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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 traveller 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 traveller, 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."
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