How to Go Back in Time and Kill Your Grandfather Without Causing a Paradox

FROM THE LECTURE SERIES: SCI-PHI—SCIENCE FICTION AS PHILOSOPHY

By David K. Johnson, Ph.D., King’s College

The grandfather paradox illustrates how time travel would make it possible for someone to annihilate themselves before they traveled in time—say by killing their grandfather before he sires their father. This would create a paradox where someone annihilates themselves if and only if they don’t.

Modern white clock with hands that spiral inward.
To solve the grandfather paradox, it is necessary to reconceive the very nature of what time travel would do. (Image: Mikhail Leonov/Shutterstock)

The Grandfather Paradox

To solve the paradox, it must be explained why time travel could never allow for the possibility of self-annihilation. To do so, it is necessary to reconceive the very nature of what time travel would do. There are two ways to do this.

The first is called branching time travel, a solution to the problem endorsed by philosophers Nuel Belnap and David Deutsch. The suggestion is this: when someone travels back in time, they do not travel to their own past.

Abstract mathematical equations and shapes on blackboard.
Branching time travel suggests when someone travels back in time, they do not travel to their own past but to the past of an alternate universe. (Image: local_doctor/Shutterstock)

They travel to the past of an alternate universe, which has a past just like the universe they left, up to the moment they traveled to it, but that also contains the event of them, the time traveler, appearing at that moment. 

They then proceed forward in time in that universe and deal with the consequences of their actions there. But they can’t kill their own grandfather, because their grandfather is in the past of the universe they left; they traveled to a different universe, with a different grandfather. If they kill him, they will simply prevent that universe’s version of them from being born. But the event of their birth will still be safe and sound, in their original universe.

This is a transcript from the video series Sci-Phi: Science Fiction as Philosophy. Watch it now, on The Great Courses Plus.

Grandfather Paradox and the Consequences of Your Actions

Although branching can make sense of the grandfather paradox, it still has two noteworthy consequences. First, it’s not clear that this is really time travel. Although it would look like time travel to the time traveler, it’s really just alternate universe creation. So to accomplish this, a machine would need to be invented capable of creating specific kinds of alternate universes, not a machine capable of traveling in time.

The second consequence of branching time travel is the realization that it might not be very useful. For example, someone can’t actually effect changes to their own timeline; they can only create a different timeline that’s more to their liking. Indeed, if they travel forward again, it will just be into the future of the new universe they created. And if they travel back again, they will just create yet another universe. They can never return to the universe they left.

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No Matter How Hard You Try, You Can’t Kill Your Grandfather

A second solution to the grandfather paradox was developed by philosopher David Lewis. He suggested that time travel would not give someone the ability to kill their own grandfather because doing so would be impossible—essentially because they’d be predestined not to do so. 

Why? Well, think back to the block world view of time, in which the entire universe, past, present, and future, exists as a whole. If time travel is possible, the universe already contains the events of time travelers traveling in time—their escapades in what would be called both the future and the past. 

That means that before a time traveler ever activates their machine, the things that they will do while in the past have already occurred. The past is already written. So they can travel there and experience the past—they can even participate, causing things to happen in the past. But it would be impossible for them to cause anything to happen other than what they had ‘already caused’.

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It’s All Set in Stone

Nebula and galaxies in space.
If time travel is possible, the universe already contains the events of time travelers traveling in time. (Image: NASA images/Shutterstock)

It may seem, from a subjective point of view, that someone can do whatever they want, but really they can only perform the actions in the future that already contain them. These can’t be changed, any more than the past could be changed. 

The universe exists as a whole—past, present, and future. Indeed, the only difference, it seems, between the time traveler and others is how the moments of the others’ lives are arranged in the block.

Theirs are neatly arranged in chronological order. The time traveler’s are cut up into pieces and scattered throughout. But since it exists, as a whole, neither the time traveler nor the others can do anything other than what the block contains.

Common Questions about How to Go Back in Time and Kill Your Grandfather Without Causing a Paradox

Q: What is the grandfather paradox?

The grandfather paradox illustrates how time travel would make it possible for someone to annihilate themself before they traveled in time—say by killing their grandfather before he sires their father.

Q: What was David Lewis’s solution to the grandfather paradox?

David Lewis suggested that time travel would not give someone the ability to kill their own grandfather because doing so would be impossible—essentially because they’d be predestined not to do so. 

Q: What is the problem of branching time travel if someone wants to kill their grandfather?

In branching-time travel, when someone travels back in time, they do not travel to their own past, they travel to the past of an alternate universe. This means the person they kill is an identical copy of their grandfather in an identical universe to their original one up until they traveled into it

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