Damascene Philosophy

ParadoxesV

The grandfather paradox

You travel back in time and prevent your grandparents from ever meeting. Then you are never born, so you never travel back. So they do meet. So you are born...

Reading time
4 min
Difficulty
Intermediate
Branch
Time
First posed
Science fiction of the 1920s to 1940s; philosophical treatment by David Lewis, 1976
grandparents meetyou are bornyou travel back
Travel back and stop your grandparents meeting. Then you are never born, so who made the trip?

The puzzle

You build a time machine and travel back to a time before your parents were born. You find your grandfather as a young man and, for reasons best not examined, you prevent him from ever meeting your grandmother.

But if your grandparents never met, one of your parents was never born, and so you were never born. And if you were never born, you could not have travelled back in time to interfere. So your grandparents did meet after all, and you were born, and you did travel back, and so on, around and around.

The grandfather paradox seems to show that travel into the past is impossible, or at least that changing the past is. Any trip back in time seems able, in principle, to undo its own cause.

Yet general relativity, our best theory of gravity, appears to permit certain spacetime geometries, called closed timelike curves, along which an object could return to its own past. Kurt Gödel found such a solution to Einstein's equations in 1949. So physicists and philosophers have had to take the paradox seriously.

Why it matters

The paradox is the main argument against the possibility of time travel, and so bears on what kinds of universe the laws of physics allow.

It also raises a surprising question about free will: if you travel back in time, can you do anything at all? You can certainly try to kill your grandfather. But it seems you will fail. Why?

Key terms

  • Closed timelike curveA path through spacetime that returns to its own past. Some solutions to Einstein's equations of general relativity allow them, which is why physicists take time travel puzzles seriously.
  • Self-consistency principleIgor Novikov's principle that any events involving time travel must be consistent with each other, so that a time traveller cannot change the past, only take part in it.
  • Branching timelineA model of time travel in which changing the past creates a new, separate history, so the traveller's original past remains intact elsewhere.
  • EternalismThe view that past, present and future events all equally exist. Opposed to presentism.
  • Causal loopA chain of events that causes itself, such as an object or piece of information carried back in time that becomes its own origin.

Proposed solutions

Proposed solution I

The past cannot be changed

Time travel may be possible, but changing the past is not. Whatever you did in the past, you already did. Your attempt will fail.

Proposed solution II

Branching timelines

Travelling back in time creates a new branch. You can change 'the past', but it is a different history from the one you left.

Proposed solution III

Time travel is impossible

The paradox shows that travel into the past cannot happen. The laws of physics, properly understood, forbid it.

Each solution, tested

The past cannot be changed

How it resolves the paradox
David Lewis argued in 1976 that time travel is consistent as long as there is only one timeline. If you travelled back, your actions were always part of history. Your grandfather survived, so your attempt to stop him failed: the gun jammed, you slipped, you changed your mind. You can kill him, in the sense that you have the ability, but you will not, since he in fact survived. Physicist Igor Novikov proposed a matching 'self-consistency principle': the laws of physics only allow self-consistent histories.
Why it doesn’t fully work
It seems strange that a determined time traveller would be guaranteed to fail, by a string of improbable coincidences, every time. Something seems to be preventing them, which looks like a mysterious force.
What defenders say
Lewis answered that nothing prevents you; it simply happens that your attempts fail, as countless ordinary attempts fail for ordinary reasons. The appearance of coincidence comes from knowing the outcome in advance.

Branching timelines

How it resolves the paradox
If you go back and prevent your grandparents meeting, you split off a new timeline in which you are never born there. But you came from the original timeline, where you were born. There is no contradiction, because two different histories are involved. Some connect this to the many-worlds interpretation interpretation of quantum mechanics, as David Deutsch has done.
Why it doesn’t fully work
This is not really travelling to your past; it is travelling to a different past. The original past remains unchanged. Many think it changes the subject.

Time travel is impossible

How it resolves the paradox
Stephen Hawking proposed a 'chronology protection conjecture' in 1992: quantum effects would prevent closed timelike curves from forming, keeping the universe safe for historians, as he joked. Many physicists suspect that a full theory of quantum gravity will rule out time machines.
Why it doesn’t fully work
The conjecture remains unproven. And the paradox alone does not show impossibility: the self-consistency view shows that time travel can avoid contradiction.

Who argued what

  • The past cannot be changed David Lewis, Igor Novikov, Kip Thorne
  • Branching timelines David Deutsch
  • Time travel is impossible Stephen Hawking

Where you stand

Is travel to the past possible, at least in principle?

Choose one to see how people answer.

Variations

The billiard ball

Physicists studying time travel replaced the grandfather with a billiard ball. A ball enters a wormhole, emerges in the past, and collides with its earlier self, knocking it off course so it never enters the wormhole. In the 1990s, calculations by Fernando Echeverria, Gunnar Klinkhammer and Kip Thorne found that for many starting conditions, there were self-consistent solutions: the ball emerges and gives its earlier self only a glancing blow that still sends it into the wormhole.

Killing Hitler

A popular version asks whether you should travel back to prevent a historical atrocity. On the single-timeline view, you would fail. On the branching view, you could succeed, but only in a new history, not in the one where the atrocity happened. Either way, the people in your own past are not saved.

Go deeper

Lewis on 'can'

Lewis distinguished two senses of 'can'. Relative to some facts (you have a gun, steady hands, a clear shot), you can kill your grandfather. Relative to a larger set of facts (including that he lived to have children), you cannot. Ordinary language shifts between such contexts constantly, Lewis argued, and the paradox exploits this shift.

Related ideas

Paradox

The bootstrap paradox

A time traveller gives Shakespeare a copy of Hamlet, which Shakespeare copies and publishes. So who wrote Hamlet?

Question

Is time real?

Time feels like a river carrying us from past to future. Physics describes it more like a map, with no moving 'now' anywhere on it.

Question

Are there other versions of you?

On one serious reading of quantum mechanics, every possible outcome happens, each in its own branch of reality, with its own copy of you.

Question

Is free will an illusion?

You feel like the author of your choices. The question is whether that feeling tracks anything real.

Question

Is the future already fixed?

If the laws of physics plus the state of the universe settle everything that happens, then everything you will ever do was settled before you were born.

Further reading and sources

Open questions

  • If you went back and tried to change something, what would stop you?
  • Is a past that can never be changed any different from a fixed future?
  • If you could change the past, would the people you saved be the same people?

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