Wikipedia Deep Dive

Temporal paradox

13 min read

The Impossible Murder You Can Never Commit

Imagine stepping into a time machine with a single, grim purpose: to kill your grandfather before he ever meets your grandmother. You succeed. The old man falls. You've done it.

But wait.

If your grandfather died before fathering your parent, then your parent was never born. And if your parent was never born, neither were you. Which means you never stepped into that time machine. Which means your grandfather lived. Which means you were born after all, and here you are again, stepping into that time machine with murder on your mind.

This is the grandfather paradox, perhaps the most famous puzzle in all of time travel fiction and philosophy. It's the reason physicists lose sleep, the reason philosophers write dense papers, and the reason every time travel movie eventually ties itself into knots. But it's just one member of a strange family of logical contradictions that emerge the moment we take time travel seriously.

Three Flavors of Impossible

Temporal paradoxes come in three broad categories, each with its own particular brand of headache.

The first is the consistency paradox, which includes our murderous grandfather scenario. Any time you change the past in a way that prevents you from going back to change it, you've created a consistency paradox. Kill your grandfather, prevent World War Two by assassinating Hitler, or simply travel back and trip yourself on the way to the time machine. The result is always the same: a logical contradiction where your action erases itself.

The second category is the bootstrap paradox, sometimes called a causal loop. This is subtler and, in some ways, stranger. Imagine a playwright who receives a beautiful pocket watch from an elderly woman. Years later, he travels back in time and meets that same woman as a young girl. He shows her the watch, and she keeps it, eventually growing old and giving it to a young playwright she meets one day. Where did the watch come from? It has no origin. It exists in a closed loop, causing itself, springing from nothing.

The third category involves free will and predestination. If a perfect predictor—perhaps one who uses time travel to observe the future—already knows what choice you'll make, did you ever really have a choice at all? These paradoxes blur the line between destiny and decision, suggesting that foreknowledge might be logically incompatible with genuine freedom.

The Bootstrap Problem: Objects from Nowhere

The bootstrap paradox gets its name from the old idiom about pulling yourself up by your own bootstraps. It's meant to describe something impossible—lifting yourself into the air by tugging on your shoes. And yet bootstrap paradoxes describe exactly this kind of self-causing situation.

Robert A. Heinlein wrote a masterpiece of bootstrap logic in his 1958 short story "All You Zombies," later adapted into the 2014 film Predestination. The protagonist is an intersex individual who, through a convoluted series of time jumps, becomes both their own mother and their own father. Every cause in their existence traces back only to themselves. They have no ancestors, no external origin. They simply are—a closed loop of identity.

Physicists Andrei Lossev and Igor Novikov gave these self-existing things a name: Jinn, after the supernatural beings in the Quran who leave no trace when they vanish. A Jinnee of the first kind is an object, like that pocket watch, that loops through time without origin. A Jinnee of the second kind is information—perhaps a poem or a theorem that a time traveler carries into the past, where it's recorded and eventually given back to the time traveler who will carry it into the past. Both kinds share the same unsettling quality. They exist, but they were never created.

The Russian physicist Sergey Krasnikov, who has spent much of his career studying the mathematics of time travel in general relativity, argues that these loops shouldn't trouble us too much. The paradox, he suggests, isn't really about the loop itself but about our expectations for how physical systems should evolve. We expect causes to precede effects, creation to precede existence. Jinn simply don't follow those rules. They're strange, but they're not necessarily impossible.

Grandfather's Murder and Its Variations

The grandfather paradox appeared in print as early as the 1920s, in letters to the science fiction magazine Amazing Stories. Readers, apparently, couldn't resist poking at the logic of time travel. If you go back and kill your grandfather, you prevent your own existence, which prevents the killing, which allows your existence, which enables the killing again, forever.

The "Hitler paradox" is a popular variation. Travel back in time and kill Adolf Hitler before he rises to power, preventing World War Two and the Holocaust. But here's the twist: if you succeed, then the war never happens. If the war never happens, you have no reason to travel back in time. If you don't travel back, Hitler lives, the war happens, and you have a reason to travel back again. The paradox isn't just about preventing your own birth. It's about preventing your own motivation.

There's an even more direct version: the retro-suicide paradox. Travel back in time and kill your younger self. If you succeed, you never grow up to build the time machine. If you never build the time machine, you never travel back. If you never travel back, your younger self survives to build the time machine and travel back to kill yourself.

Physicist John Garrison and his colleagues designed a thought experiment involving an electronic circuit connected to a time machine. The circuit sends a signal backward through time, and that signal activates a switch that shuts off the circuit before it can send the signal. The circuit receives the instruction to shut down before it ever gives the instruction. It's the grandfather paradox translated into electronics—a contradiction you could build in a lab if only you had a time machine.

Why the Past Can't Be Changed (Logically Speaking)

Here's the philosophical core of the problem, stripped down to pure logic.

If something happened, then it happened. This sounds trivial, but it's actually a statement about the nature of truth and time. The past is fixed. It's not waiting to be determined. The Battle of Hastings occurred in 1066. Julius Caesar crossed the Rubicon. Your grandfather met your grandmother. These aren't possibilities anymore. They're facts.

Modal logic—the branch of philosophy that deals with necessity and possibility—tells us that if something necessarily happened, then it's impossible for it to have not happened. The past, having already occurred, is necessary in this sense. You can't change 1066 to 1067. You can't unmake the Rubicon crossing. And you can't prevent your grandmother from meeting your grandfather, because that meeting already happened.

This is why philosopher Bradley Dowden once argued in his textbook on logical reasoning that time travel to the past is simply impossible. If traveling to the past lets you change the past, and changing the past creates contradictions, then time travel creates contradictions. Since contradictions can't exist, time travel can't exist.

But Dowden later changed his mind after corresponding with philosopher Norman Swartz. The key insight was subtle but important: maybe time travel doesn't let you change the past. Maybe it only lets you participate in the past that already happened.

The Novikov Self-Consistency Principle

Igor Novikov, a Russian theoretical physicist, proposed what might be the most elegant solution to the grandfather paradox. His self-consistency principle says simply this: if time travel is possible, then only self-consistent events can occur.

What does this mean in practice? It means that if you travel back in time to kill your grandfather, something will stop you. Your gun will jam. You'll slip on a banana peel. You'll have a heart attack. You'll shoot the wrong person. Whatever happens, the universe will conspire to prevent you from creating a paradox, because paradoxes can't exist.

This isn't some mystical force. It's a constraint built into the physics itself. Just as you can't travel faster than light, and you can't make energy from nothing, you can't create a paradox. The laws of physics simply don't allow it.

Physicist Joseph Polchinski tested this idea with a thought experiment involving a billiard ball and a wormhole. Imagine shooting a billiard ball toward a wormhole at exactly the right angle so that it travels back in time and knocks its past self off course, preventing it from ever entering the wormhole. This is the grandfather paradox in miniature, played out with physics instead of murder.

Kip Thorne, the Nobel Prize-winning physicist who consulted on the film Interstellar, worked with his students Fernando Echeverria and Gunnar Klinkhammer to analyze this problem. They found something remarkable: there was always a self-consistent solution. The ball might glance off its past self at a slightly different angle, redirecting rather than stopping. There might be multiple consistent solutions, even infinitely many for some initial conditions. But there was never a case where the ball had to knock itself off course in a way that prevented its own entry into the wormhole.

The universe, it seems, can always find a way to make time travel work without contradiction.

The Many Worlds Escape Hatch

Quantum mechanics offers another potential solution, though it's controversial and mind-bending in its own way.

The many-worlds interpretation of quantum physics suggests that every quantum event causes the universe to split into multiple branches, each representing a different outcome. In this view, when you travel back in time to kill your grandfather, you don't actually change your own past. Instead, you enter a different branch of reality—a parallel universe where your grandfather dies but you still exist because you came from a different branch where he lived.

This neatly sidesteps the paradox. You can kill someone who looks exactly like your grandfather, in a universe that looks exactly like yours used to look, but you're not changing your own history. You're just visiting a different one.

Stephen Hawking wasn't satisfied with this solution. He argued that even if many worlds exist, we should expect time travelers to experience a single self-consistent history. You shouldn't hop between universes. You should stay in your own world, subject to the constraints that prevent paradoxes.

The physicist David Deutsch took a different approach, using quantum computation to argue that time travel could only produce self-consistent solutions. The math is dense, but the intuition is that quantum systems naturally find consistent states, avoiding contradictions the way water flows downhill—following the path of least resistance to logical coherence.

Allen Everett raised a disturbing objection: if Deutsch is right, then any macroscopic object traveling through time might be torn apart, with different particles ending up in different quantum branches. Your time machine might arrive intact, but you might not.

What If Time Is an Illusion?

The mathematician and logician Kurt Gödel, famous for his incompleteness theorems that shook the foundations of mathematics, also contributed to the physics of time. He discovered a solution to Einstein's equations of general relativity that described a rotating universe where time travel was possible. More than that, he suggested the solution showed something profound: time might be an illusion.

In what's called the block universe view, time is just another dimension like space. Past, present, and future all exist simultaneously, frozen in a four-dimensional block. You experience time because your consciousness moves through this block, but from the outside—from God's-eye view—everything that ever happened or will happen is already there.

If time is an illusion, then the grandfather paradox takes on a different character. You can't change the past because the past isn't behind you. It's beside you. It's part of the same static structure as the present and future. Everything that happens is already built into the block, including any time travel you might do. Your trip to kill your grandfather is already part of history, and so is whatever happens when you try.

Some modern physicists take an even more radical view, suggesting that time isn't a fundamental property of the universe at all but emerges from the laws of entropy—the tendency of systems to move from order to disorder. In this framework, traveling to the past means reconfiguring your local region of space to match an earlier entropic state, not moving backward along a timeline. The original sequence of events remains preserved in the universe's broader progression. You can visit what looks like the past, but you can't change the causal history that produced you.

The Cosmic Censorship Hypothesis

Perhaps the universe has its own way of keeping paradoxes private.

The cosmic censorship hypothesis, proposed by physicist Roger Penrose, suggests that singularities—the infinitely dense points at the heart of black holes—are always hidden behind event horizons. You can never see a singularity from the outside because light can't escape to show it to you.

Some physicists have extended this idea to time travel. Maybe every closed timelike curve—every path through spacetime that loops back on itself—passes through an event horizon. If so, then even if time travel creates paradoxes, we could never observe them. They would be hidden behind cosmic censors, kept from view by the structure of spacetime itself.

This doesn't solve the paradoxes. It just hides them.

Why This Matters Beyond Science Fiction

You might wonder why anyone bothers with these puzzles. Time machines don't exist. We can't kill our grandfathers. What's the point?

The point is that temporal paradoxes reveal deep truths about logic, causation, and the nature of reality. They force us to think carefully about what it means for something to cause something else, what it means for the past to be fixed, what it means to have free will in a universe where the future might already be determined.

The grandfather paradox isn't just about time travel. It's about the logical structure of possibility itself. When we find that changing the past creates contradictions, we learn something about what change means, about the relationship between truth and time, about the difference between what is and what could have been.

And the solutions—from Novikov's self-consistency to many-worlds to emergent time—each reveal different ways of thinking about the universe. Is the cosmos a rigid block where everything is predetermined? Is it a branching tree of parallel realities? Is time fundamental or emergent? These aren't just questions for physicists. They're questions for anyone who has ever wondered whether their choices matter, whether the future is open, whether the past could have been different.

So the next time you watch a time travel movie and notice the plot holes, remember: those holes connect to some of the deepest questions humans have ever asked. The grandfather paradox isn't a bug in our stories about time travel. It's a feature—a window into the logical structure of existence itself.

And maybe, just maybe, the reason we can't stop thinking about it is that somewhere, somewhen, a version of ourselves already figured out the answer.