew of whose culture had not taken a completely perverted turn—was wholeness of spirit. It was purely a question of taste whether that made up for all they lacked. The fact remained, they were one with earth and sky and sea in a way that those who set the gods apart from themselves, or who denied any gods, could never be. When the Indo-Europeans brought their patriarchal pantheon to a land, they brought much that was good; but they created a new and lonely kind of man.

     There was no sharp dichotomy. The old ones endured. After a time, they blended with the aliens, transfigured them, until ageless forms stood clear again and only names had changed. Dyaush Pitar, with his sun chariot and battle axe, became Thor, whose car was drawn by honest earthy goats and whose hammer brought the rain which was life. No blood was offered the Redbeard; he was himself a yeoman.

     And into this great slow conflict and interweaving of two world-views the time war had entered. Rangers engineered the march of the war making tribes and their militant gods; Wardens found secret ways to keep what was old and make the invaders over into its image. Rangers urged on the tomahawk people, who obliterated the cult of the passage grave; but Neolithic herdsmen became Bronze Age farmers and seafarers, and the sun was no longer a fire spirit but earth’s guardian and fructifying husband. Christendom entered, with books and logic and the first god who ever punished incorrect beliefs about his own nature—and erelong the people’s hearts belonged to Mary. The Reformation brought back Jehovah, armed with a terrible weapon against instinct—the printing press—but religion itself was subtly divided, discredited, emasculated, until the world five or six hundred years hence felt its-own barrenness and yearned for a faith which went deeper than words. Lockridge looked into the century after his own and did not see science triumphant; he saw men gathered on hills in the name of a new god or of an ancient one reborn.

     Or a goddess?

     The equinoctial nights, Lockridge thought; those belonged to the earth gods (the Wardens). Summer and winter solstice were the sun’s —the Rangers’.

     He didn’t want to pursue that thought. Instead, he speculated on what enclaves the Rangers maintained. In Akhnaton’s court? Caesar’s? Mohammed’s? The Manhattan Project?

     “Matters are not that simple,” Mareth answered. “In the whole period of history from Luther to—beyond your time—the Rangers are ascendant. Warden forces are concentrated elsewhen. We (the Wardens) maintain only a few agents like myself in this century.”
     “But in this present century (1535 CE), Denmark is not where our real European strength lies. Rather we are concentrated in Britain. King Henry has forsaken the Roman Church; but we saw to it that he did not go over to Lutheranism either, and for us his kingdom is pivotal. What you know as the episode of the two Queen Marys is a time of gain for the Wardens; the Rangers will resurge with Cromwell, but we will drive them out at the Restoration.

     “The corridor was built chiefly for access to this very era. Its future end terminates in the eighteenth century, when we have another strong point in India. The Rangers are especially active in England between the Norman Conquest and the Wars of the Roses, so we have no gates there opening on the Middle Ages at all—nor many in earlier epochs, when the critical regions, the theatres of major conflict, are elsewhere. In fact, gates throughout the Neolithic and Bronze Age North serve as little more than transfer points. It is largely a fortunate coincidence that we do have one here with a temporal overlap on the one in Denmark.”

     Storm nodded. “Yes. I have been thinking about that,” she said. “We can decoy the enemy into believing we have promptly moved away ourselves, while actually remaining. Brann was quite right about this being a good place to operate from (in 1827 BCE). Attention is all on Crete, Anatolia, India. The Rangers think the destruction of those civilisations will hurt us severely. Well, let them continue to think so. Let them spend themselves in helping along an Indo-European conquest that is foredoomed to happen. Both sides have tended to forget the North.”
     Her cloak swirled as she strode. She smote fist into palm and cried: “Yes! Piece by piece, we’ll withdraw forces hither. We can quietly organise this part of the world just as we please. There is no proof that we never did; the possibility stands gate-open. How much word will ever reach the South about the doings of barbarians in these far hinterlands? When the Bronze Age comes, it will bear our shape, furnish us men and goods, guard Warden bases. The final great futureward thrust may well be pivoted here!”

     They met no one on their trip futureward. But Hu took them out in the seventh century A.D. “At this gate, Frodhi rules the Danish islands,” he explained. “Also here on the mainland is peace, and the Vanir—the older gods of earth and water—are still at least coequal with the Aesir. A little further on, the Rangers will drive us back and the Vikings begin to sail. We are too likely to encounter enemy agents in that part of the bore.”

     Languages: two major ones, Eastern and Western, Warden and Ranger; others survived among the lower classes of either hegemony. Religion: here a mystical, ritualistic pantheism, with Her the symbol and embodiment of all that was divine; among the enemy, only a harsh materialistic theory of history. Government: he was sickened by the rush of data on Ranger lands, underlings made into flesh-and-blood machines for the use of a few overlords. Not much came to him concerning the Wardens. This was clearly not a democracy, but he got the impression of a benevolent hierarchical structure, its law derived rather from tradition than from formal innovation, power divided among aristocrats who were at one with their people, more like priests or parents than masters. Priestesses, mothers, mistresses? Women dominated. At the apex were the Koriachs, who were—well—something in between a Pope and a Dalai Lama? No, not that either. Odd, how sketchy the account was. Maybe because visitors got the local scene explained to them viva voce.

     Near the limit of their trip, Lockridge spied a dove-grey tower. At the fifteen hundred foot summit, two wings reached out beneath a golden wheel, to make the ankh which signifies life. “Is that on the edge of a city?” he asked.
     Hu spat. “Don’t speak to me of cities. The Rangers build such vile warrens. We let men live next to the earth their mother. That’s an industrial plant. None but technicians are quartered there. Automatic machinery can do without sunlight.”

(ed note: Lockridge is sent on a mission into the Ranger's home territory. Yes, it is pretty much a huge city covering an entire continent; full of pollution, squalor, poor citizens mesmerized by TV, secret police, and a militaristic culture.)

(ed note: Lockridge is delivered into the Warden's time period by agents from the far far future. But not into the part were the aristocrats live, rather into the area of the downtrodden peons. He is hunted by Warden aristocrats as if he was a beast. He is saved by a poor old peasant lady who says that Lockridge reminds her of her dead son Ola.)

     Lockridge slipped the tunic over his head. “Was Ola your son?” he asked as softly.
     “Yes. The last. Sickness got the rest in their cribs. And this year, when he was no more than seventeen, the lot chose him.”
     Her mood changed with intoxicated swiftness. “Ola, now,” she said, “he got to be the Year Man.” She knuckled her eyes. “Goddess forgive me. I know his life is in the land. If only I could forget how he screamed when they burned him.”
     … yet even so, I sometimes wonder if the Goddess won’t ever make us a better way.”
     Oh, yes, Lockridge thought sickly, a better way can be made.
     Though not in this age. I see it quite plain. I see that bewildered old workman I knew, two thousand years ago, laid off because he couldn’t handle a cybernetic machine. What do you do with your extra people?
     If you’re a Ranger, you dragoon them into a permanent army. If you’re a Warden, you keep them ignorant serfs, with some out-and-out savages as a check, and a religion that— No, there’s the worst of the matter. The Wardens themselves believe.

(ed note: Lockridge is taken by the Warden aristocrats. He expresses his disgust at how the peasants are treated.)

     Afterward Yuria had held lengthy discussions with him, not the least ill-tempered. Her position was that, imprimis, his background did not equip him to understand a totally different civilization; secundus, what he had seen was not a fair sample; tertius, tragedy must be integral to any human life which was to realise its full nobility; quartus, granted, abuses did occur, but they were correctable, and under a wiser government they would be.

     Her smile faded. She regarded him long before she set her glass down again. “I know what you are thinking,” she said.
     “That the Wardens are no better than the Rangers, and to hell with ‘em both? Yeah, I reckon so.”
     “But it isn’t true,” she said earnestly, never releasing his eyes. “Once you mentioned the Nazis of your time as a case of absolute evil. I agree. They were a Ranger creation. But think—be honest—suppose you were a man from the Neolithic now, transported to 1940. How much difference between countries could you have seen?”
     “Your cousin Yuria used some such line of argument.”
     “Ah, yes. Her.” Briefly, the full mouth hardened. “Someday I must do something about Yuria.”

From THE CORRIDORS OF TIME by Poul Anderson (1965)

(ed note: This does not quite match, since there is only one evil side doing the manipulating. The other side is the natural build-in progressive instinct of the human race.)

"You can trace the same basic struggle right down through history," he told them. "Two opposed ideologies—the feudalism of the aristocracies on one side, and the republicanism of the artisans, scientists, and city-builders on the other. You had it with the slave economies of the ancient world, the intellectual oppression of the Church in Europe in the Middle Ages, the colonialism of the British Empire, and, later on, Eastern Communism and Western consumerism."

"Keep 'em working hard, give 'em a cause to believe in, and don't teach 'em to think too hard, huh?" Caldwell commented.

"Exactly." Pacey nodded. "The last thing you want is an educated, affluent, and emancipated population. Power hinges on the restriction and control of wealth. Science and technology offer unlimited wealth. Therefore science and technology have to be controlled. Knowledge and reason are enemies; myth and unreason are the weapons you fight them with."

Lyn was still thinking about the conversation an hour later when the three of them were sitting around a small table in a quiet alcove that opened off one end of the lobby. They had opted for a last drink before calling it a night, but the bar had seemed too crowded and noisy. It was the same war that Vic, consciously or not, had been fighting all his life, she realized. The Sverenssens who had almost shut down Thurien stood side by side with the Inquisition that had forced Galileo to recant, the English nobility who would have ruled the Americans as a captive market, and the politicians on both sides of the Iron Curtain who had seized the atom to hold a world to ransom with bombs. She wanted to contribute something to his war, even if only a token gesture to show that she was on his side. But what? She had never felt so restless and so helpless at the same time.

Downstairs in the alcove to one side of the lobby, Lyn was thinking about Egyptian pyramids, medieval cathedrals, British dreadnoughts, and the late-twentieth-century arms race. Were they all parts of the same pattern too? she wondered. No matter how much more wealth per capita improving technology made possible, always there had been something to soak up the surplus and condemn ordinary people to a lifetime of labor. No matter how much productivity increased, people never seemed to work less, only differently. So if they didn't reap the fruits, who did? She was beginning to see lots of things in ways she hadn't before.

"We've been analyzing the rates of development of the Lunarian civilization and Earth's," she said. "The difference is staggering. They were into steam power and machines in a matter of a few thousand years after starting to use stone tools. We took something like ten times as long. Why do you think that was?"

(Dr. Danchekker says something dismissive and condescending)

Heller seemed to have been expecting something like that, and didn't react. "Maybe you're thinking too much like a biologist," she suggested. "Try looking at it from a sociological angle, and asking the question the other way around."

Danchekker's expression said that there couldn't be any other way around. "What do you mean?" he demanded.

"Instead of telling me what speeded the Lunarians up, try asking what slowed Earth down."

Danchekker stared darkly down at his plate for a few seconds, then raised his head and showed his teeth. "The upheavals caused by the Moon's capture," he pronounced.

Heller looked at him in open disbelief. "And regressed them to a point that needed tens of thousands of years to recover from? No way! A few centuries at the most, maybe, but not that much. I couldn't buy it. Neither could Showm. Neither could Calazar."

"I see." Danchekker looked a bit taken aback. He attacked his bacon in silence for a while and then said, "And what alternative explanation, if any, are you offering, might I ask?"

"Something you haven't mentioned so far," Heller answered. "The Lunarians developed rational, scientific thinking early on, and relied on it totally from the beginnings of their civilization. By contrast Earth went off into thousands of years of believing that magic, mysticism, Santa Claus, the Easter Bunny, and the Tooth Fairy would solve its problems. It only started to change comparatively recently, and even today there's still a lot of that around. We got visar to estimate the effects, and it eclipses all the other factors put together. That's what caused the difference!"

Danchekker thought about it for a while, then replied a trifle grudgingly, "Very well." He thrust his chin out defensively. "But I fail to see the need for any melodramatic suggestion that it poses a different question. It's as valid to argue that the early adoption of rational methods accelerated one race as it is to say that its absence retarded the other. What point are you making?"

"I've been thinking a lot about it since I talked to Calazar and Showm, and asking what the reason was. Vic says everything has to have a reason, even if it takes some digging to find it. So what would the reason be for a whole planet clinging obstinately to a lot of nonsense and superstitions for thousands of years when even a little bit of observation and common sense should have shown it doesn't work?"

"I think perhaps you underestimate the complexities of scientific method," Danchekker told her. "It takes centuries . . . scores of generations to evolve the techniques necessary to distinguish reliably between facts and fallacies, and truth and myth. Certainly it couldn't happen overnight. What else did you expect?"

"So why didn't that stop the Lunarians?"

"I have no idea. Have you?"

"That was the question I was leading up to." Heller leaned forward to look at him intently across the table. "What do you think of this for a suggestion: The reason that belief in myths and magic became so deeply rooted in Earth's cultures and persisted for so long could be that, in the earliest stage of our first civilizations, it did work?"

Danchekker gagged over the mouthful of food that had been about to swallow and colored visibly. "What? That's preposterous! Are you suggesting that the laws of physics that dictate the running of the Universe could have changed in the last few thousand years?"

"No, I'm not. All I'm—"

"I've never heard such an absurd suggestion. This whole matter is already complicated enough without introducing attempts to explain it by astrology, ESP, or whatever other inanities you have in mind." Danchekker looked about him impatiently and sighed. "Really, it would take far too long to explain why if you are unable to distinguish between science and the banalities dispensed in adolescent magazines. Just take my word that you are wasting your time . . . mine too, I might add."

Heller maintained her calm with some effort. "I am not suggesting anything of the kind." An edge of strain had crept into her voice. "Kindly listen for two minutes." Danchekker said nothing and eyed her dubiously across the table as he continued eating. She went on, "Think about this scenario. The Jevlenese (the bad guys) have never forgotten that they're Lambians, and (us Earthmen) we're Cerians. They still see Earth as a rival and always have. Now put them in the situation where they've been taken to Thurien and are making the most of the opportunity to absorb all that Ganymean technology, and the rivals on Earth have been slowed down by the Moon showing up. They've gained control of the surveillance operation, and probably by this time they can do their own instant moving of ships and whatever around the Galaxy because they've got their own independent computer, JEVEX, on their own independent planet. Also they're human in form—physically indistinguishable from their rivals (us Earthmen)." Heller sat back and looked at Danchekker expectantly, as if waiting for him to fill in the rest himself. He stopped with his fork halfway to his mouth and gaped at her incredulously.

"They could have made magic and miracles work," Heller went on after a few seconds. "They could have put their own, shall we say, `agents' into our culture way back in its ancient history and deliberately instilled systems of beliefs that we still haven't entirely recovered from—beliefs that were guaranteed to make sure that the rival would take a long, long time to rediscover the sciences and develop the technologies that would make it an opponent worth worrying about again. Meanwhile the Jevlenese have bought themselves a lot of time to become established on their own system of worlds, expand JEVEX, milk off more Ganymean know-how, and whatever else they've been up to."

From GIANT'S STAR by James Hogan (1983)

Time Radio

A Time-Radio is a specialized time machine which allow information to be sent, without gross physical interaction, from the future to the past. There are many examples in science fiction.

The time agents in the movie Millennium carry some kind of temporal walkie-talkie that allows them to talk in "real time" to the main base eighty-thousand years future. Presumably the "real time" aspect is maintained by allowing the walkie-talkie to send and receive only at a fixed interval into the future. So the elapsed time from when the agents arrive in the past and elapsed time from when the agents departed from the future stays synchronized.

In the movie Frequency a Heathkit single-sideband ham radio suddenly acts like a time radio during an occurrence of the aurora borealis. In the episode of Darkroom entitled Stay Tuned, We'll Be Right Back, a crystal radio starts acting like a time radio for no logical but very dramatic reasons.

(ed note: The Dirac communicator was named after Paul Dirac who predicted antimatter. It communicates instantly and has infinite range. So all sentient creatures in all the galaxies can listen in to what you say. As it turns out it is even worse than that. Each transmission starts with a "beep" noise. As it turns out, the beep is the sum total of all Dirac messages ever sent in all the past and all the future. By demultiplexing you too can receive messages from the future and violate causality.)

She paused and smiled. "I have heard," she said conversationally, "the voice of the President of our Galaxy, in 3480, announcing the federation of the Milky Way and the Magellanic Clouds. I've heard the commander of a world-line cruiser, traveling from 8873 to 8704 along the world line of the planet Hathshepa, which circles a star on the rim of NGC 4725, calling for help across eleven million light-years — but what kind of help he was calling for, or will be calling for, is beyond my comprehension. And many other things.

From BEEP by James Blish (1954)

      Cornelius shook his head. "We didn't expect success so easily. We just had to eliminate the most obvious possibility."
     She glanced around. "These are radio telescopes. Right? You're expecting to pick up back-to-the-future messages by radio waves?"
     "We're trying to build a Feynman radio here, Emma," Dan said.
     "Feynman? As in Richard Feynman?"
     Malenfant was smiling. "Turns out," he said, "there's a loophole in the laws of physics."
     Cornelius held up his hands. "Look, suppose you jiggle an atom to produce a radio wave. We have equations that tell us how the wave travels. But the equations always have two solutions."
     Dan scratched his belly and yawned. "Like taking a square root. Suppose you have a square lawn, nine square yards in area. How long is the side?"
     "Three yards," she said promptly. "Because three is root nine."
     "Okay. But nine has another square root."
     "Minus three," she said. "I know. But that doesn't count. You can't have a lawn with a side of minus three yards. It makes no physical sense."
     Dan nodded. "In the same way the electromagnetism equations always have two solutions. One, like the positive root, describes the waves we're familiar with, traveling into the future, that arrive at a receiver after they left the transmitter. We call those retarded waves. But there's also another solution, like the negative root—"
     "Describing waves arriving from the future, I suppose."
     "Well, yes. What we call advanced waves."
     Cornelius said, "It's perfectly good physics, Ms. Stoney. Many physical laws are time-symmetric. Run them forward, and you see an atom emitting a photon. Run them backward, and you see the photon hitting the atom."
     "Which is where Feynman comes in," Dan said. "Feynman supposed the outgoing radiation is absorbed by matter, gas clouds, out there in the universe. The gas is disturbed, and gives off advanced waves of its own. The energy of all those little sources travels back in time to the receiver. And you get interference. One wave canceling another. All the secondary advanced waves cancel out the original advanced wave at the transmitter. And all their energy goes into the retarded wave."
     "It's kind of beautiful," Malenfant said. "You have to imagine all these ghostly wave echoes traveling backward and forward in time, perfectly synchronized, all working together to mimic an ordinary radio wave."
     Emma had an unwelcome image of atoms sparsely spread through some dark, dismal future, somehow emitting photons in a mysterious choreography, and those photons converging on Earth, gathering in strength, until they fell to the ground here and now, around her.
     "The problem is," Cornelius said gently, "Feynman's argument, if you think about it, rests on assumptions about the distribution of matter in the future of the universe. You have to suppose that every photon leaving our transmitters will be absorbed by matter somewhere—maybe in billions of years from now. But what if that isn't true? The universe isn't some cloud of gas. It's lumpy, and it's expanding. And it seems to be getting more transparent."
     "We thought it was possible," Dan said, "that not all the advanced waves cancel out perfectly. Hence all this. We use the radio dishes here to send millisecond-pulse microwave radiation into space. Then we vary the rig: we send out pulses into a deadend absorber. And we monitor the power output. Remember the advanced waves are supposed to contribute to the energy of the retarded wave, by Feynman's theory. If the universe isn't a perfect absorber—"
     "Then there would be a difference in the two cases," Emma said.
     "Yeah. We ought to see a variation, a millisecond wiggle, when we beam into space, because the echo effect isn't perfect. And we hope to detect any message in those returning advanced echoes—if somebody downstream has figured out a way to modify them.
     "We pick cloudless nights, and we aim out of the plane of the Galaxy, so we miss everything we can see. We figure that only one percent of the power will be absorbed by the atmosphere, and only three percent by the Galaxy environment. The rest ought to make it—spreading out, ever more thinly—to inter-galactic space."
     "Of course," Cornelius said, "we can be sure that whatever message we do receive will be meaningful to us." He looked around; his skin seemed to glow in the starlight. "I mean, to the four of us, personally. For they know we are sitting here, planning this."
     Emma shivered again. "And did you find anything?"
     "Not to a part in a billion," Cornelius said.

(Stephen Baxter said: The "Feynman radio" idea of using advanced electromagnetic waves to pick up messages from the future is real. This has actually been attempted, for example by I. Schmidt and R. Newman (Bulletin of the American Physical Society, vol. 25, p. 581, 1979))

From MANIFOLD: TIME by Stephen Baxter (1999)

Time Viewer

A Time Viewer passive form of time machine which typically displays, but allows no interaction with, scenes from the past. There are many examples in science fiction. TV Tropes calls it a Chronoscope.

The viewer in Isaac Asimov's The Dead Past can see anywhere since it uses neutrinos. Which means it is also a spy ray, since it can view things that happened two milliseconds ago (i.e., more or less in real time). The authorities, knowing this technology will end privacy forever, strictly control its use and emphasize its utility in observing historical events (hoping everybody will overlook the question of when exactly "the past" starts). The same situation happened in reverse in Arthur C. Clarke and Stephen Baxter The Light of Other Days. In that novel all privacy was lost first, then they discovered how to use it to view the past.

The time viewing technology in T. L. Sherred's E for Effort shows how panic stricken such a device will make the corrupt political leaders of the various nations of the world.


     (Wilmont told Denny) "Mere probability is all we have left. And my first actual invention was a geodesic tracer, designed for probability analysis. It was a semi-mathematical instrument, essentially a refinement of the old harmonic analyzer. Tracing the possible world lines of material particles through time, it opened a window to futurity."
     The hoarse whisper paused, and old Wil McLan limped to the side of the dome. His scarred trembling hands lifted a black velvet cover from a rectangular block of some clear crystal mounted on the top of a metal cabinet.
     "Here is the chronoscope," he said. "A sort of window into time. It creates special fields, that bend radiation into the time-axis. We get a stereoscopic image in the crystal screen—there's a selective fluorescence to the beat frequencies projected from below."
     The old man snapped a switch, manipulated dials at the end of the crystal block. It lit with a cloudy green. The green cleared, and a low cry escaped Lanning's lips. Within the crystal, microscopically clear, he saw a new world in miniature.

     "It happened," the hoarse voiceless gasp went on, "that Gyronchi was the first future world, out of those possible, that the chronoscope revealed. Happened that I found Sorainya, splendid in her armor, fencing with one of her human ants.
     "You can see that she is—well, attractive. At first the range of the instrument was limited to her youth, where scenes of such barbarity are less frequent. Remember, Denny, I was thirty years younger when I first saw her, back in 1945. Her glorious beauty, the military pomp of her empire—I was swept away.
     "Neglecting all the other possible worlds, I followed her, for months—years. I didn't know, then, all the harm the temporal searchbeam was doing." His white head bowed; for a moment he was speechless. "But no process whatever can reveal the state of an electron without changing that state. The quanta of my scanning ray were absorbed by the atoms that refracted them. The result was an increase in the probability factor of Gyronchi—that is the root of all the tragedy."
     The scarred face made a grimace of pain.
     "The blame is mine. For, before I was aware of it, the absorption had cut down the probability of all other possible worlds, so that Gyronchi was the only one the limited power of my instrument could reach. That blinded me to the crime that I was doing.
     "But you can see the future," broke in Lanning. "Can't you tell?"
     "The chronoscope reveals no certainties," said McLan. "Only probabilities—which it changes even as it reveals them." His white head shook. "I know, though, that the balance of probability is far in favor of Sorainya."

From THE LEGION OF TIME by Jack Williamson (1952)

It was known that the Overlords had access to the past, and more than once historians had appealed to Karellen to settle some ancient controversy. It may have been that he had grown tired of such questions, but it is more likely that he knew perfectly well what the outcome of his generosity would be….

The instrument he handed over on permanent loan to the World History Foundation was nothing more than a television receiver with an elaborate set of controls for determining coordinates in time and space. It must have been linked somehow to a far more complex machine, operating on principles that no one could imagine, aboard Karellen's ship. One had merely to adjust the controls, and a window into the past was opened up. Almost the whole of human history for the past five thousand years became accessible in an instant. Earlier than that the machine would not go, and there were baffling blanks all down the ages. They might have had some natural cause, or they might be due to deliberate censorship by the Overlords.

There were drawbacks, of course, though they were willingly accepted. One had to be very old indeed to realize that the papers which the telecaster printed in every home were really rather dull. Gone were the crises that had once produced banner headlines. There were no mysterious murders to baffle the police and to arouse in a million breasts the moral indignation that was often suppressed envy. Such murders as did occur were never mysterious; it was only necessary to turn a dial—and the crime could be seen re-enacted. That instruments capable of such feats existed had at first caused considerable panic among quite law-abiding people. This was something that the Overlords, who had mastered most but not all the quirks of human psychology, had not anticipated. It had to be made perfectly clear that no Peeping Tom would be able to spy on his fellows, and that the very few instruments in human hands would be under strict control. Rupert Boyce's projector, for instance, could not operate beyond the borders of the Reservation, so he and Maia were the only persons inside its range.

From CHILDHOOD'S END by Arthur C. Clarke (1953)

      "We tackle our problems wherever we find them," Lanning answered, opening the cover and disclosing a chunky and complicated piece of apparatus mounted on an aluminum tripod. "I have no theories at all about these things. I'm here find out the truth, why this thing (Stonehenge) was built."
     "Admirable," Barker said, and the coolness of his comment was lost in the colder wind. "Might I ask just what this device is?"
     "Chronostasis temporal-recorder." He opened the legs and stood the machine next to the Altar Stone. "My team at MIT worked it up. We found that temporal movement—other than the usual 24 hours into the future every day—is instant death for anything living. At least we killed off roaches, rats, and chickens; there were no human volunteers. But inanimate objects can be moved without damage."
     "Time travel?" Barker said in what be hoped was a diffident voice.
     "Not really; time stasis would be a better description. The machine stands still and lets everything else move by it. We've penetrated a good ten thousand years into the past this way."
     "If the machine stands still—that means that time is running backwards?"
     "Perhaps it is; would you be able to tell the difference? Here, I think we're ready to go now."
     Lanning adjusted the controls on the side of the machine, pressed a stud, then stepped back. A rapid whirring came from the depths of the device: Barker raised one quizzical eyebrow.
     "A timer," Lanning explained. "It's not safe to be close to the thing when it's operating."
     The whirring ceased and was followed by a sharp click, immediately after which the entire apparatus vanished.
     "This won't take long," Lanning said, and the machine reappeared even as he spoke. A glossy photograph dropped from a slot into his hand when he touched the back. He showed it to Barker.
     "Just a trial run, I sent it back 20 minutes."
     Although the camera had been pointing at them, the two men were not in the picture. Instead, in darkish pastels due to the failing light, the photograph showed a view down the Avenue, with their parked truck just a tiny square in the distance. From the rear doors of the vehicle the two men could be seen removing the yellow box.
     "That's very … impressive," Barker said, shocked into admission of the truth. "How far back can you send it?"
     "Seems to be no limit, just depends on the power source. This model has nicad batteries and is good back to about 10,000 B.C."
     "And the future?"
     "A closed book, I'm afraid, but we may lick that problem yet."

From THE SECRET OF STONEHENGE by Harry Harrison (1968)

      Flag ship of the Astrarch’s space fleets, the Warrior Queen lay on her cradle, at the side of the great field beyond the low gray forts. A thousand feet and a quarter of a million tons of fighting metal, with sixty-four twenty-inch rifles mounted in eight bulging spherical turrets, she was the most powerful engine of destruction the system h.od ever seen.
     Brek Veronar’s concern was almost forgotten in a silent pride, as a swift electric car carried them across the field. It was his auto-sight—otherwise the Veronar achronic field detector geodesic achron-integration self-calculating range finder—that directed the fire of those mighty guns. It was the very fighting brain of the ship—of all the Astrarch’s fleet.
     For an instant, the short man’s smile seemed genuine. “The Astrarchy is indebted to you for the autosight. The increased accuracy, of fire has in effect quadrupled our fleets.” His eyes were sharp again, doubtful. “Are further improvements possible?”
     Brek Veronar caught his breath. His knees felt a little weak. He knew that he was talking for his life. He swallowed, and his words came at first unsteadily.
     “Geodesic analysis and integration is a completely new science,” he said desperately. “It would be foolish to limit the possibilities. With a sufficiently delicate pick-up, the achronic detector fields ought to be able to trace the world lines of any object almost indefinitely. Into the future—”
     He paused for emphasis. “Or into the past!”
     An eager interest flashed in the Astrarch’s eyes. Brek felt confidence returning. His breathless voice grew smoother.
     “Remember, the principle is totally new. The achronic field can be made a thousand times more sensitive than any telescope—I believe, a million times! And the achronic beam eliminates the time lag of all electromagnetic methods of observation. Timeless, paradoxically it facilitates the exploration of time.”
     “Exploration?” questioned the dictator. “Aren’t you speaking rather wildly, Veronar?”
     “Any range finder, in a sense, explores time,” Brek assured him urgently. “It analyzes the past to predict the future—so that a shell fired from a moving ship and deflected by the gravitational fields of space may move thousands of miles to meet another moving ship, minutes in the future.
     “Instruments depending on visual observation and electromagnetic transmission of data were not very successful. One hit in a thousand used to be good gunnery. But the autosight has solved the problem— now you reprimand gunners for failing to score two hits in a hundred.”
     Brek caught his breath. “Even the newest autosight is just a rough beginning. Good enough, for a range finder. But the detector fields can be made infinitely more sensitive, the geodesic integration infinitely more certain.
     “It ought to be possible to unravel the past for years, instead of minutes. It ought to be possible to foretell the position of a ship for weeks ahead—to anticipate every maneuver, and even watch the captain eating his breakfast!”
     The Earthman was breathless again, his eyes almost feverish. “From geodesic analysis,” he whispered, “there is one more daring step—control. You are aware of the modern view that there is no absolute fact, but only probability. I can prove it! And probability can be manipulated, through pressure of the achronic field.

     Brek Veronar sat at the curving control table. Behind him, in the dim-lit vastness of the armored room, bulked the main instrument. Banked thousands of green-painted cases—the intricate cells of the mechanical brain—whirred with geodesic analyzers and integrators. The achronic field pick-ups—sense organs of the brain—were housed in insignificant black boxes. And the web of achronic transmission beams—instantaneous, ultrashort, nonelectromagnetic waves of the subelectronic order—the nerve fibers that joined the busy cells—was quite invisible.
     Before Brek stood the twenty-foot cube of the stereoscreen, through which the brain communicated its finding. The cube was black, now, with the crystal blackness of space. Earth in it, made a long misty crescent of wavering crimson splendor. The Moon wsis a smaller scimitar, blue with the dazzle of its aitificial atmosphere.
     Brek touched intricate controls. The Moon slipped out of the cube. Earth grew—and turned. So far had the autosight conquered time and space. It showed the planet's Sunward side.
     Softly, in the dim room, a gong clanged. Numerals of white fire flickered against the image in the cube. An arrow of red flame pointed. At its point was a tiny fleck of black. (the first of the six-ship rebel fleet)
     He watched them, rising, swinging around the huge, luminous curve of the planet. They were only six mathematical points, tracing world lines through the continuum, making a geodesic pattern for the analyzers to unravel and the integrators to project against the future

(ed note: Battle is joined, but Tony Grimm's rebel fleet has a superior auto-sight. The Astrarch's fleet is destroyed, and the crippled flag ship is falling into the sun. But Brek thinks that time can be changed.)

     The Astrarch shook his pale head. “I’m the madman,” he whispered. “To speak of changing even two minutes of the past!” His hollow eyes clung to Brek. “Though you have done amazing things, Veronar.”
     The Earthman continued to stare at his huge creation. “The autosight itself brought me one clue, before the battle,” he breathed slowly. “The detector fields caught a beam of Tony Grimm’s, and analyzed the frequencies. He’s using achronic radiation a whole octave higher than anything I’ve tried. That must be the way to the sensitivity and pePetration I have hoped for.”
     Hope flickered in the Astrarch’s eyes. “You believe you can save us? How?"
     “If the high-frequency beam can search out the determiner factors,” Brek told him, “it might be possible to alter them, with a sufficiently powerful field. Remember that we deal with probabilities, not with absolutes. And that small factors can determine vast results.
     “The pick-ups will have to be rebuilt. And we’ll have to have power. Power to project the tracer fields. And a river of power—if we can trace out a decisive factor and attempt to change it. But the power plants are dead.”
     “Rebuild your pick-ups,” the Astrarch told him. “And you’ll have power—if I have to march every man aboard into the conversion furnaces, for fuel."
     Calm again, and confident, the short man surveyed the tall, gaunt Earthman with wondering eyes.
     “You’re a strange individual, Veronar," he said. “Fighting time and destiny to crush the planet of your birth! It isn’t strange that men call you the Renegade.”

     The wreck dropped Sunward. A score of expert technicians toiled, under Brek’s expert direction, to reconstruct the achronic pick-ups. And a hundred men labored, beneath the ruthless eye of the Astrarch him.self, to repair the damaged atomic converters.
     They had crossed the orbit of Venus, when the autosight came back to humming life. The Astrarch was standing beside Brek, at the curved control table. The shadow of doubt had returned to his reddened, sleepless eyes. “Now,” he demanded, “what can you do about the battle?”
     “Nothing, directly,” Brek admitted. “First we must search the past. We must find the factor that caused Tony Grimm to invent a better autosight than mine. With the high-frequency field—and the full power of the ship’s converters, if need be—we must reverse that factor. Then the battle should have a different outcome.”
     The achron-integrators whirred, as Brek manipulated the controls, and the huge black cube began to flicker with the passage of ghostly images. Symbols of colored fire flashed and vanished within it.
     “Well?” anxiously rasped the Astrarch.
     “It works!” Brek assured him. “The tracer fields are following all the world lines that intersected at the battle, back across the months and years. The analyzers will isolate the smallest—and hence most easily altered—essential factor.”
     The Astrarch gripped his shoulder. “There—in the cube—yourself!”
     The ghostly shape of the Earthman flickered out, and came again. A hundred times, Brek Veronar glimpsed himself in the cube. Usually the scene was the great arsenal laboratory, at Astrophon. Always he was differently garbed, always younger.
     Then the background .shifted. Brek caught his breath as he recognized glimpses of barren, stony, ocher-colored hills, and low, yellow adobe buildings. He gasped to see a freckled, red-haired youth and a slim, tanned, dark-eyed girl.
     “That’s on Mars!” he whispered. “At Toran. He’s Tony Grimm. And she’s Elora Ronee—the Martian girl we loved.”
     The racing flicker abruptly stopped, upon one frozen tableau. A bench on the dusty campus, against a tow adobe wall. Elora Ronee, with a pile of books propped on her knees to support pen and paper. Her dark eyes were staring away across the campus, and her sun-browned face looked tense and troubled.
     In the huge dim room aboard the wrecked warship, a gong throbbed softly. A red arrow flamed in the cube, pointing down at the note on the girl’s knee. Cryptic symbols flashed above it. And Brek realized that the humming of the achron-integrators had stopped.
     “What’s this?” rasped the anxious Astrarch. “A schoolgirl writing a note—what has she to do with a space battle?”
     Brek scanned the fiery symbols. “She was deciding the battle—that day twenty years ago!” His voice rang with elation. “You see, she had a date to go dancing in Toran with Tony Grimm that night. But her father was giving a special lecture on the new theories of achronic force. Tony broke the date, to attend the lecture.”
     As Brek watched the motionless image in the cube, his voice turned a little husky. “Elora was angry—that was before she knew Tony very well. I had asked her for a date. And, at the moment you see, she has just written a note, to say that she would go dancing with me.”
     Brek gulped. “But she is undecided, you see. Because she loves Tony. A very little would make her tear up the note to me, and write another to Tony, to say that she would go to the lecture with him.”
     The Astrarch stared cadaverously. “But how could that decide the battle?”
     “In the past that we have lived,” Brek told him, “Elora sent the note to me. I went dancing with her, and missed the lecture. Tony attended it—and got the germ idea that finally caused his autosight to be better than mine.
     “But, if she had written to Tony instead, he would have offered, out of contrition, to cut the lecture—so the analyzers indicate. I should have attended the lecture in Tony’s place, and my autosight would have been superior in the end.”
     The Astrarch’s waxen head nodded slowly. “But—can you really change the past?”
     Brek paused for a moment, solemnly. “We have all the power of the ship’s converters,” he said at last. “We have the high-frequency achronic field, as a lever through which to apply it. Surely, with the millions of kilowatts to spend, we can stimidate a few cells in a schoolgirl’s brain. We shall see.”
     His long, pale fingers moved swiftly over the control keys. At last, deliberately, he touched a green button. The converters whispered again through the silent ship. The achron-integrators whirred again. Beyond, giant transformers began to whine.
     And that still tableau came to sudden life.
     Elora Ronee tore up the note that began, “Dear Bill—” Brek and the Astrarch leaned forward, as her trembling fingers swiftly wrote: “Dear Tony—I’m so sorry that I was angry. May I come with you to father’s lecture? Tonight—”
     The image faded.

(ed note: Time re-sets itself, the battle happens only this time the Astrarch has the superior autosight. Alas for the dictator, the battle turns out the exact same way. Apparently they live in a universe with a self-healing time line.)

     Looking at the muttering men, Brek shook his head. “I was mistaken,” he said deliberately. “I failed to take account of the two-way nature of time. But the future, I see now, is as real as the past. Aside from the direction of entropy change and the flow of consciousness, future and past cannot be distinguished.
     “The future determines the past, as much as the past does the future. It is possible to trace out the determiner factors, and even, with sufficient power, to cause a local deflection of the geodesics. But world lines are fixed in the future, as rigidly as in the past. However the factors are rearranged, the end result will always be the same.”

From HINDSIGHT by Jack Wiliamson (1940)

Time-Machine Computer

First off, remember that "closed-timelike curves" (CTC) is a code word for "time machine", that is, a way to travel backwards in time.

As it turns out, if you have a way of sending things backwards in time, you can use this to make a really fast quantum computer. "Really fast" is defined as "able to quickly solve problems that would take a conventional computer longer than the lifespan of the universe".

In 2002 Dr. Todd A. Brun wrote a paper suggesting that a computer having access to CTC could actually solve solve NP-complete (NP + NP-hard) and PSPACE-complete problems. In 2003 Dr. Dave Bacon wrote a paper suggesting that a quantum computer using CTC could easily solve NP-hard problems.

But anytime closed-timelike curves are involved, the dread spectre of Temporal Paradox rears its ugly head.

In late-breaking news, Dr. Xiao Yuan et al came up with a method to use closed-timelike curves that avoided the horror of temporal paradox. In a 2015 paper they demonstrated that a computer can use quantum entanglement to prevent a time-traveling particle from interacting with the past (thus avoiding temporal paradox) yet the particle's entanglement with another particle in the present will create a gain in computational power.

You see, if you prevent the particle from interacting with the past, it becomes what is called a Deutschian open timelike curve, free from temporal paradox. But if the particle does not interact, there is no gain in computer power.

Dr. Yuan's technique prevents the particle from interacting (thus preventing paradox) but cleverly uses quantum entanglement to get a gain in computer power.


(ed note: in this story the CTC computer just rampantly breaks causality all to heck. They already have problems with causality being shattered by their faster-than-light drive. So they figure causality is highly over-rated)

     But Nilis had assigned her to another part of the project, the development of his "CTC computer," as he called it. his closed-timelike-curve time-travel computing machine. Nilis made it clear that he considered the CTC-processor work just as important as experiments with the Xeelee ship, and she had to accept the assignment.

     "I'm one of his advisors, yes." Pila waved a hand at the prototype. "Very impressive. And it's all based on time travel?"
     "Closed-timelike-curves, yes." Torec pointed to the ducts. "Pilot Officer Pirius—Pirius Blue—defeated the Xeelee because his fellow pilot used her FTL drive to bring tactical information back from the future. So we have miniature bots in those tubes. The bots are the components of the processor. They fly back and forth, and actually jump through short FTL loops."
     "Little starships in tubes! And these bots travel back in time and tell you the answer before you even pose the question?"
     "Something like that."
     "How marvelous."
     The dummy problem they were hoping to run today concerned protein folding. Proteins were the structural elements of life, but remained beyond the capability of humans to design optimally. There were more proteins a hundred components long than there were electrons in the universe; to work out how many ways a long protein molecule could fold up was an ancient problem, previously insoluble even in principle. "But we hope to crack it," Torec said. She pointed to a large blank Virtual screen. "The results will be displayed there."

     "This is only the first step. A proof of principle. Eventually we will have to cram this down into a unit small enough to be carried on a greenship."
     "A clever answer," the woman murmured. "And what is your key problem?" Torec shrugged. "Control of those flying bots. obviously. We've a list of issues."

     "Describe your algorithm."
     Torec took a breath. Despite the way she had hammered away at her techs to get them to talk to her comprehensibly, the theory of the CTC software was still her weakest point. "We give the system a problem to solve, in the case of our prototype to find a particular protein geometry. And we give it a brute-force way to solve the problem. In the case of protein folding, we instruct the processor simply to start searching through all possible protein geometries. And we have a time register, a special cache that stores a flag if a signal has been received from the future.
     "The basic CTC program has three steps. When the processor starts, the first step is to check the time register. If a signal has been received—if the solution to the problem is already in memory—then stop. If not, we go to step two, which says to carry out the calculation by brute force, however long it takes. When the answer is finally derived, we go to step three: go back in time, deliver the solution and mark the time register."
     Luru nodded. "So the timeline is redrafted. In the first draft timeline, the problem is solved by brute force. In the final version of the timeline, the answer is sent back through time to the moment when the question is posed. So it isn't necessary to run the computation at all."
     Luru sighed. "The joy of time-travel paradoxes. You can get the answer to a problem without needing to work it out! But there must be a good deal more to your design. Your closed-timelike-curves must be pretty short."
     "Actually just milliseconds."
     "Surely you can solve no problem which would take longer to solve than that length of time."
     Torec smiled, her confidence growing. "No. By breaking a problem down into pieces you can solve anything." She described how the problem was broken up into a hierarchy of nested subcomponents. At the base level were calculations so trivial they could be handled within the processor's short CTC periods. The answers were passed back in time to become the input for the next run-through, and so on. That way an answer was assembled piece by piece and looped back repeatedly to the zero instant, until the overall problem was resolved. "The technical challenge is actually decomposing the problem in the first place, and controlling the information flow back up the line," she said.
     Luru laughed, an odd. hollow sound. "You're computing with multiple time loops, and you think that's the only challenge? Ensign, you're a true pragmatist. ... I think it's nearly time."
     Over the glittering, much-patched array of the prototype processor, the bots hovered, utterly motionless against the greater lunar stillness. Behind the prototype, the blank Virtual screen hovered, waiting to display the solution.
     The last seconds wore away.
     And at zero, the screen filled with a molecular diagram. Just like that, with no time elapsed. It was almost anticlimactic, Torec thought.

     The bots descended on the prototype complex, checking its physical integrity. But ironically. Torec knew, there should be little for them to find, for as the processor's paradoxical operation had worked, there had been no need for the problem to be brute-force solved, and no need even for the little toy ships to chatter back and forth on their FTL hops—in this draft of the timeline anyhow. The curves in time had served their purpose—and had rendered their own existence unnecessary. It was another peculiar advantage of a time-travel computer. If it worked correctly, it never actually ran at ail—and so it should never wear out. Some of the techs had even debated whether they could get away with the economy of making the processors shoddily, almost at the point of failure—for that failure would never be tested.
From EXULTANT by Stephen Baxter (2004)

Literary Crutches

There are some standard limits on time travel used by science fiction authors. These do not exist because physics demand them, they exist because the SF writers and readers would become hopelessly confused otherwise.

A common one is the Linearity Principle or San Dimas Time. This makes a scientifically ridiculous (but reader comforting) one-to-one correspondence between past time and present time. If the time travelers go back in time to meet Alexander the Great and spend a week in the past, they will return to the present exactly one week after they left. If Time Patrol agents are dispatched from Project Tic-Toc to 1914 (monitoring the start of World War 1), the agents used their Time RadioTM to call Tic-Toc Control in the present to request some vital historical research, and the research takes two days, when the info is sent back to the agents in 1914 the agents will get the message exactly two days after they made the request.

In reality travelers should be able to return to any time they want (even an hour or so before they left), and Tick-Tock should be able to sent the info to the agents before the agents ask for it. But this gets very confusing, and quickly sets up some nasty temporal paradoxes.

A related literary crutch is the Delayed Ripple Effect. So the moron in A Sound of Thunder goes back in time to the late Cretaceous Era for some big-game T-Rex hunting, and like a total idiot steps on a butterfly and thus alters history. In reality all of history should be changed "instantly" (if that means anything) as soon as the butterfly is crushed.

However in movies such as Millennium there is a ridiculous delay. The Time Control Central in the far future sends time agents back to 1989, where they screw up and create a paradox that will result in Time Control Central never existing in the first place. But from the viewpoint on Time Control Central, the "time quake" created by the paradox progresses up the timeline at the rate of a few second of delay per timeline year. The end result is that Time Control Central has about ten minutes to do something before they cease to exist. Scientifically bogus, but dramatically exciting.

Left unanswered is the question of exactly when the ten minute count-down starts at Time Control Central. Especially considering that the time agents screwed up in 1989, several thousand years prior to the current "Now" at Time Control Central.

Sometimes in literary time-travel, the universe has a Dynamic timeline but it acts like a Self-Healing timeline or even a Fixed timeline. This means the timeline contains Clock Roaches or even Time Police. This creates an Enforced Fixed timeline.

Remember the weakly god-like AI called the Eschaton in Charles Stross's novel Singularity Sky, who commands: "Thou shalt not violate causality within my historic light cone. Or else."

Yet another common trope is a time traveler bent on changing history learning the hard way the law of unintended consequences. A sub trope is somebody else having to clean up the mess you made.

A literary trope that I have not managed to figure out is the "You Can't Use Time Travel To Meet Your Older/Younger Self". In reality there is nothing to prevent this. In fact, that trope is routinely violated in Poul Anderson's novel There Will Be Time and violently violated in Robert Heinlein's All You Zombies.

The Terminator movies and The Time Traveler's Wife made up the rule that time travelers could be sent through time, but they could not carry anything with them. So they arrived at their destination stark naked. This seems more like a rule made up to increase the movie ratings more than anything based on real science.

A good summary of the various ways time-travel seems to work in various SF novel can be found in the Guide To SF CHRONOPHYSICS and of course the infinite time-sink TV Tropes.


(ed note: In Andre Norton's Time Trader series the time agents sent back in time thousands of years are doing everything possible not to alter history. They need a way to cover up so that the natives do not discover that they are impostors. One good technique is to pretend that you are a trader from a far away land.)

     “Trade, eh?” Renfry nodded. “Heard how you boys on the time runs play that angle.”
     “Its’ a good cover, one of the best there is. A trader moves around without question in a primitive world. Any little strangeness in his speech, his customs, his dress, can be legitimately accounted for by his profession. He is supposed to come from a distance, his contacts don’t expect him to be like their fellow tribesmen. And a trader picks up news quickly. Yes, trade was a cover the project used from the first.”
     “You were a trader, back in time?” Travis asked.
     Ashe appeared willing enough to talk of his previous ventures. “D’you ever hear of the Beaker Folk? There were traders for you — had their stations from Greece to Scotland during the early Bronze Age. That was my cover, in early Britain, and again in the Baltic. You can really be fascinated by such a business. My first partner might have retired a millionaire — or that period’s equivalent to one.”

From GALACTIC DERELICT by Andre Norton (1959)

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