The Grandfather Paradox is a famous time travel paradox in which traveling to the past creates a logical contradiction by changing events that have already occurred. The classic example involves a person traveling back in time and killing their own grandfather before one of their parents is conceived — which would prevent their own birth, and with it, the journey they just made.
The Grandfather Paradox is not limited to a person’s grandfather, however. It applies to any situation in which changing the past creates an inconsistency in the timeline, raising fundamental questions about causality, free will and whether history can ever truly be altered.
From Joseph Polchinski’s billiard-ball thought experiment to Marty McFly’s fading family photograph in Back to the Future, the paradox has become one of the defining puzzles associated with backward time travel in both physics and science fiction.
Grandfather Paradox (Quick Definition)
The Grandfather Paradox is a consistency paradox in which an action taken in the past prevents the cause of that action from ever occurring.
Etymology of the Grandfather Paradox
Versions of the ancestor problem were circulating in American science-fiction circles by the late 1920s. Amazing Stories had built much of its readership around debating the scientific plausibility of the stories it published, at a time when Einstein’s theory of relativity had transformed scientific ideas about space and time.
Reader letters printed in the magazine in 1927 and 1928 already worked through the basic scenario — one asks what happens if an inventor tries to shoot his younger self, while another describes traveling back sixty years to kill a grandfather before a parent’s conception. These weren’t private exchanges between writers: the paradox was being worked out publicly by science-fiction readers in the magazine’s own letters pages, years before either the idea or the phrase had settled into its now-familiar form.
French writer René Barjavel later gave the idea one of its best-known early literary treatments in Le Voyageur imprudent (Future Times Three). Interestingly, Barjavel’s traveler doesn’t set out to kill his grandfather at all. He travels into the past intending to assassinate Napoleon, only to accidentally kill one of his own ancestors instead — with unfortunate consequences for his own existence. Barjavel is therefore often associated with the Grandfather Paradox, although the basic idea clearly predates his story.
The phrase itself took shape gradually within the science-fiction community. Hugo Gernsback, the founder of Amazing Stories, explored the same idea in a 1929 essay titled “The Question of Time-Traveling.” By 1939, a fan writing in the science-fiction fanzine Fantasy Scout could refer to the “well known grandfather paradox” as an established idea needing no introduction. The name was fully crystallized by 1950, when a short story titled simply Grandfather Paradox appeared in Fantastic Adventures, and the term continued to appear in science-fiction writing over the following decades — including in Larry Niven’s 1971 short-story collection All the Myriad Ways.
And despite the name, there is nothing special about grandfathers. A grandmother, parent or more distant ancestor would work just as well. “Grandfather” simply became a memorable shorthand for a much broader problem.
A Consistency Paradox, Not a Closed Loop
The Grandfather Paradox, along with the Hitler Paradox and Polchinski’s Paradox, is an example of a consistency paradox — a scenario that results in a self-inconsistent version of the timeline’s history. If a time traveler killed his own grandfather, he would never have been born, and so would never have been able to travel back through time and commit the murder in the first place. This is the paradox: the same history requires the grandfather both to survive and not to survive.
The Predestination Paradox and the Bootstrap Paradox, by contrast, are examples of closed causal loops, in which cause and effect form a circular pattern without contradicting themselves. A Bootstrap loop may leave an object, a piece of information, or a person with no obvious point of origin, while a Predestination loop may cause the very event a time traveler was trying to prevent. In both cases, however, the history remains fixed, consistent and internally coherent, however strange it looks from the outside.
Examples of the Grandfather Paradox
The Hitler Paradox swaps family history for world history. A traveler goes back and kills Hitler before the events that motivated the journey ever happen — but erase that history, and with it, the very reason the journey happened at all.
An even simpler version needs no person at all. Imagine an electronic circuit that sends a signal into the past telling itself to switch off. If the signal arrives before the circuit sends it, the circuit switches off and never sends the signal. But if no signal is sent, what caused it to switch off?
Physicist Joseph Polchinski posed the same basic problem using a billiard ball and a hypothetical wormhole, prompting Kip Thorne and colleagues to investigate whether the resulting trajectories could remain self-consistent. The ball enters the wormhole and emerges in its own past, where it collides with its younger self and knocks it away before it can enter. But if the younger ball never enters the wormhole, the older ball can never emerge to knock it away. And if the older ball never emerges, nothing stops the younger ball from entering after all. Polchinski’s example strips the Grandfather Paradox down to its essentials. There is no grandfather, gun or human decision. The contradiction lies in cause and effect themselves.
Movie and TV Examples
Science fiction has naturally had fun with the same idea. In Back to the Future (1985), Marty McFly accidentally interferes with his parents’ first meeting and begins threatening his own future existence. The paradox is given a wonderfully simple visual form: as the possibility of his birth recedes, Marty and his siblings begin disappearing from a family photograph.
The Terminator franchise offers a darker variation: Skynet sends a machine back to kill Sarah Connor before her son John — the future leader of the human resistance — is ever born. If John is never born, however, the future that motivated Skynet to send the Terminator back would itself be changed.
The details change, but all these examples lead back to the same problem: if traveling into the past allows an effect to erase its own cause, history can no longer remain consistent.
The Problem of Causal Consistency
At first glance, the Grandfather Paradox can sound like a puzzle about one particularly reckless time traveler. Polchinski’s billiard ball shows why the problem goes deeper: if backward time travel were physically possible, it wouldn’t just be people who could create inconsistencies — matter or information interacting with its own past could potentially create the same difficulty. Any workable theory of backward time travel therefore has to answer a fundamental question: how can a history containing time travel avoid contradicting itself? One possible answer is that the past simply cannot be changed. But that creates another problem the moment we put the traveler back into the story.
Can You Change History?
Suppose you travel into the past and find your grandfather standing right in front of you. You know he survived, because his survival is part of the history that led to your own existence. If the past really cannot change, then whatever you choose to do must somehow be compatible with the history you already know. That creates an unusual tension between an unchangeable past and our ordinary idea that we are free to choose our actions.
Can You Kill Your Grandfather if History Says You Didn’t?
Philosopher David Lewis tackled this question in his influential 1976 paper The Paradoxes of Time Travel. Imagine standing in front of your grandfather with a loaded gun. You know how to fire it, you have the opportunity, and nothing obvious prevents you from pulling the trigger. In the ordinary sense, you can kill him. Yet in another sense you cannot — your own existence is evidence that your grandfather survived.
Lewis argued that the two statements need not contradict each other. Whether something “can” happen depends partly on which facts are being held fixed. Given your ability, weapon and opportunity, killing your grandfather is possible. Given the complete set of historical facts — including his survival and your subsequent birth — killing him is not compatible with that history. This is related to the philosophical concept of compossibility — whether different facts or events can coexist within the same possible history.
So perhaps you remain perfectly free to pull the trigger. You simply don’t produce the contradictory result: you miss, the gun jams, somebody intervenes. Events unfold in some other way that remains compatible with the history from which you came — which gives us one possible route out of the paradox.
Solutions to the Grandfather Paradox
Nobody has traveled into the past and attempted to change history, so there is no experimentally established solution to the Grandfather Paradox. Several solutions have been proposed, but most fall into three broad possibilities.
Solution 1: History Cannot Be Changed
The first is that traveling into the past may be possible, but changing established history is not. This is the central idea behind the Novikov Self-Consistency Principle, associated with physicist Igor Novikov.
Under this view, there is only one history. If a time traveler appears in 1940, their arrival and everything they subsequently do were always part of what happened in 1940. Any action they take must therefore be consistent with the history that eventually led to their journey. Try to kill your grandfather and you fail — not because some mysterious force necessarily appears to save him, but because events cannot produce the contradiction.
Polchinski’s billiard ball shows how this might work without involving human choice. Instead of knocking its younger self completely away from the wormhole, the older ball could strike it at an angle that still sends it into the wormhole, and the younger ball later emerges to make the very collision that sent it in. The sequence is strange, but nothing contradicts itself: under Novikov’s principle, the traveler doesn’t change history. The traveler was always part of history.
This solution preserves one single past by making it self-consistent. Another approach abandons the assumption that there is only one history.
Solution 2: You Change a Different Timeline
Traveling into the past could instead take you into a different branch of history — essentially a parallel timeline — while the timeline you came from continues to exist.
In this version, you really do kill your grandfather. You don’t miss, the gun doesn’t jam, and you don’t disappear. That’s because the grandfather you kill belongs to the parallel timeline, not the original timeline in which you were born. In your original timeline, your grandfather survived, your parent was born, and eventually so were you. In the parallel timeline, you kill your grandfather before he has children, so you will never be born there. But that doesn’t erase the traveler who pulled the trigger, because you came from the original timeline where Grandfather survived. The contradiction disappears because the two incompatible events — your grandfather surviving and your grandfather dying — happen in two different timelines rather than the same one.
This type of branching or multiple-history solution is often associated with the Many-Worlds Interpretation of quantum mechanics, but the two ideas shouldn’t simply be treated as the same thing. Many Worlds does not establish that traveling into the past would create or carry someone into a parallel timeline. Rather, some theoretical models use multiple histories, sometimes drawing on ideas related to quantum branching, to show how time travel might remain self-consistent.
Novikov preserves consistency by allowing one history that cannot be changed. Multiple-history models preserve it by allowing more than one history. Both, however, assume that reaching the past is physically possible. The third solution questions that assumption.
Solution 3: You Can Never Reach the Past
Perhaps nature avoids the Grandfather Paradox by never allowing the meeting with Grandfather to happen.
In 1992, physicist Stephen Hawking proposed his Chronology Protection Conjecture, suggesting that physical effects might prevent the kinds of spacetime structures required for backward time travel from ever becoming usable time machines. As Hawking memorably put it, there seemed to be a “Chronology Protection Agency” making “the universe safe for historians” — preventing travel into the past before it can ever create a paradox in the first place.
If something like chronology protection operates in nature, history doesn’t need to arrange a missed shot, and the universe doesn’t need another timeline to accommodate a successful one — the contradiction never gets started.
This remains a conjecture rather than experimental proof that backward time travel is impossible. But it raises an interesting question: if travel into the past creates such obvious problems, why does modern physics leave the subject open at all?
Why Does Physics Take the Grandfather Paradox Seriously?
The reason is that Einstein’s general theory of relativity does not simply rule out every possible route into the past. Certain solutions to its equations contain closed timelike curves — paths through spacetime that loop back to an earlier point. If such paths could physically exist and somehow be used, questions such as the Grandfather Paradox would become unavoidable. That, however, is a very long way from having a workable time machine.
Physicists have investigated hypothetical wormholes, self-consistent systems such as Polchinski’s billiard ball, and quantum models designed to explore what happens when cause and effect loop back upon themselves. In 1991, physicist David Deutsch proposed an influential quantum treatment of closed timelike curves in which consistency conditions can avoid the classical paradox problem.
Some aspects of proposed quantum CTC models have also been reproduced in laboratory simulations without actually sending anything into the past. Such experiments investigate the mathematical behavior predicted by the models; they are not evidence that real closed timelike curves or backward time travel exist.
The question is also still actively being worked on rather than settled. In 2020, physicists Germain Tobar and Fabio Costa developed a mathematical framework in which complex interactions involving closed timelike curves could remain logically consistent while still allowing observers freedom over their local actions. In other words, being free to make a choice need not necessarily mean being free to create a paradox.
In 2024, physicist Lorenzo Gavassino approached the problem from a very different direction. In his model of an object traveling around a closed timelike curve, the system must eventually return to its initial state when the loop closes. Among the stranger consequences is that memories formed during the journey would ultimately have to disappear, while increases in entropy would eventually reverse. Within the model, paradoxes are avoided because the system cannot complete the loop in a state inconsistent with the one in which it began.
There may still be a deeper physical principle that rules backward time travel out altogether. But research like this shows that physicists aren’t simply treating the Grandfather Paradox as a science-fiction curiosity. They are still exploring, from very different directions, whether a universe containing closed timelike curves could remain logically and physically consistent.
For a fuller look at wormholes, closed timelike curves, and the physics involved, see Can We Really Travel Back in Time?
Does the Grandfather Paradox Prove Time Travel Is Impossible?
No. What the Grandfather Paradox shows is that if backward time travel is possible, something about our ordinary understanding of history has to give.
Perhaps the past exists as one fixed history that cannot be changed. Perhaps changing the past leads into another timeline instead. Or perhaps nature prevents backward time travel before either possibility can arise. The paradox itself cannot tell us which answer is correct — what it does show is that we cannot simply add backward time travel to an ordinary, changeable past and expect cause and effect to behave exactly as they did before.
Grandfather Paradox vs Bootstrap and Predestination Paradoxes
The Grandfather, Bootstrap and Predestination Paradoxes all disrupt our ordinary understanding of cause and effect, but the problem is different in each case.
| Paradox | What Happens? | The Puzzle |
|---|---|---|
| Grandfather Paradox | An action in the past prevents the cause of that action. | History contradicts itself. |
| Bootstrap Paradox | An object or information travels around a closed causal loop. | Where did it originally come from? |
| Predestination Paradox | Trying to prevent an event helps cause that same event. | The attempt to change history fulfills it. |
The crucial difference is consistency: with the Grandfather Paradox, the classic scenario requires incompatible events to occur within the same timeline.
A Bootstrap Paradox can remain consistent. You might take the plans for a time machine into the past and give them to the person from whom you originally obtained them. The puzzle isn’t a contradiction but the apparent lack of an original source for the plans.
A Predestination Paradox can also remain consistent. You travel into the past to prevent an accident, only to discover that your intervention causes the very accident you wanted to stop.
The three paradoxes therefore twist causality in different ways: Grandfather breaks the consistency of history, Bootstrap leaves cause or information without an ordinary point of origin, while Predestination turns an attempt to change history into part of history itself.
Conclusion
The Grandfather Paradox starts with an almost absurdly simple scenario: travel back in time and kill your grandfather. Follow that idea through, however, and it leads to some remarkably deep questions about causality, free will and the nature of history itself. Perhaps the past cannot be changed. Perhaps changing it leads to another timeline. Or perhaps the laws of physics prevent us from reaching the past at all.
What the Grandfather Paradox makes clear is that backward time travel would require more than a machine capable of reaching yesterday. The universe would also need some way of keeping cause, effect and history from contradicting one another.
Frequently Asked Questions
What is the Grandfather Paradox?
The Grandfather Paradox is a time-travel consistency paradox in which changing the past prevents the cause of that change from occurring. The classic example involves traveling back and killing your grandfather before your parent is conceived, thereby preventing your own birth and therefore the journey itself.
What happens if you go back in time and kill your grandfather?
That depends on how backward time travel works — if it is possible at all. In a single self-consistent timeline, you could not successfully change the past in a way that prevents your own journey. In a branching-timeline model, you might kill your grandfather in a different timeline without erasing the history you came from. A third possibility is that the laws of physics prevent travel into the past altogether.
Can the Grandfather Paradox be solved?
There is no experimentally confirmed solution because backward time travel itself has never been demonstrated. Proposed answers include the Novikov Self-Consistency Principle, branching or multiple-timeline models, and the possibility that the laws of physics prevent backward time travel.
What’s the difference between the Grandfather and Predestination Paradoxes?
The Grandfather Paradox creates a contradiction: an action in the past prevents its own cause. A Predestination Paradox forms a self-consistent causal loop in which an attempt to change the past instead helps cause the event the traveler was trying to prevent.
Is the Grandfather Paradox scientifically possible?
We don’t know because backward time travel has never been demonstrated. General relativity contains solutions involving closed timelike curves, and physicists can investigate their consequences using theoretical models, but there is currently no evidence that a person or object can actually travel into its own past.
What if history simply rewrites itself?
Science fiction sometimes uses another model in which there is only one timeline, but changing the past causes history to rewrite itself. This can appear to solve the paradox if the traveler survives long enough to make the change before disappearing. The difficulty is explaining how an erased history can still produce the traveler who erased it.
