Copyright � 2026 by Shane
Tourtellotte
Thanks to tremendous scientific leaps and more than one stroke of luck, the physical barriers to practical time travel have fallen. This now leaves us confronted with the metaphysical barriers to time travel. They aren’t as absolutely forbidding, but they are real and they can bring disaster.
With time travel introduced, the normal order of the universe is loosened into an unfamiliar new form. We’re used to a universe where cause precedes effect, but time travel makes it possible for effects to happen before their own causes. This opens the door to logical inconsistencies, or, to be more frank, impossibilities.
To enter the world of time travel is to move within half a step of the terrifying realm of paradox, where actions can prevent themselves from happening, or effects can take place without any cause. Nature abhors paradoxes even more than it proverbially abhors a vacuum, and if you cause one, it’s going to abhor you too.
The nature of paradoxes, and what they do to the universe, is an indispensable field for the would-be time-traveler to study and understand. What can happen, how it can happen, what its effects would be, and how to avoid those effects will guide you in making your time-travel adventures safer. Fortunately, paradox avoidance is not a straitjacket or a tightrope. With foreknowledge and sensible precautions, you can make your travels in confidence rather than fear.
Categorically Impossible
There are numerous types of paradox, some of them well known even to people with only a passing knowledge of time-travel theory. Mostly, though, they are subsets of two main categories:
1 – The Consistency Paradox. A time-travel action prevents a cause already seen. This would be a time-traveler (say, me) going back and preventing an event I knew had happened before I set out.
2 – The Closed Causal Loop. A time-travel action produces a cause already seen. This is me traveling back in time and causing an event that happened before I went back to intervene.
I will give several examples of each category, to let you see the wide assortments of ways that smart people fear you will do something impossible and mess up their tidy, ordered universe.
1A – Polchinski’s Paradox. Theoretical physicist Joseph Polchinski conceived of a way that effect could prevent its own cause using the thought experiment of a billiard ball and a wormhole. I was never any great shakes at billiards, but I am pretty decent at miniature golf, so I will render his paradox with my own spin on it.
I’m at the 7th hole of my local mini-golf course. My shot from the tee funnels into a tube which curls through 270 degrees, sending the ball shooting across its earlier path and onto the green, where it’s an easy tap-in to make my par.
However, the owner, Mr. Polchinski, added a fresh challenge to the hole. The tube now contains a wormhole which sends my ball several seconds back in time. When it emerges from the tube, the ball is just in time to hit its earlier self, sending it off-course. It misses the tube, rolls into the penalty area, and condemns me to a double-bogey at best.
Only … if it missed the tube, it couldn’t have come around to strike itself. That means it would have gone into the tube … and come out to prevent its entering the tube, and so on. The event becomes a Möbius strip: if it happens, then it won’t happen, which means that it will then happen1.
This is the essence of the consistency paradox, pared down to the bare bones of mechanical determinism. The next version of the paradox fleshes it out in stark human terms.
1B – The Hitler Paradox. Many people have heard of this scenario, with or without its paradoxical implications. Travel back in time to kill a young Adolf Hitler, and by derailing the horrors of the Second World War and the Holocaust, you save millions of lives and make the world a better place. A common version poses it as a moral dilemma: could you kill an innocent infant destined to grow up to be the enormously guilty Hitler?
I will remove the moral paradox to make the existential one clearer.
It's October 31st, 1914. German forces have just driven off British defenders at a tiny Belgian village named Gheluvelt, and are waiting for further orders to exploit their victory. Among the German units is the 16th Bavarian Reserve Regiment, which counts among its numbers Private Adolf Hitler.
You, the intrepid pre-avenging time-traveler, are holed up in a well-chosen sniper’s nest. You spot Private Hitler2, and take aim. There are no moral qualms about shooting a willing combatant in the First World War. You make the shot, and decamp before the British counterattack that sends the Germans running, apart from the one you just killed.
Congratulations. You slew the monster. There are only two problems. The first one I will elide, as we’ll take a closer look at it later. The second is the world history you just changed so massively. You just went back in time to do exactly what? Kill an anonymous soldier, because he would have started a war that didn’t take place3? Where could you have gotten that idea? You would have had no motivation for your act of wanton murder. You would have done something better with your time, in both senses.
Thus the Möbius strip twists and connects again. Killing Hitler would have erased your motivation for killing Hitler, so you wouldn’t have done it, restoring your motivation, and so on ad infinitum.
In case that hasn’t driven the lesson home, the next example is even more personal.
1C – The Grandfather Paradox (Version One). Even more people have heard about this scenario than the Hitler Paradox. The setup is simple: you go back in time, and kill your grandfather as a youth, before he’s had a chance to sire your parent4. It’s usually assumed that this is murder, but the situation unfolds just the same if it’s an accident.
With grandpa dead, his children will never come into being, and neither will his grandchildren. They, of course, include you. You never existed, so obviously you never went back in time to kill him, so nothing prevented him from having his family. We are back on the Möbius highway, and there is no off-ramp.
This is, by the way, the other problem with the Hitler Paradox. Killing Hitler changes world history so much that it becomes awfully unlikely that your parents met and mated the way they did in the original timeline5. That means you don’t come to be, so you didn’t travel back to expunge that feckless artist, and this is becoming all too familiar by now.
Let’s relieve the tedium, and look at examples from the other category of time paradoxes, the closed causal loops.
2A – The Ontological Paradox. This is also known as the Bootstrap Paradox, or in the specific case I’m going to give, the Inventor Paradox.
You’re in your workshop, struggling over a seemingly intractable problem with the circuitry of your time machine. Who should appear but your elder self, in a time machine? Elder-you gives Now-you a look at the inner workings of the machine, or maybe gives Now-you a copy of the circuit diagram, and departs. Enlightened, you complete your time machine and go off on your adventures, which one day include that trip to help your past self.
This sounds like a problem creatively solved, except for one nagging question. Who devised the circuitry?
You didn’t: you needed help. The person who helped you didn’t, because that person was you, and we already established that you didn’t.
This is the reverse of the Möbius highway with no exit ramp. Now we have an untwisted loop, but with no on-ramp. There is no place where the information can enter the loop6. How did it reach you?
There is a paradox related to this, but I’ll postpone that examination to look at our next enigma.
2B – The Predestination Paradox. You’ve traveled back to October 8, 1871, to the house of Patrick and Catherine O’Leary on the West Side of Chicago. You’re hovering near their barn fronting De Koven Street, hoping to prove or disprove a legend. Did Mrs. O’Leary’s cow really kick over a lantern to start the Great Chicago Fire?
The answer is negative, because as you sneak into their barn, you stumble and knock over that lantern7. After a few frantic seconds of trying to extinguish blazing hay, you panic and skedaddle -- and the city burns behind you8.
The closed causal loop has struck again. The effect -- the famed fire -- has produced its own cause -- you going back to witness, but also start, the fire. You wouldn’t have gone back to that time and place without the motivation of seeing the event you yourself were to cause, and by your perception had not yet caused. How could you have done it before you gave yourself reason to?
Note that it doesn’t help matters for you to make another journey to 1871, this time to steer your alter-self away from the O’Leary barn. You’d just be engaging in a different version of Polchinski’s Paradox, as the ball knocking yourself off-course. Resolving one paradox by invoking another is a lateral move at best.
Speaking of making moves you probably shouldn’t, we come to the next paradox.
2C – The Grandfather Paradox (Version Two), AKA I Am My Own Grandpa. This one applies only to male time-travelers, so any females reading this may lean back and feel smug.
This one has a familiar start: you travel back to a time before your grandfather had sired your parent. In this instance, however, you make the acquaintance of your grandmother-to-be, and your feelings go in a direction much different from the familial. Hormones take their course, and the result is that grandma ends up with your bun, not grandpa’s, in the oven9.
The timestream may not care about your warped sex drive, but it sure cares about paradox. How can you be your own progenitor, your own descendant? If some mischance had killed you before you went on your time-traveling prowl, would you cease ever to have existed?
There’s even a further objection beyond the paradoxical -- and I’m not even speaking about the moral issues, which could fill their own book. How could your lucky sperm combine with grandma’s ovum to produce the exact same result as grandpa’s no-longer-lucky sperm? It couldn’t, right?
Wrong. In the interest of strict fairness, and of stretching out one of the most salacious sections of this book, I must state that this scenario is not excluded solely by biological laws. It actually works better a generation downstream, by replacing your father rather than your grandfather, but I’ll stay with the current example.
We all learned in biology class that we get half our genes from our mothers and half from our fathers. While true, this is a statement of probability, not an ironclad law of even division. It’s like saying half the time you flip a coin it comes up heads, and the other half it comes up tails. It can diverge wildly in short runs, but flip the coin often enough, and the result will very probably come close to the fifty-fifty split.
Time-travel genetics can strain those laws without breaking them. The genes that the child in this case got from its mother (your grandmother), she can still provide. The others, that couldn’t come from her, you can provide. If those genes match the ones your grandfather would have provided, you get the same result from two differing causes, and genetically the timeline is unchanged.
Having it all fall into place this way is very, very improbable -- which is to say, it’s not impossible. It can happen in the genetic sense. All we need to worry about now is the paradox. And the morality.
To give your brain a break from that, I’ll rewind to that cousin to the Inventor Paradox I promised you. If you’ve ever wondered how to get rich from time travel, this is and then is not one method.
2D – The Bilking Paradox. You possess a time-travel device that can only send information twenty-four hours into the past. You use it to send last night’s Powerball numbers to yourself last morning. Past-you buys the ticket, hits the jackpot, and goes on an epic bender to celebrate. Waking up hungover some thirty hours later, you realize you were supposed to have sent your past self a message with the winning numbers, but forgot in the excitement. It’s too late now: Past-you won’t get the information in time.
If you’re a mean drunk, this could even be deliberate. You could refuse to send back the numbers as a taunt of your previous poor self. If you’re a creatively mean drunk, you might send back the Mega Millions numbers instead, claiming they’re the Powerball numbers10. The result is the same: you don’t win the lottery.
This paradox starts as a feint toward the closed causal loop, but it cannot really be one. The vital information does get into the loop, by watching the drawing. It swerves instead into a consistency paradox, in a way now very familiar to you. How did you both succeed and fail to perform the same task at the same time? Which result happens, winning or not winning?
The Inventor Paradox would become this paradox if you changed your behavior. Once you got the time machine diagrams from Future-you, you could decline to make the future trip back to complete the loop. Who’s going to make you? This contrary act of free will turns a causal loop paradox into a consistency paradox, another of those pesky lateral moves.
Paradox Lost
From all I’ve thrown at you, your head may be reeling from logic-bending paradoxes. You may be glad for something, anything, to show that the maddening and self-contradictory implications don’t really exist. You want the paradoxes resolved. So have plenty of other people, who thought hard about why they wouldn’t really come to pass, or wouldn’t break the universe as we know it.
Perhaps the clearest hypothesis against paradoxes originated with Soviet physicist Ivan Novikov. His Self-Consistency Principle states, with the abstruse physics shorn away, that local solutions to the laws of physics must be globally self-consistent. Any event that would cause any change to the past thus has a probability of zero. In short, paradoxes cannot happen.
This principle was stated in even simpler terms by science fiction writer Larry Niven: “If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.”
This is a fine declarative statement, whichever one you use, but it’s all that it is. There’s no enforcement mechanism; there’s no penalty for violations. If you try to accelerate faster than the speed of light, you get the relativistic effects of time dilation and increasing mass that keep light speed forever beyond your reach. Neither Novikov nor Niven posits any such thing for time travel creating a paradox: you just cannot do it.
Both men were saying, Novikov by implication and Niven more directly, that time travel cannot happen at all. This was a simple and clear solution, until we invented time travel. With the enforcement mechanism gone, all they can rely on now is that penalty for violations.
Others have made some effort to impose that penalty. Stephen Hawking posed what he called the Chronology Protection Conjecture, which mainly states his belief that the natural universe could not survive if time travel was possible. He quipped in support that the best evidence of time travel being impossible was that we were not overrun by tourists from the future.
This flash of wit was actually poor evidence for his assertion, since the theoretically practical methods for time travel then recognized did not allow travel to before the point at which the time machine was created, as noted earlier in this book. If someone invented the time machine the day after Hawking made his comment, it would be two days after that he might have seen the flood of time tourists. The actual invention of time travel rescued Hawking from this fallacy, but left other statements about the Chronology Protection Conjecture intact.
Hawking posited, though he could not prove by mathematics, that closed timelike lines would create infinite stress-energies between themselves and the regular areas of the universe where time wasn’t being bent. In short, space-time would be torn apart in something called a back reaction, which would propagate backward in time and annihilate any time machine before it could produce any time travel.
This idea, in slightly less cataclysmic terms, has made its own niche in science fiction. Paul Levinson’s novelette “The Chronology Protection Case,” published four years after Hawking offered the Chronology Protection Conjecture, imagined physicists about to crack time travel, and others only hearing about their work, falling victim to a series of unlikely accidents that deterred them from pursuing the discovery. Robert R. Chase’s “Immune Response” from 2023 covers similar ground, with a scientist deliberately discrediting his own career so others wouldn’t pursue his time-travel work and die the suspicious deaths his colleagues did.
This strong version of the Chronology Protection Conjecture is now fairly disproven: too many people have survived their jaunts through time. I will soon have more to say about a weaker interpretation of the conjecture.
Another solution to the problem of paradoxes uses one of the most famous ideas in speculative physics, the Many-Worlds Hypothesis. To condense immensely for those who haven’t heard of it, Many-Worlds says that each time something can happen one of two ways, it happens both ways. The universe splits, one version having you wait for the next light, the other having you dash across, or try to. With all the possible branchpoints in everyday life, down to the quantum level, this produces an immense number of universes in a very short time, and the numbers only keep going higher at dizzying speed.
The time-travel upshot is that when your rifle bullet meets Hitler’s skull, reality splits into one universe where you killed him and one where you didn’t. Which one you return to when you leave the past is not resolved in this theory, though it matters quite a lot to you. Return to your original present, and it’s as if you failed. Return to the split-off present, and you succeeded, but everything you once knew, such as your family, will be unrecognizable if it even exists at all. You probably won’t have to worry about the awkwardness of meeting your parallel-universe self, because you probably didn’t come to exist in that parallel universe. Still, you have created a Führer-less world -- which would have come into existence anyway by Adolf dying of a billion-to-one stroke on the same day, so it isn’t so much of an accomplishment.
If the many-worlds solution to time paradoxes was not dispiriting enough, this next one will be. It doesn’t start so bad, merely being an assertion that the past does not happen twice. Whatever happened in your past will still happen when it becomes your present or your future. If you affected the past, that effect was always part of the flow of time.
The problem is that this leads to something called the “block universe.” By this concept, all of spacetime is a pre-existing entity, fully formed and determined from its inception. Nothing can change what happens within it, not even the conscious beings that occupy increments of its time and space. By saying not “this will happen” but “all things have already happened in an external objective sense,” it leaps beyond determinism into absolute fatalism. There’s nothing left even for God to do: it was all done before the clock of this universe began ticking.
This could be the case -- but what’s the point of thinking so? If you believe it’s true, and if your belief is correct, you were compelled to believe it. If instead free will does exist in the universe, you still may choose to believe in the fatalist model, but you’ll be wrong. Beholden or mistaken: neither is complimentary to you. Believing in free will ends up akin to Pascal’s Wager: there’s no penalty if you’re wrong, and plenty of benefit if you’re right11.
Paradox Protection, by Increments
Even if you embrace your power to affect the universe, there is still the pesky matter of the universe abhorring paradoxes. If it doesn’t obliterate time machines before they can be built, it can still affect how they operate, and what those carried by them can accomplish. The mechanism is posited by what is called the Timeline Protection Hypothesis.
This hypothesis states that, where a paradox could be created, probability distortions will arise to thwart the paradoxical incident. Improbable events will occur to prevent an impossible event. In the more lenient interpretations, your rifle will jam before you can shoot Hitler, or your sniper’s nest will be discovered and you’ll have to flee, your mission aborted. In the harsher interpretations, your time machine will blow out before your trip -- or during it. Alternatively you could have a stroke and drop dead.
This certainly can serve to prevent the big banner-headline paradoxes, while still permitting some time travel. What the hypothesis does not explain is how it can allow the million tiny paradoxes naturally attendant to time travel.
If you travel to the past, you are going to affect the past. The random pedestrian dodging around you in 1917 Petrograd may, through his new course, end up jogging the elbow of Lenin a kilometer away. The dirt you tread in rural medieval England will bear your print, and perhaps cause an otherwise sure-footed farmer to stumble. The very air will be shifted by your passage, if not actually breathed in and then expelled with less oxygen and more carbon dioxide than it originally held. Everything you do changes history, at least on the smallest scale, and the universe doesn’t recognize anything as too small to worry about.
How, then, can it make sense? How can big effects be prevented, but many little effects be allowed to propagate until they perhaps create their own big effects? Time machines work, but doesn’t the Timeline Protection Hypothesis say they shouldn’t?
The answer is that it doesn’t. A big, obvious improbability will be necessary to prevent a big, obvious paradox. For the myriad tiny ones, a series of tiny, subtle shifts in probability will do the job. The longer ago your actions in the past are, the more time there is to adjust things so that the world ends up just as we observe it today.
Your swerving pedestrian in Petrograd will end up keeping out of the path of someone else fifty meters down the street, and thus miss his encounter with a baleful Vladimir Ilyich. Quicker erosion or earthworms or a wayward sheep will fill in your footprint enough that it won’t impede the farmer. Your exhaled carbon dioxide will make its way to nearby plants that take it in and then replace your used oxygen. They’ll grow a little more robustly for it, something else will come along later to smooth out that change, and so on. Each iteration trades the extant change for a smaller one, until it vanishes in a puff of irrelevance.
The established timeline effectively has an attractive force. It “wants” to be the way it originally was; it “wants” to regain its original shape. The world “wants” to be the one you knew before you embarked on your time trip, and if you don’t jostle it out of place too severely, it will be, without severely jostling you right back.
Here you have another ally, in the fuzziness of the world you know. Quantum indeterminacy is a time-traveler’s friend, and a forgiving one, and not only by producing those useful Planck-level wormholes.
We see how the world is today, but we don’t see it past a certain level of resolution. We don’t know the exact geological contours or chemical composition of a patch of old farmland in England. If there’s some subtle concavity or a six-century-old trace of shoe sole rubber that shouldn’t have been around six centuries ago, it probably makes no difference in the world we perceive. If someone eventually notices some peculiarity there, that doesn’t affect our present, only our future.
This can hold true on a larger level. History, as written and told by humans, is our record of what has happened. It is not exactly the same thing as what has happened, due to millions of distortions and omissions both deliberate and innocent, along with the entropy of time destroying historical documents. Indeed, seeing past all those distortions and gaps is one of our compelling reasons to want to travel to the past and experience it for ourselves.
The changes you create in the past may slip right through the cracks. Most will be small enough that they would not get into the historical record, or get people talking about what you did. Anything that did get into a book or a newspaper or local gossip might still be small enough that it would vanish. The book would fall into obscurity; the newspaper would be tossed away the next day; tales of the odd things that happened when you met Walter Raleigh would grow distorted with retellings until any truth in them was effaced, assuming people didn’t merely stop talking about them.
Even if you did bump some larger-scale history off course, it might not affect recorded history. The new version you created might not be noted down. Somebody with an agenda might portray it as his preferred version, the one we knew all along. It could even be that history didn’t really unfold as written to begin with. Your alteration might simply change the true version that was going to be suppressed for a convenient fiction all along.
All this does not mean that you don’t need to worry about producing paradoxes. It does mean that the past is not one gargantuan minefield where any step you take will blow you up. If you take sound precautions not to make a spectacle of yourself or to shift the course of someone’s life massively -- such as by ending it prematurely -- you have a very good chance of keeping the timeline clean, or at least clean enough that it can tidy itself up.
Still, even if you don’t have to worry about little paradoxes, there remain those big ones. What keeps you from shooting young Hitler or knocking over that lantern in the O’Learys’ barn? What prevents you from either killing or cuckolding Grandpa? Does it even get prevented?
Here we are forced into greater speculation. Evidence for how a paradox is avoided, for a convergence of reasons, is difficult to come by. The few reports that have trickled in from time-travelers, or people familiar with them, do give some shape to what appears to be the mechanism.
The Timeline Protection Hypothesis does appear to work on this larger scale. Strange coincidences happen to prevent actions that might cause consistency paradoxes. They can be small incidents in the past being visited, or larger ones in the present before a time journey is undertaken. A sizable number of the incidents in question are accidents, medical crises, or outright deaths occurring to someone about to travel back in time. Precise rate statistics are virtually impossible to produce, but they clearly happen far more often than simple chance can explain.
The diverting events in the past don’t appear as threatening to life and limb, though again precision is impossible because you cannot get someone who travelled to the past and was never heard from again to take a survey12. The ones we’ve learned about have tended toward peculiar flukes, like a highly unlikely equipment malfunction in a time-sensitive situation, or falling prey to laryngitis before getting to speak to a noted historical figure13. Nothing that observers might interpret as supernatural or miraculous has been reported, which is what one would expect. Avoiding one big unconcealable change in history by creating another one solves nothing.
It is probable that the process must cycle a number of times before it succeeds. It isn’t a sentient timeline making a specific adjustment to itself: it’s a reset from an original state, with a fresh set of random chances for something to happen that disrupts the impending paradox. The events may run through the Möbius loop five, twenty, a hundred, or a million times before the right bit of freak luck comes along. The participants, and instigator, will perceive it happening only once, the final time that breaks the loop.
Do not take this to mean that producing a paradox is safe or cost-free. Every iteration through the loop means a fresh chance that the incident breaking the loop will happen to you. You’ll get sideswiped by a runaway ox-cart, or not dodge that one fellow’s sword thrust so nimbly. You’ll trip over your tongue and say the wrong thing to the Emperor, or just have a fatal aneurysm14. The paradox won’t be a disaster for history, but it may easily become a disaster for you. Avoiding it remains absolutely imperative.
Some readers are now turning a couple pages back and objecting that I extolled their free will there, but am denying it here. What good is personal autonomy if you can’t go put one between Adolf’s eyes? It might as well be a block universe after all.
This shows a fundamental misunderstanding. You have a right to your free will; you don’t have a right to succeed. It may be your will to take a running start and leap to the Moon, but the limits of your body and the physical laws of the universe will thwart you. It may be your will to turn the Crash of 1929 into a mixed day of trading on Wall Street, but laws as immovable as gravity, just less obvious, will prevent this. They may do this even more painfully than the fall you take from jumping for the Moon, so watch it.
Footnotes:
This would make it the second strangest occurrence I’ve seen on a miniature golf course.
Be careful about your identification. Hitler has a wider mustache in 1914 than the one you’re familiar with.
All right, it could have taken place, but with Hermann Göring or Ernst Röhm leading the Nazis, its character and course would have been far different.
The scenario works equally well if it’s your grandmother being killed, but come on: who would want to kill grandma? It works more directly still if it’s a parent you’re killing, but the moral opprobrium of that act presumably would dissuade you more than if it were a grandparent.
Unless they got together sometime before 1941 or so. If that’s the case, I salute the gumption of somebody of your advanced years intending to go Hitler-hunting. Beats the heck out of shuffleboard or pickleball, doesn’t it?
Unless you looked up the information in the interim, but you would remember that, so we have to assume that you didn’t. After all, what motivation would you have to look up knowledge you already possessed?
The poor defamed cow is on the other side of the barn. She’s innocent, though that has ceased to interest you.
Among other effects, this renders you ineligible to sing along to Billy Joel or the Foo Fighters.
Robert Heinlein did this paradox more directly, and far more pruriently, in his classic story “All You Zombies ….” I’m trying to keep things a little more focused.
I know the two lotteries don’t hold drawings on the same days. Drunk-you may not care.
No, it’s not exactly alike. I’m not trying to argue theology here. That should be its own book, and I will not be writing that one.
Unless you pursue that person into the past with a clipboard. If this is how you’re using the fascinating opportunities offered by time travel, I don’t know whether to pity or respect your dedication.
Descriptions of the incidents have been blurred to protect the reputations of the time-travelers in question. You may come to appreciate such a policy of forgiveness in due course.
Which will be a cleaner death than the one the Emperor devises for you. Seriously, his mother?
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