This is the famous grandfather paradox.
Let's say that the time traveler goes back in time a hundred years and kills his grandfather when his grandfather is only a little boy. What happens?
On one hand if the grandfather was killed when he was a little boy, there is no way he could have been a grandfather, and so the time traveler could not have been born. On the other hand if the time traveler had not born, the grandfather would not have died....

The grandfather paradox is the classic example of the paradoxes and problems inherent in time travel.

Here's the form in which I first heard it:

Three men are sitting together, and one of them announces "Right now, in my basement, I have a working time machine!" One of the other men wants to use it, and being asked why, explains that he had always hated his grandfather, who had abused his grandmother and ruined her life. "I have always regretted," he said, "that my grandfather died of old age before I was a grown man and could kill him for what he did to her. If I could travel back in time to before they met and kill him, not only could I avenge my grandmother, but I could save her from having to ever endure him."

The first man, not being very smart despite having invented a time machine, agrees to this, and the other man goes back in time with a rifle to the night before his grandparents met.

Two men are sitting together, and one of them announces "Right now, in my basement, I have a working time machine!" . . .

That's the way my father told it, and I seem to remember his saying that he had read it as a short story in a sci-fi book a long time ago. It seems to have an antique flavor, which makes me think there is an ur-version out there, but nobody who mentions it seems to have any sources. Perhaps it did spontaneously generate in the common knowledge.

The whole story strikes me as very Freudian, but someone else can elaborate on that . .

The paradox, of course, is that having killed his grandfather before his parent was concieved, he was never born, and so he didn't go back in time, and so his grandparents married, and so he was born, and so he went back in time, and so he killed his grandfather, and so he was never born, and so on.

There are many solutions put forth for the paradox, which I try to cover here, in a spectrum from most physical to most metaphysical, with the major problems with each one:

  1. Time travel leads to paradoxes, and therefore is not possible.
    Objection: Modern physics seems fairly certain that time travel is possible, at the very least at the subatomic level, and probably happens all the time. Besides, this answer is no fun.
  2. Time is not linear, but a series of branching possibilities, and all outcomes of any choice exist: The Multiple-Timelines hypothesis
    Objections: Beloved as they are of writers of speculative fiction, multiple universes are frowned on by physicists at the moment. Besides, this really doesn't satistfy us from a human perspective: Sure, there are infinite timelines, in some of which the man exists and in some of which he doesn't, but does he exist in our timeline?
  3. Time travel cannot be invented; if the man kills his grandfather, ripples of that change ocsilate back and forth through history until a change occurs that prevents the first man from inventing the machine. Then there is no longer any paradox and history becomes stable again. (I didn't explain this one very well: Think about it a little, and if your brain doesn't explode, it might make sense.)
    Objections: This solution is itself a paradox! (I think) Besides, it's no more fun than the first one, once you've untangled it.
  4. You can't cause paradoxes: Varying implementations, from the Morphail effect to the Novikov principle. Basically, if he goes back in time, something will happen to prevent him from actually doing the deed. Perhaps a virtual particle, appearing for an instant, will knock the bullet off course just enough; perhaps he will be violently expelled either back to his own time or into oblivion; perhaps something in between, but anyway the entire universe will be conspiring against him. This can be implemented as a variant of 3: The timeline oscillates until a causality is reached where the paradox never happened.
    Objections: This seems to require a universe that is actively and intelligently trying to prevent paradoxes- a thing for which we have no explanation, justification, evidence, or mechanism, (unless we assume it evolved that way; or use the mechanism of 3, which carries with it all of 3's objections)
  5. Traveling in time removes one from causality: If he goes back in time and kills his grandfather, he was never born; but by removing himself from the normal timeline, he no longer has to be born; he just is. If he goes back home no one will remember him, and he may or may not lose his memories of a past that never happened, but he still exists, because he's outside time.
    Objections: Perhaps the most fun of all the suggestions so far. Of course one gets tangled up in what causality is and creative physics and whether anything can be known if causality doesn't really apply, but none of that can really be reasoned about. No real mechanism for this one either.
  6. Paradoxes simply don't matter: This is the one that the story above illustrates. If he kills his grandfather, he doesn't exist. That's that. But the grandfather's still dead. Beloved of writers who don't want to bother explaining anything.
    Objection: But it doesn't explain anything!
  7. There are more things in heaven and earth: So there's a paradox. So what? If we weren't temporal beings ourselves, that wouldn't bother us. All the confusion is caused by our limited minds. Basically, this means that whatever happened didn't seem odd to any of the people involved, but whatever really happened is beyond their comprehension.
    Objection: Yeah, and if you believe that, I have some psychotropic drugs to sell you. Seriously, this one can't be argued against because it's basically a deus ex machina. Not much fun for the same reason it's hard to argue against.
  8. We are such stuff as dreams are made on: Paradoxes like this just prove the nonexistence of objective reality. Rather, all universes and timelines are merely creations of sentience and none is "realer" than another. Therefore, as long as the man who killed his grandfather believes that is what he did, he did; since there is no specific reality the illogic doesn't matter.
    Objections: Dream on, sister.

There. I think that covers about everything.

Of course, everyone knows that the real solution is that anyone who would go back in time solely to kill his own grandfather is a bastard anyway . . .

Resolution of Paradox

Imagine a wormhole in the shape of a horse shoe, shaped like a U but bent round further. A ball travels through space towards one end of the hole and enters it. The ball flies round the horse shoe.

As it goes round it travels in distance and displaces in time; and when it comes out, and flies off through space, it is slightly behind the time when it entered the wormhole.

It flies off and hits itself on the way towards the wormhole - knocking itself off course so that it misses the entrance and never enters the hole.

This is a paradox and the thing is that it is not about having a sex change, going back in time, and sexually assaulting yourself, but about balls and suchlike simple things. Being simple we may do our sums, or so it is claimed.

Said process breaks the Law of Least Action, perhaps the most basic law in physics, it is claimed. We are invited to conclude that all these paradoxes do likewise.

The purpose of a paradox, if it has one, is to display the limitations of our habits of thought.

Given our normal habits of thought, the question: "Assuming time travel is possible, what happens if someone goes back in time and kills their grandfather?" appears difficult. We are tempted to ask ourselves what could stop them doing it, and find nothing.

A similar question "Assuming carpentry is possible, why should someone not make a round square table?" appears less difficult: there is no need for anything or anyone to stop round squares being made, because if something is round then it is not square, and vice versa.

So let us rephrase the time-traveller question to "Assuming time travel is possible, why should someone not be both alive and dead on the date of conception of a time-traveller's parent?" This is the time-traveller's intention, since his grandfather was alive on that date and he desires him also to have been dead. The answer is the same as that to the question concerning the carpentry of round squares: there are no contradictions. Carpentry lets you shape wood, but within limits, including those of logic. Time travel lets you shape the past, but within limits, including those of logic.

The habit that leads us into difficulty is our natural tendency to be impressed by stories: normally, knowing how something happens is useful for understanding it. The purposeful activity involved in climbing into a time machine and engaging in targeted assassination appears to make the desired outcome comprehensible. But this is only an appearance: the desired outcome is still a contradiction.

Here is a similar story: there once was a joiner who loved to make round tables. In fact, he only ever made round tables. But it happened that he was very poor, and a rich man offered him a lot of money to make a square table. So the joiner thought things over, and had a wonderful idea: he made a beautiful round table for himself, and also made it square for his customer. The customer was very happy, and paid him twice, because, as he said, he had both a round and a square table.

They say that the joiner had a grandson who built a time machine...

Apt /apt/

Adj. Appropriate or suitable under the circumstances.

“Listen,” Jay said. “If this were a time travel thing – and I'm not saying it is – but if it is, then everything we do from this moment on would have already happened to another couple of Dave and Jason's later on down the timeline...”

I re-evaluated again the smoking, flipped-over Brinks truck in the ditch. “So what do we do?”

He twisted the red scarf in his hand again and then glanced at the duffel bag on the side of the road. The wind pulled dollar bills of different value away from the bag – several notes already making a run for it across the ditch.

“Look,” He passed me the scarf. “I can't tell – is that your handwriting?”

“Dude, this could be anybody's handwriting–”

“Why's the truck empty?”


“And where the hell is the driver?”

Sirens were localizing somewhere to the west, and getting closer. The sun hadn't fully set yet, and the damp leaves refused to let go of anything. I looked around, searching the deep blue tree-line on either side of the road for a sign of life, finding nothing. “I don't know...”

Jay walked back to the message gouged into the side of the road and scratched his head for the millionth time. I reread the other message written onto the scarf, scrawled in messy magic marker, wondering how the hell–

–A second duffel bag suddenly fell from the sky.

I dove for the ditch, screaming bloody death as more dollar bills exploded in a pale green cloud of particulates. A single white leaf of eight by eleven paper spiraled out of the primary confetti of bank notes, gently weaving its way through the air, finally smacking me in the face.

“W– what the hell was that!?” Jay ran over with his hands on his head.

“Unghh,” I stammered, picking myself up off the ground. “Another bag...?”

Jay looked up, squinting at what looked like a tiny teardrop passing high overhead – I'm talking really high, several thousand feet. He looked back and forth between the new bag of cash and the sky, pacing in a tight circle, wagging his finger at the unreasonableness of the situation. “We – we have to get out of here,” he pointed at the sirens. “They're gonna think we tried to rob this truck – we're gonna – we gotta go, Dave.”

I wiped dirt from my chin, deciding that he was right, that we needed to hightail it fast. Jay stopped and kicked the tar again – then pointed. “Quick, come here and read this again please – just so I know that I'm not losing my vegetables –”

I peeled the piece of paper off my chest and stumbled over to Jay.

“Dave and Jay,” it read. “Uniforms in truck, leave bag number two and grab the red scarf....”

Jay snorted, barely controlling his breath. “How is this even possible? That tar is old, Dave....”

I knelt and brushed the neatly engraved words with my fingers, feeling the sirens getting closer. “I don't know, but we have to get out of here–”

I stood, noticing the hook of an upper case J on the piece of paper in my hand. Unfolding it, I said, “Awe, no...”

Jay turned and started to say something, then stopped.

I held up my hand for him to let me finish.“To Dave and Jay,” I read. “Why are you two mentally-insufficients still standing there? Take bag number one and grab the uniforms inside the truck – en route, thirty seconds.”

“En route... what does that mean?”

I shrugged. “Dude, let's get outta' here.”

He nodded and we moved briskly toward our hatchback – but something in the first duffel bag caught my eye – a flash of scarlet and another hooked, uppercase letter. Since we were passing it anyway, I jogged to the first duffel bag, eying the second one, still feeling the sirens biting at every passing second. The bag was open – an exact replica of the one that fell from the sky, which was lying a few feet away from the empty, turned-over Brinks truck. Nestled inside of what must have been about five hundred thousand bucks, was a red scarf.

Jay waved me along, shooting worried looks where the road disappeared toward the sirens. “Dave, what the hell–?”

I pulled the scarf out of the bag – and read the same message, scribbled in black magic marker, Dave and Jay, this is real.

“Uh, Jay...?” I walked back to him hopping in the roadway, dragging my jaw with me.

“Oh lord Jesus, what now–?”

I passed him the two scarves, each with an identical message written along one side, in messy magic marker. “Dave and Jay,” he read. “This is real…”

A police car suddenly burst over the hill and sped toward us, lights and sirens blaring. Jay shoved the scarves back into my arms, and I just stood there like an idiot, not knowing what to do. The cop car screeched to a halt near the Brinks truck, swinging its doors open wide, and out stepped a pair of officers from either side of the vehicle, each holding a very large shotgun.

“Hold it right there!” The driver yelled, “You scum eating, puke slurping, video-game-playing, Douglas Adams-Readin' sons-a-guns!”

My fingers went suddenly numb, so I let go of the two scarves – one fell to the ground as the wind wrapped it around my thigh, and the other took off with a gust, away from the crash scene, somewhere into the tree-line. Jay threw his hands up, while I fought the sudden urge to wet myself, thereby making my meat unappetizing to any predators in the area.

The officer on the passenger side said, “Quit screwing around, man – we don't have time.”

“On the contrary, my friend.” The primary officer racked his shotgun and then pointed it back at our faces, “as it turns out, we have loads of it.”

“Dude, you know what I mean–”

“Yeah, Yeah...”

Jay and I exchanged a couple of aorta pinching glances as another Dave walked briskly in front of the cop car – smiling ear to ear – wearing a crisp, neatly pressed Portland Police uniform. “Relax guys – it's us. Take off your clothes.”

“Uh...?” I said, not wanting to move.

I quickly walked toward the tipped over Brinks truck... meaning he did... basically, we walked over to it, impossibly, while one of us stood in the middle of the road wrestling with our bladder, and the other grabbed one of the duffel bags filled with an obscene sum of money. The second Jay disappeared into the back of the Brinks truck, as I – that is to say, he – walked across the road and tossed the duffel bag into the backseat of our hatchback.

Jason and I watched this happen through a lens of refracted shock. The whole time, Jason kept his hands up, and we just stood there making popping sounds with our lips, not knowing what to say.

“Dude,” Jay finally said. “What the holy hell is happening?”

The other Jay – the Jay in the police outfit – emerged from the wreckage carrying a pair of gray security uniforms. He stumbled out of the truck and walked toward the edge of the ditch, facing the tree-line. “It's clear guys!”

I walked back over and started unbuttoning my shirt – he did, I mean, the other me – and said, “Come on guys, take off your clothes. We only have a few minutes before the real cops start showing up.”

“We didn't have anything to do with this,” Jay stammered, pointing at the smoking truck. “We were just driving by, thought somebody needed help–”

“Well,” I said, shaking my head – he said, rather. “You didn't have anything to do with this yet.”

I started pulling my jacket off, but Jay kept standing in place, in a state of shock.

“Listen,” I said... he said... “Trust me – I know what you're thinking – I remember thinking the exact same thing – but you're going to have to do exactly what I say, because if you don't, this time-loop will end and our past selves will each be spending the next ten ad infinitum miserable years in prison.”

“You're us,” I said. Me this time.

“That's right,” he winked. “We're you from about four days into the future.”

“Uh huh,” I nodded, stepping out of my pants.

“Dude,” Jay said. “What are you doing, let's go...?”

The whole time my thoughts kept circling back to the bag of cash that the other me set in our back seat. “That's a lot of money,” I said.

I smiled – rather, he smiled, and I smiled back, and we were suddenly in this weird but excellent hall of repeating mirrors.

“How screwed are we?” I asked the other me.

My smile – his smile, actually – widened. “Pretty screwed,” He said, passing me his cop uniform. “Here, put this on – but, if this works you're going to be stinking, filthy, obscenely, disgustingly rich.”

The other Jay jogged up with the security uniforms draped over his arm, eyeing his past self through a sly grin, and I saw a pair of shadows separate from the tree-line not far from the Brinks truck. The second Jay handed me – handed the other me – the uniforms and started undressing. “Fellas,” he said, as the largest smile that I had ever seen broke across his lips.

Jay finally gave up trying to figure everything out, and quietly donned the police uniform with an unsettled look on his face. There was a bit of trouble with some of the belt keepers, but we eventually wiggled our way in without too much of a struggle. The other us's started pulling themselves into the clean Brinks outfits.

“Here,” I said – the other Dave said, passing me a rather large key-ring. “These are the keys to pretty much every room at the station. In about five hours, they're going to lock that bag of cash in evidence room C – as in cat – you got it? It will be sitting there for exactly forty eight minutes, until another Brinks crew comes to collect it. You two are going to take the cop car and head to the Ramada Inn on highway 10. When you get there, you're going to get a room, stow the uniforms, and meet up with a woman named Alice at the hotel bar, who'll explain everything. She's got another bag of cash for you–”

As the other Jay took pictures of the duffel bag with his cellphone, the other me reached down and pulled the red scarf out of my pile of clothes. “You're going to give her this to identify yourselves. She'll tell you how to proceed from there.”

“Wait,” I said, taking the scarf. “What’s happening…?”

The two silhouettes from the tree-line finally made it to the road. My heart dropped when I realized that those two shadows were another, separate Dave and Jason, wearing a couple of tattered, ripped up and singed Brinks security uniforms. Our faces were covered with grit, and the side of my head – his, rather – was singed and missing a patch of hair. But he was still smiling like a maniac.

They met up with us on the side of the road, and I passed – he passed, the third Dave – me a set of keys and squinted. “Which one are you again?”

“Those are for me,” the second Dave said, the me from the police car.

“Right,” the singed Dave nodded. “I'm just going to run through this real quick, to make sure we have everything.”

“Sure,” I said – the second Dave.

“The receipts are all in the bag of cash over there,” dirty Dave said, pointing at the duffel bag near the Brinks truck. “The first bag is in the backseat of our car,” he pointed at our hatchback. “So when the police get here, all of the cash will technically be accounted for...”

“Yep,” Dave number two said. “So you two are going to take their clothes,” he pointed at us. “The car and the cash, and drive out of here like nothing happened.”

“Right,” the filthy Dave said, sliding into my pants. “So that means no All-Persons-Bulletin for any rust colored hatchback that just robbed the evening Brinks truck. Nobody will be looking for anything...?”

The other Jay – the second Jay – finished putting on the clean security uniform and hurried over to the Brinks truck, where he carefully crawled inside.

I slowly eased my sore body back into my jacket and took a deep, shuddery breath. “I can't believe that worked.” I shook my head and took several wobbly steps toward the hatchback – relief and victory nearly knocking me to the ground.

The dirty Dave and Jason hopped into our car with a crisp, anonymous bag of cash in the back seat, and started the engine.

Dave number two leaned into the window and said, “remember guys, you have to meet up with Alice at the gas station on 169 and Bass Lake Road. When the market opens tomorrow, you're going to place an order for one hundred and fifty thousand shares of FSI to sell at $7.50. The options expire in two days – remember, two days – at $3.25.”

“Yeah,” I said quietly, smiling. “We got it.”

“Oh,” I said, “and think of a better message delivery for the next loop – one that's a bit less stroke inducing, okay? These guys should have been ready to go as soon as we got here, not scratching their heads.”

“Sure, uh... we’ll try.” After a couple of long and deep, unrestrained breaths, Dave and Jay number three hit the gas in my hatchback, and casually meandered out of our lives. They each stuck an arm out of a window and waved us luck, moving into the autumn sunset with our future wealth safely tucked in the back seat. I walked over, finishing the top button and straightened my badge. “Any questions?”

“Yeah, um–”

“Any questions about getting to the Ramada?” I interrupted.

We shook our heads.

I passed myself a couple of neatly folded pieces of paper. “Job applications,” I said. “For Brinks.” I patted myself on the shoulder and nodded. “You guys are going to do fine…”

And with that, I left us there. I walked to the wrecked Brinks truck to meet up with the other Jay, stopping every couple of steps to do a little dance, and at least one pirouette. “Remember,” I called back. “Alice at the Ramada Inn, highway 10.”

Jay and I moved toward the police car in a daze, watching our feet as we walked.

“Hokay, then.” Jay finally said, still unsettled, but relaxed. “Are you driving?”

I smiled and did my own little dance. “There had to have been a couple hundred grand in there!”

Jay turned around and watched our car disappear over the horizon. “You think that'll be us in a few days?”

We hopped into the police cruiser and pulled away from Dave and Jay's Brinks truck, thinking about the adventure ahead, not saying anything, smiling as flashing red and blue lights steadily filled the roadway. That was about five minutes until the rest of our lives, I realized. A complete circle collapsing onto a moment, like an artifact of memory. Each version of myself infinitely different than the one before it, irrevocably affected by that essence of us that dared to step out of the thoroughfare of time. Jay and I moved ahead anyway, watching the second red scarf in the rear view mirror float to the ground, where it finally curled itself into the damp curtilage of the shoulder, wondering where it was going to come from.

“Well,” I said, flipping on the headlights. “There's only one way to find out.”

fin/début/fin ...

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