Time Travel II PDF

TIME TRAVEL II PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Different conceptions of time travel Parallel timelines The Grandfather Paradox  INTRODUCTION In the previous class, we said that time travel should not...

TIME TRAVEL II PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Different conceptions of time travel Parallel timelines The Grandfather Paradox  INTRODUCTION In the previous class, we said that time travel should not be understood to involve past events happening a second time around. Otherwise, we get contradictions: In 44BCE, you were not there to see Caesar killed. In 44BCE, you were there to see Caesar killed. For the same reason, time travel should not be understood to allow the past to be changed.  INTRODUCTION If you went back and stopped Caesar being killed, we’d end up with another contradiction. In 44BCE, Caesar was killed. In 44BCE, Caesar was not killed. But is there any way of understanding time travel which allows us to change the past?  PARALLEL TIMELINES / UNIVERSES A popular view of time travel involves creating a new timeline. Suppose that in 2030 you decide to travel back in time to 44BCE to stop Caesar from being killed. 44BC E On this view, when you travel back in time, you create a new timeline which branches off from our own timeline at 44BCE.  PARALLEL TIMELINES / UNIVERSES On this new timeline, there is somebody exactly like Caesar, and you can stop his enemies from killing him. This conception of time travel doesn’t allow you to change the past in our own timeline. It only allows you to create a “different history” in a new timeline. We won’t be focusing on this conception of time travel. We’ll be focusing on time travel into the past of our own time-line (as with Godel’s model of time travel). PARALLEL TIMELINES / UNIVERSES  THE GRANDFATHER PARADOX The most widely debated puzzle about time travel is the “grandfather paradox”. Imagine that your grandfather was a really evil man who committed many horrible crimes, and you’d like nothing better than to kill him. Unfortunately, your really evil grandfather died happily and peacefully in bed in 1990 before you were born, so it looks like you’ll never get the chance. But then in 2030 you discover the secret to time travel!  THE GRANDFATHER PARADOX You decide to go back in time to 1940 when grandfather was just a boy and kill him before he becomes evil. You take the best gun available with you and travel back to the past. You spend months training so that you are an extremely talented sharpshooter. You learn granddad’s route to school and know his exact movements each day.  THE GRANDFATHER PARADOX Finally, you pick the perfect day when weather conditions are just right; you find a perfect spot and wait for the boy (grandfather) to walk by on his way to school. What happens? You cannot kill grandfather! Think what it would mean if you killed your own grandfather as a boy.  THE GRANDFATHER PARADOX Grandmothe Grandmothe Grandfather Grandfather r r Father Mother You If you killed grandfather as a boy, then you would never have been born —the bloodline would stop.  THE GRANDFATHER PARADOX In short, we get more contradictions: You are born and travel back in time. You are never born because your grandfather was killed as a boy. Your grandfather lived to be an evil man who died peacefully in old age. Your grandfather was killed as a boy. Does this mean time travelers can’t kill anybody in the past?  THE GRANDFATHER PARADOX No, time travelers can kill people in the past; but they cannot just kill anybody. Suppose you went back to 1966, got into a fight with somebody called “Harry” and accidentally killed him. That’s possible, but only if it was already true that Harry was accidentally killed in a fight in 1966 (by a time traveler, i.e., you.) You couldn’t go back in time and kill Harry in 1966 if history tells us correctly that Harry lived until 1975.  THE GRANDFATHER PARADOX In short, you cannot go back in time and change the past, i.e., make it different from how it already was. But you can go back and influence the past by helping to make it the way it already was. When you go back to the past, you do the things that you already did thereby contributing to history the way it already happened.  THE GRANDFATHER PARADOX That’s not the end of the grandfather paradox though; it raises more puzzling questions. Why can’t you kill grandfather exactly? What stops you from killing him? Are there guardian angels that protect the past from being changed?! Do the laws of logic intervene to stop you creating a contradiction by killing granddad?!  THE GRANDFATHER PARADOX Plus, it looks like there may be a contradiction here already. There’s a clear sense in which you CAN kill the boy—you’ve got the gun and the bullets; you’re a great shot; you’ve got a clear view etc. But because you are a time traveler aiming at your own grandfather, you CANNOT kill him. Contradiction: You CAN kill the boy (you’ve got what it takes). You CANNOT kill the boy (he’s your grandfather).  THE GRANDFATHER PARADOX AGAIN David Lewis offers us some answers. To say that somebody can do something is to say that it is compossible with certain facts. E.g., an athlete in jail can complete the London marathon given the fact that she is very well-trained and in excellent physical condition. But she cannot complete the London marathon given the further fact that she is stuck in jail.  THE GRANDFATHER PARADOX AGAIN Similarly, you can shoot the boy given the fact that you have a gun and bullets etc. But you cannot kill the boy given the further fact that he lived to be an old man. You can do it consistently with some of the facts, but you cannot do it given all of the facts—there is no contradiction here. Lewis also argues that we don’t need anything mysterious like guardian  THE GRANDFATHER PARADOX AGAIN Something ordinary will stop you from killing him. Maybe you’ll slip on a banana skin. Maybe your gun will jam. Maybe you’ll lose your nerve. Maybe the boy will suddenly get into a car. Maybe a bird will fly by and distract you. No mysterious forces are needed, according to Lewis.  IMPROBABLE TIME TRAVEL Paul Horwich argues that Lewis’ solution makes time travel highly improbable even if not impossible. Imagine again that you’re trying to kill grandfather as a boy. You aim and shoot, but the gun jams. You reload and slip on a banana skin. You try again and a bird distracts you. You look back and the boy is getting into a car. The next day, you try again, and there’s another sequence of unlikely events to get in the way.  IMPROBABLE TIME TRAVEL This sequence of events is not impossible, but we know it is extremely unlikely. If time travel gives rise to these kinds of sequences of events, then it seems to follow that time travel is extremely unlikely.  MYSTERIOUS RING-FENCING Others have argued that Lewis’s defence of time travel involves the inexplicable. We could explain individual coincidences that stop you from killing grandfather—e.g., the gun jams because it needs more oil, you slip because But whensomebody left a bananaoccur, strings of coincidences skin etc. that looks like ring-fencing of the past to stop you from changing it. This ring-fencing is inexplicable and mysterious.  SUMMARY Getting clear which view of time travel we’re interested in Not creating a new timeline Instead, travelling back into the past of our own timeline The Grandfather Paradox You can influence the past, but you cannot change the past You cannot go back in time and kill granddad as a boy You CAN kill granddad and you CANNOT kill grandad—how so? Lewis: you can kill him relative to some of the facts but you cannot kill him relative to all the facts  SUMMARY What stops you from changing the past? Lewis: ordinary events will stop you (no need for guardian angels etc.) Horwich: this makes time-travel improbable Others say Lewis’ view involves an inexplicable ringfencing of past events  QUESTIONS Explain the grandfather paradox in your own words. Explain the idea that time travellers can influence the past but they cannot change it. How does Lewis try to solve the contradiction that you can kill the boy (you’ve got the gun) and you cannot kill granddad (since he died an old man)? What will stop you from killing the boy, according to Lewis? Do you think Lewis’s explanation is satisfactory?