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314Lecture13-TimeTravelOnline - Lecture 13 Time Travel...

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Lecture 13: Time Travel According to philosopher David Lewis, time travel is a possibility. Time paradoxes such as the Grandfather Paradox illustrate that a world in which time travel is possible is a very strange world indeed, and perhaps, fundamentally different from the way we experience our world. However, the paradoxes do not negate the possibility of time travel. There are many paradoxes that arise when thinking about the nature of time. In a previously lecture we discussed the Grandfather Paradox: if a person travels back in time and kills his biological grandfather before he met his biological grandmother, then the time traveler would never have existed. So this means the time traveler couldn’t have gone back in time, which means he couldn’t have killed his grandfather, which means the time traveler exists (since his grandfather wasn’t killed by time traveler). But if his grandfather is still alive, then the time traveler could go back in time to kill his grandfather. Each possibility seems to imply its own negation. Michio Kaku presents another example of a time travel paradox in his book Physics of the Impossible (p. 216) : In the novel Janus Equation writer Steven Spruill explored one of the harrowing problems with time travel. In this tale a brilliant mathematician whose goal is to discover the secret of time travel meets a strange, beautiful woman, and they become lovers, although he knows nothing about her past. He becomes intrigued about finding out her true identity. Eventually he discovers that she once had plastic surgery to change her features. And that she had a sex change operation. Finally, he discovers that “she” is actually a time traveler from the future, and that “she” is actually himself, but from the future. This means that he made love to himself. And one is left wondering, what would have happened if they had had a child? And if this child went back into the past, to grow up to become the mathematician at the beginning of the story, then is it possible to be your own mother and father and son and daughter? David Lewis takes his cue from science-fiction writers. In some sci-fi accounts time travel is presented consistently. While some science-fiction writers are not constrained by scientific theory, we can learn from the stories that are well-thought through and consistent with the latest scientific findings. What is time travel? Lewis says it is the “discrepancy between time and time”. When a traveler departs at time 0 , he will arrive at time 1 . The journey takes time 1 - time 0 . This is the duration of the journey. 1
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This is the commonsense or intuitive description of a journey through time. However, if you are a time traveler, the separation in time between departure and arrival does not equal the duration of the journey. Let’s say you travel for one hour, you leave at 4:00pm (=time
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314Lecture13-TimeTravelOnline - Lecture 13 Time Travel...

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