The Philosophical Complications of Time Travel and How to Solve Them

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

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

If a person’s friend gave them a pen, and two days later the person traveled back in time with that pen and gave it to the friend before they gave it to their past version, it was apparently never manufactured. A self-created object like this is called a Jinnee.

Sand running through the bulbs of an hourglass.
Einstein’s relativity reveals that motion, and consequently simultaneity, are relative to a reference frame. (Image: Min C. Chiu/Shutterstock)

The Argument against Time Travel into the Past

The possibility of Jinn (plural of Jinnee) seem to make time travel into the past impossible. After all, causes always precede their effects, right? So if A caused B, A came before B. But if B also caused A, then B came before A. But, by the principle of transitivity, that means that B came before itself, and that’s logically impossible. 

The problem with Jinn actually goes deeper. Regardless of what temporal direction the causal arrow points, it seems that causal explanations have to bottom out somewhere. Unless the stopping point is arrived at, nothing has really been explained. But with Jinn, there is no stopping point—A caused B, which caused A, which caused B, and on and on and on. 

Newton’s Metal Cradle on a white background.
The possibility of Jinn seem to make time travel into the past impossible. (Image: Dotted Yeti/Shutterstock)

This, it seems, leaves Jinn without an explanation for their existence. They have no causal origin. And if there is no explanation for why they exist, then logically they can’t exist, right? Since time travel into the past would entail that they could exist, time travel into the past must be impossible. That’s the argument.

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Einstein’s Mind-Bending Relativity

Well, experience shows that there is a fact about what moments are past, present, and future at any given moment. In the beginning, the Big Bang was the present moment, and everything else was the future. Later, the JFK assassination was in the present moment, and everything else was either past or future. And if two events are simultaneous, that’s because they happened—they were both present—at the same time.

But Einstein’s relativity reveals that this is not correct. Because motion, and consequently simultaneity, are relative to a reference frame. Think of two spaceships about to collide—say the Millennium Falcon and the starship Enterprise. From the Falcon’s perspective, or frame of reference, it’s stationary and the Enterprise is moving toward it. But from the Enterprise’s perspective, it’s stationary and it’s the Falcon that’s moving. 

And, from the perspective of someone on, say, Battlestar Galactica watching this whole thing, the Falcon and Enterprise are moving toward each other. What relativity explains is that no one of these perspectives is ‘more correct’ than another. These are all equally accurate ways of describing what’s happening. 

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

Think of Time as a Non-linear Concept

The classic thought experiment used to explain this involves two people—one inside a ‘moving’ train and one outside it—and lights that turn on, on the front and back of the train, whose light beams meet in the middle of the train. From the perspective of the person inside the train, the lights turned on at the same time. 

Man writing mathematical problem solution on blackboard.
The classic thought experiment used to explain time as a non-linear concept involves two people, one inside a ‘moving’ train and one outside it. (Image: fran_kie/Shutterstock)

But, from the other person’s perspective, the back light turned on first—since the train is moving toward the front light and away from the back light, that’s the only way for their light beams to meet in the middle of the train and the speed of light not to change.

Because simultaneity is relative, for any given event A, and any other event B that happens after it, there will be another event C such that C is simultaneous with A in one reference frame, but also simultaneous with B in another. But the only way that can be the case is if all the events exist together. 

And that’s why relativity contradicts the common progressive view of time. Because the only way that can be true is if all moments (what is called past, present, and future) exist—as a whole—in a giant block. A block that has different reference frames running through it, all dictating different things about which events are simultaneous with others in different frames—but none capturing the full truth.

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As Weird as It Seems, Time Exists All at Once

Realizing that this is the true nature of time allows the realization that there is nothing paradoxical at all about Jinn. They seem paradoxical because they were running around in a circle. A caused B, but B caused A—the causal explanation doesn’t end anywhere. 

They have no causal origin. But if all of time is a giant block, which contains all moments in time, they do have a causal origin. The Jinn came into existence, along with everything else, as the entire block came into existence. Or it could be thought of this way. When it comes to a Jinnee, nothing is unexplained. A explains B, B explains A—and there is nothing else to explain.

Common Questions about the Philosophical Complications of Time Travel and How to Solve Them

Q: What is a Jinnee?

A self-created object is called a Jinnee. Objects like these are usually in time travel stories where somebody has an object, and then time travels into the past and gives themselves that object, so the object never gets created; it just gets handed to them by themselves.

Q: Why does Einstein’s relativity contradict the common progressive view of time?

Einstein‘s relativity contradicts the common progressive view of time because the only way that it can be true is if all moments (what is called past, present, and future) exist—as a whole—in a giant block.

Q: How are Jinn used in an argument against time travel?

If there is no explanation for why Jinn exist, then logically they can’t exist. Since time travel into the past would entail that they could exist, time travel into the past must be impossible. That’s the argument.

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