The grandfather paradox is a potential logical problem that would arise if a person were to travel to a past time. The name comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible. So, if time travel is possible, it somehow must avoid such a contradiction.
The logical inconsistency of time travel is a common theme in time-warping fiction, but it's also of interest to philosophers. In early versions of the grandfather paradox, some tried to argue that time travel was impossible on logical grounds, said Tim Maudlin, a philosopher at New York University, who frequently writes about physics and philosophy. "In a way, that's like asking why, right now, I can't be wet and completely dry," he said. "Well, that's just logically impossible. What are you asking about?"
But contradictions such as the grandfather paradox don't mean that time travel is impossible. The logical consistency of time travel largely depends on the concept of time, and physicists have many different ways of conceptualizing time. For example, if some laws of physics are considered probabilistic, rather than precisely determined, it opens the possibility of multiple outcomes from a trip back in time, some of which may not be contradictory.
"It's actually more difficult than you think to come up with a situation where there are no consistent solutions," Maudlin said. For a logically consistent time travel story, he gives the example of a traveler going back in time to shoot themselves. They aim to kill, but miss because of a tremor in their hand. The non-lethal shot strikes a nerve in the past version of the traveler, causing a tremor in their hand for the rest of their life.
The concept of time travel can also be separated from the idea of changing the past, or backward causation. But Maudlin doesn't think backward causation is possible. "I think it's contrary to the nature of time itself," he said. "That's not a majority view."
What is time?
People in Western cultures tend to think of time as a line. The present is a point somewhere in the middle, the past stretches in one direction and the future unfurls in the other. There may or may not be a beginning or an end. There's also a Newtonian idea of global time: The same time line applies to everywhere in the Universe.
But philosophers, scientists and fiction writers have dreamt up other versions of time, where it has more dimensions and features. Visualizations have taken the form of loops, circles, hourglasses, mobius strips, and cardboard tubes. There are also numerous, less serious, fictional accounts that nonetheless allow for the possibility of time travel, such as "Jeremy Bearimy."
Einstein's theory of relativity has been particularly influential. Relativity theoretically allows for space and time to fold over onto itself in structures that have been dubbed closed time-like curves. If these time loops exist, they would be a form of time travel, since people inside the loop would revisit the same moment in time.
With relativity, Newtonian global time is also out the window. In that case, "nothing in particular counts as 'right now,'" Maudlin said. "There are lots of different things that count as 'right now.'"
All time travel stories work under an assumption that time travel is possible, but their creators don't necessarily worry about paradoxes. Authors might ask interesting what-if questions about personal or historical events. Maudlin said he appreciates efforts that are logically consistent, but also said that time travel is often just a storytelling device. "The point is not really time travel, but counterfactuals," he said.