Alternate Universes and Time Travel Paradoxes in Science Fiction


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

Time travel and alternate universes are used a lot in science fiction. The problem with time travel in such stories is the paradoxes they create that the viewers just have to ignore. However, not all time-traveling stories end up in paradoxes.

A 3D rendering of time travel through space.
Branching time travel is a solution to the problem of time travel endorsed by philosophers Nuel Belnap and David Deutsch. (Image: andrey_l/Shutterstock)

I’m Sorry Doctor, Who?

In the popular TV series Doctor Who there is an episode from 2007 called ‘Blink’. In it, Sally Sparrow discovers an alien race called the Weeping Angels. The angels freeze into stone when they are being observed, but can kill someone when they’re not by sending them back in time and feeding off the potential energy of the future days they would have lived.

A blue police telephone box in the street.
In an episode of Doctor Who, the Doctor is trapped in 1969 without his time machine, the TARDIS, which is stuck in 2007. (Image: bmszealand/Shutterstock)

This is how they’ve trapped the Doctor in 1969 without his time machine, which is stuck in 2007. To get it back, he arranges for Easter eggs to be planted on 17 different DVDs—the 17 that Sally Sparrow owns.

These Easter eggs show the Doctor carrying on half a conversation. Where’s the other half? It is in 2007, being carried on by Sally Sparrow. The Doctor is able to maintain his half of the conversation because he has a transcript of what she will say 38 years before she’ll say it. How?

Well, when Sally finally watches the Easter egg, she figures out that the Doctor is talking to her and carries on her half of the conversation. Sally’s friend Larry starts writing it down. Then at the end of the episode—after Sally sends the TARDIS back to the Doctor in 1969—she runs into the Doctor at a moment in his life before he is trapped in 1969. She gives him the transcript so that one day in his future when he needs it to create the Easter eggs, he will have it.

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

Branching Realities: Alternate Universes

Branching time travel is a solution to the problem of time travel 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. 

They travel to the past of an alternate universe, which has a past just like the universe they left, up to the moment at which they traveled, 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.

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Branch Too Much, You Lose Your Touch

Although it does solve many potential paradoxes, branching time travel cannot be used to make sense of every time travel story—even ones that explicitly try to explain their plot with the branching view.

Back to the Future's DMC DeLorean on display.
In the movie Back to the Future II, Doc Brown and Marty travel from the peaceful town of Hill Valley in 1985 to Hill Valley in 2015. (Image: The Community- Pop Culture Geek/CC By SA/2.0/Public domain)

Take the second Back to the Future. Doc Brown and Marty travel from the peaceful town of Hill Valley in 1985 to Hill Valley in 2015. While they’re there, Biff steals the DeLorean to travel back to 1955 and gives himself a Sports Almanac, and he then returns the time machine to 2015. Doc and Marty then use it to return to Hill Valley in 1985—only to find that it is now a lawless, dystopian wasteland. 

Doc explains this by saying that Biff’s giving his younger self the Almanac created an alternate timeline in which he gained wealth that enabled him to ruin the town—and that is the timeline in which they are now in.

But that’s not how branching time travel works. If in 2015 Biff created an alternate universe when he stole the DeLorean to travel to 1955, then when he went back to the future, he should have arrived at the 2015 of that alternate universe—one where that universe’s Biff is old and rich. He should not have returned to the future of the universe he left, where he got the DeLorean from Marty and Doc in the first place—but that’s what happens in the movie.

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The Trippiest Movie Ever

To make sense of other stories, the branching theory has to be tweaked a bit. Take Donnie Darko, for example. It tells the story of a boy who is out for a walk one night, when, out of nowhere, while he’s communicating with a psychedelic rabbit, a jet plane engine lands on his bedroom. 

Somewhat thankful to be alive, he continues to live his life, falling in love with a girl named Gretchen. But his meeting her, ultimately—although accidentally—leads to her death. When presented with the opportunity to go back and be in his bedroom when the engine falls on it, he takes it and dies, thus saving her life.

But, to explain the plot in terms of branching time travel, there is one little hitch. If Donnie’s traveling back in time created a duplicate universe, just like the one he left, right up to moments before the engine fell on his room—well, that universe would also contain a duplicate Donnie out for a walk, who would go on to live his life.

Common Questions about Time Travel Paradoxes and Alternate Universes in Science Fiction

Q: If the alternate universe theory is true, what would have actually happened to Marty and Doc in Back to the Future?

In the movie, in 2015, Biff steals the DeLorean to travel to 1955. In doing so, he creates an alternate universe, so when he travels back to 2015, he shouldn’t just go back to the universe he came from. He should have gone to 2015 of the alternate universe he traveled to when he went to 1955.

Q: How does the branching realities theory work?

If someone were to time travel into the past, they would be both traveling in time and changing universes. So they would land in an alternate universe identical to theirs except it would be in the past, and its future wouldn’t be identical to the original universe anymore since this one would now have a time traveler in it.

Q: In the Doctor Who episode ‘Blink’, how does the doctor have a transcript of his conversation with Sally Sparrow?

When the Doctor is trapped in 1969, he needs the transcript he’s using to talk to Sally. When Sally watches the DVD with the Doctor’s part of the conversation, she figures out that the Doctor is talking to her and carries on her half of the conversation and her friend writes down her answers. Then in the future, the Doctor and Sally meet each other and she gives him the manuscript so he can use it later on.

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