Time Travel III PHI101 Introduction to Philosophy

TIME TRAVEL III PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Causal Loops Identity and Time Travel  INTRODUCTION In the last class, we looked at the grandfather paradox. If it is possible for you to return to the past when...

TIME TRAVEL III PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Causal Loops Identity and Time Travel  INTRODUCTION In the last class, we looked at the grandfather paradox. If it is possible for you to return to the past when your grandfather was a boy, it looks like you should be able to kill him. But that would mean you would never be born! What could stop you from killing grandfather? This week, we will look at two further puzzles about time-travel.  CAUSAL LOOPS Another objection to the possibility of time travel is that it would allow causal loops in which things come from nowhere. Consider the following story. In 2030, Ahmed chances upon a secret room in an old Dubai souk. In that room he finds a time machine. He steps inside and travels back in time to 1971 to experience the founding of the UAE. But while there, Ahmed gets sick. The last thing he does before dying is hide the time machine in a neglected room in a Dubai souk.  CAUSAL LOOPS Question: where did Ahmed come from? Question: where does the time machine come from? If we trace the time machine’s passage through time, it goes round in a circle with no beginning and no end.  CAUSAL LOOPS Ahmed Time Machine 1971 1995 2030  CAUSAL LOOPS Consider this alternative story: As a boy, an old man with a long beard tells you a simple way to make a time machine. But you can’t make it until you have all the parts. You spend your whole life gathering the parts until finally, as an old man, you are able to make the time machine. The first thing you do is go back in time to meet yourself as a boy to tell him how to make the time machine. Where did the knowledge of how to make the time machine come from? It seems to have come from nowhere.  CAUSAL LOOPS One view is that time travel would give rise to causal loops, but causal loops are impossible, so time travel must be impossible too. Another view is that time travel is possible as long as it doesn’t give rise to causal loops, which are impossible. Lewis takes a third view: causal loops may arise through time travel, but that’s okay because causal loops are merely strange—not impossible.  CAUSAL LOOPS They’re strange because we cannot explain where the loop comes from. But we can’t explain where the big bang comes from either—this doesn’t mean the big bang is impossible. Perhaps you think that God explains the big bang. But then you can explain causal loops in the same way—God created them, and that’s where they come from.  IDENTITY Older, wiser self Young foolish Another puzzle about time travel concerns identity. self Imagine you go back in time and meet your earlier self. We want to say that You25 = You15; you are the same person. Identity obeys the following principle: Leibniz’ Law: if x=y, then any property x has, y has too. But suppose You25 is wise and You15 is not wise. Now it looks like: You25 ≠ You15!  IDENTITY Lewis solves this problem with his theory of temporal parts. On Lewis’s theory, you are spread out in time, made up of lots of temporal parts, much like a road spread out in space made up of lots of spatial parts. For Lewis, you are a space-time worm stretching across many years.  IDENTITY Suppose the road is damaged on the top stretch but not damaged along the bottom stretch. That doesn’t violate Leibniz’ Law. The top stretch and the bottom stretch are not identical. Top They are different parts of the same road. Bottom  IDENTITY Older, wiser self Lewis would say something similar about You25 and You15. Young, foolish self There is no violation of Leibniz’s Law because You 25 and You15 are not identical. You25 and You15 are different temporal parts of the same person—the same space-time worm—spread out over time. IDENTITY Time traveller Time  IDENTITY But what if you don’t think we are space-time worms made up of temporal parts? What if you think that You25 and You15 are identical (the very same person), not just two different temporal parts of a stretched-out person? The idea might be that there is just one person present—You—in two different places and at two different stages of life. That leaves us with the problem involving Leibniz’s Law: how can You25 and You15 be identical if one is wise and the other is not wise?  IDENTITY One option is to relativize your properties to places. We could say: Place A You25 is wise-at-place-A and You15 is not wise-at-place-B. Place B These properties are shared, so there is no violation of Leibniz’s Law: You25 and You15 are both wise-at-place-A and You25 and You15 are both not wise-at-place-B.  IDENTITY Here’s a simpler way to say the same thing. You25 = You15 = You! Place A You are 25 and wise at place A, and Place B you are 15 and foolish at place B.  IDENTITY So, to handle identity in cases of time travel, we need to: EITHER Accept the complicated view of objects as space-time worms made up of temporal parts OR Accept a complicated view of properties as relativized to places.  SUMMARY Causal loops Time travel allows causal loops in which things come out of nowhere Does this make time travel impossible? Time Travel and Identity Leibniz’s Law: if x=y, then any property x has, y must have too When you go back in time and meet your earlier self, how can you be identical with your earlier self when you have different properties (like being wise and being foolish)? Lewis’s temporal parts theory Properties relativized to places  QUESTIONS Describe one of the stories involving a causal loop in your own words and explain why it is puzzling. What is Lewis’s response to causal loops? Do you agree with him? Why or why not? What is the problem of identity and Leibniz’s Law for time travel? Explain the two solutions mentioned. Which do you think is better and why?