If you were to travel back in time to kill your grandparents — let's ignore the 'why' here, for the sake of argument — you would never have been born. Which means there was nobody to kill your grandparents. Which means you were actually born after all, which... hold up, what's going on here?!
These kinds of brain-breaking paradoxes have been puzzling us forever, inspiring stories ranging from "Back to the Future" to "Hot Tub Time Machine."
Now, New Scientist reports that physicists Barak Shoshany and Jacob Hauser from the Perimeter Institute in Canada have come up with an apparent solution to these types of paradoxes that requires a very large — but not necessarily infinite — number of parallel universes.
They uploaded a paper of their research to the preprint archive arXiv last month. It describes a model in which a person could theoretically travel from one timeline to another by stepping through a hole in space-time, or wormhole, in a way that they claim is "mathematically possible."
"The parallel universes approach that we suggest says there are different parallel universes where things are roughly the same, and each one is mathematically on a separate space-time manifold," Shoshany told New Scientist. "You can go between those manifolds when you travel back in time."
Multiple timelines would allow you to travel to a different timeline and kill your grandparents without causing a paradox. But the number of timelines doesn't have to be infinite for this to work, the researchers calculated.
The model does have a major drawback, at least for narrative purposes: time travel won't do any good for your own timeline.
"What time travel means here is stepping between those histories — that’s even freakier," astrophysicist and dark matter expert Geraint Lewis at the University of Sydney, who was not involved in the research, told New Scientist. "At some level it doesn’t even feel like time travel anymore, because what’s the point of going back and killing Hitler if the second world war still plays out in the universe you’re from?"
READ MORE: Time travel without paradoxes is possible with many parallel timelines [New Scientist]
More on time travel: A New Model Debunks a Popular Argument Against Time Travel