A new computer program lets you go back in time and rewrite the past

by Robert K. J. Killheffer

If you’ve seen Back to the Future, Looper, or one of a hundred other time travel movies, then you’re aware of one of time travel’s classic paradoxes: meeting yourself. It’s a paradox because your original memories don’t include the presence of a future-you, but if future-you goes back in time, then future-you will have been there, which means you should have remembered it all along.

Paradoxes like this lead physicists to believe that travel into the past is probably not possible—at least not in the real world. But in virtual reality, all bets are off, as demonstrated by a team of researchers from the University of Barcelona and the Interdisciplinary Center Herzliya in Israel. They designed a VR environment that would present test subjects with a moral dilemma—something that has been done in several other studies, but they added a crucial twist. Their system tracked the actions of participants in detail and then allowed them to go “back in time” to watch themselves do whatever they had done before—and to intervene and change the past if they liked.

The virtual scene was a small art gallery with two floors and an elevator between them. The participants could control the elevator to give visitors access to the different levels, and they could also sound an alarm that would stop the elevator wherever it was. As the scenario progressed, five visitors chose to go up to the second floor, while one remained on the lower level. At that point a seventh visitor arrived, asked to go up, and once there began shooting the five museum patrons on the upper floor. (The VR system even provided realistic screams of pain and fear.)

The second time through, the participants had decisions to make: Do something differently to try and stop the gunman? Or stand by and let history take its course? Refusing to take the would-be killer to the second floor might lead to him killing the single visitor on the first floor. Keeping more visitors on the first floor might just make them easier targets. Most participants preferred to have one person killed rather than five, but only a few found a way to save everyone: Hit the alarm while the elevator is between floors, trapping the gunman inside.

The experiment, conducted in 2014, relied on standard high-end VR equipment, including full body motion-capture suits and hi-res headsets. But the time travel element required some new software: a history-tracking system that could store each participant’s actions, and a “narrative engine” capable of replaying those histories. In each new session, as participants changed their course of action, the software generated alternate outcomes and also stored variant histories.

In recent, still-unpublished work, computer scientist Doron Friedman of the Interdisciplinary Center Herzliya, a leader of the Barcelona group, explores the most famous of all time travel conundrums, the “grandfather paradox,” in which a time traveler kills her own grandparent before her parent is conceived, thus causing herself to have never existed. Friedman explains that he built the basic rules of the grandfather paradox into his logic engine, right down to a person having to be alive in order to procreate. Friedman simplified the scenario slightly in order to focus on the central logic problem (in his program, the time traveler kills his father, not his grandfather). Based on the rules, his algorithm searched for solutions that were logically consistent.

“I didn’t think I’d really solve it,” Friedman says. “I did it as an exercise in AI.” But the program worked, coming up with two solutions to the paradox. In the first, the time traveler simply becomes his own father. Icky, maybe, but it works. The second solution is less creepy but more complicated: The traveler’s father has a time machine, too, and just before the traveler kills him, the father pops forward in time and begets the traveler, then returns to be killed. Neither solution may be particularly plausible—or savory—in real life, but they are interesting, and logical within the context of Friedman’s AI world.

Virtual time travel is embryonic today, but could have a bright future. “I’d love to see it work as part of a virtual reality game, or a massive multi-user world,” says Friedman. “Imagine you spend a lot of time in some video game, and maybe you’ve built some serious relationships. If you’ve got my time travel plug-in for it, you could correct your mistakes or maybe build your relationships differently.”

And it’s not just games. Friedman thinks his system could become a tool for scientists and other researchers, whether they are interested in time travel or not. “I had one physicist contact me about using it to try to simulate real physics,” he says. “I think that could be really interesting. I think it can be useful for modeling theories and letting someone experience what their ideas mean, rather than just looking at the formulas. Once you interact with your model you can get new insights.”

The work might also lead to real-world applications, including therapeutic systems that return PTSD patients to a semblance of the traumatic scene, and training programs that allow students to study their past errors. In fact, we’re more likely to have virtual time travel before we have the real thing, if we ever do. But that isn’t so bad: in VR, you can’t disrupt the course of real history, or kill your real grandfather, or do anything else that might poof yourself right out of existence.

Featured Photo: SHUTTERSTOCK.COM / MICHAL BEDNAREK