# Doctor Who: How Does the TARDIS Work? (Is Time Travel Possible?)

29.06.2018

Episode #4 of the course Sci-Phi: Philosophy through science fiction by David Kyle Johnson, PhD

Einstein’s relativity suggests space and time exist as one entity, spacetime, and that spacetime can be warped and curved by mass and acceleration. This raises questions about whether we could alter the rate or direction in which we travel in time. And indeed, we could—and we can see how in one of my favorite sci-fi TV shows, Doctor Who.

Traveling to the Future

For example, massive objects like black holes warp spacetime in such a way that the closer you get, the slower time passes. And the effect can be quite pronounced. In the Doctor Who episode, “World Enough and Time,” a 400-mile-long spaceship is trying to reverse away from a black hole. While those on the top of the ship (closest to the black hole) age only a few hours, those in the bottom of the ship age years. (In the extra time they have, those in the bottom of the ship become the Doctor’s archenemy: the Cybermen.) Because the gravitational effects of a black hole increase exponentially as you approach it, such a disparity in time dilation is possible (if the black hole has the right mass).

Acceleration can also dilate time. According to relativity, if you were to travel away from the Earth at high speeds for decades, turn around, and then come back—by the time you returned, centuries would have passed on Earth. This method of time travel is used in classics like Planet of the Apes to facilitate travel into the future but could also be used to explain how The Doctor’s TARDIS allows him to travel both forward and backward in time. How?

Traveling to the Past

In short, if you use this method to time-dilate one end of a wormhole (a bridge between two distant spacetime points), you could travel into the future or the past by stepping into one end of the wormhole and emerging from the other. If the Doctor’s race—the Time Lords—figured out how to keep wormholes open (Kip Thorne says doing so would require “negative energy”), they could create a whole collection of them, call it a “time vortex,” build ships called TARDISes capable of traversing it, and thus travel anywhere and anywhen they wanted.

Now, this is all just theoretical, but the mere possibility of backward time travel seems to raise a logical paradox. As the Doctor’s companion Martha observes in “The Shakespeare Code,” with the ability to travel to the past, one could kill one’s grandfather before he sires one’s father. Such “self-annihilation” paradoxes are thought to entail that backward time travel is impossible. And contrary to the Doctor’s reply to Martha, one cannot solve them by simply not including grandfather killing in one’s time travel plans. It’s logically impossible for one to make it the case that one never existed, and logical impossibilities can’t even possibly be true.

But philosophers have concocted two ways to describe backward time-travel that makes self-annihilation impossible. The first is that of David Lewis, who suggests that backward time travel would not enable one to change the past; so if you traveled back in time and tried to kill your grandfather, you would necessarily fail. Indeed, since the actions you “will take” while in the past were already a part of the past before you hopped in your time machine, if you travel back to try to prevent something, you might find that you were actually the cause of it. In 50-plus years of Doctor Who, The Doctor has turned out to be the cause of everything from the extinction of the dinosaurs to the eruption of Mt. Vesuvius.

The second possible solution, belonging to Nuel Belnap and David Deutsch, suggests that traveling back in time would create an alternate timeline—one identical to the one you left up to the moment in the past to which you traveled. Any changes you make there will affect that timeline, but not the one you left. Thus, if you kill your grandfather, you won’t negate your own existence—but prevent the birth of someone with your DNA in the new timeline you created.

Now, one might argue that this looks more like “alternate universe creation” than time travel, but it also raises another question: Would a genetically identical doppelgänger of you in another universe actually be you? This is a question about personal identity, and we shall use another long-running science fiction TV show—Star Trek—to explore it next lesson.

Recommended books

Doctor Who and Philosophy: Bigger on the Inside edited by Courtland Lewis and Paula Smithka

Who Is Who? The Philosophy of Doctor Who by Kevin S. Decker

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