Is Time Travel Logical?

Professor Ryan Christensen explored the metaphysical possibilities of time travel at the Philosophy Lecture Series.

PROVO, Utah (Feb. 16, 2017)—For centuries, the concept of time travel has captured the human imagination. From the ever-popular Doctor Who TV show to books like Washington Irving’s Rip Van Winkle or Madeleine l’Engle’s A Wrinkle in Time, it’s evident that we as a society are still fascinated with the potential implications of time travel.

Philosophy professor Ryan Christensen tackled the topic of time travel as part of the Philosophy Lecture Series. “I’m not asking whether it’s physically possible,” Christensen clarified. “I’m asking whether it’s metaphysically possible – does it even make sense to talk about time travel?”

He started out defining time travel, imagining the chair at the front of the room was his “time machine.” “I get in my time machine. I’m going to wait. And now, it’s five seconds later [when I get out of the chair]. That is not time travel.” The kind of time travel Christensen references is the difference between internal and external time. He used the following diagrams to illustrate:

“These lines represent external time, and the circles represent your own time.” In other words, every time someone internally perceives one second passing, the same amount of time passes externally.

“But if you fast forward,” Christensen continued, “then your time, and the time that’s measured by the clock on the wall and other people around you are no longer in sync.” In his diagram, for each two seconds that pass externally, the person fast-forwarding time would only internally experience one second. He said, “If I pushed fast forward and you were watching me, it would look like I was going slowly.”

Christensen then introduced what he calls “slow forward,” which represents internal time moving more quickly than external time. He described how in that scenario, everything external would slow down, including the air molecules, making them feel like liquid instead of gas. “I think that’s one difficulty with this kind of time travel,” Christensen concluded. “You wouldn’t really be able to move your body through the external world.”

Christensen proposed the difficulty of determining what separates internal and external time. “If the boundary is on your skin, imagine what that would feel like [if time were frozen]. You’d be submersed in frozen nitrogen.” He suggested that one option was putting the line further in, between mind and body.

He brought up an interesting phenomenon that would occur if time travel did involve physical bodies – the collision of two of the same person at one point in time as time rewinds. At that point, he explained, “my body as it’s going forward is in the same physical location as my body going backward.” This kind of rewinding time, called Putnam time travel, gives rise to the presence of two or three sets of the same people at one point in time – one set going forward, one rewinding, and a third going forward after having gone back in time.

Many of these metaphysical issues can be solved by portals – “wormholes are the scientific way of talking about this,” Christensen said. “[They] connect two space-time points.” Stepping from one moment of time to another is easier than rewinding or fast forwarding – there are not multiple of the same person, the air-friction dilemma is solved, and there is no discrepancy between internal and external time before or after entering the portal.

But if you can get to the past through a wormhole, what about what you do once you get there? This is called the grandfather paradox – if you hypothetically traveled to the past and killed your grandfather before he had children, which would be the “real” past? The one where he died without children or grandchildren, or the one where he didn’t, which eventually led to you? “Any change in the past would be a contradiction,” Christensen concluded. “When you go back to change the past, you can’t do it, because it’s already happened.” But what could stop you from killing your grandfather if you did arrive in the past? Philosophers say that “banana peels” – Normal things that stop someone from doing something, like slipping on a banana peel– would be responsible for stopping you.

The last concept Christensen explored was the concept of future time as a garden of forking paths, given birth in Borges’ short story of the same name. In this way of thinking, the future is not defined; there are innumerable possible futures, and the “real” future of an individual is the one he or she creates through their choices. This makes time travel to the future difficult because the “real” future hasn’t been created – what future does the time traveler end up in – will that future ever be realized? Christensen again asked the question, “Is time travel possible?” He followed the question with the opinion, “It seems very costly to believe that it is.”

Olivia Madsen (B.A. French language, ’18)

Olivia covers events for the Philosophy Department of the College of Humanities. She is a senior pursuing a degree in French language with a minor in international development.

All images courtesy of Ryan Christensen, cover photo via