Paradoxes of Time Travel

David K. Lewis American philosopher (1941- 2001). Student of Quine. Important work in the foundations of logic, modal logic, probability theory, game theory, philosophy of language and mind, and metaphysics

Basic Discrepancies I get in my time machine in 2017. I travel for one year. I arrive in 1017. 2017 + 1 = 1017?

Personal Time Wristwatch time ≠ personal time. Aggregate – and not really time.

The Grandfather Paradox

Agustin Rayo Philosopher at MIT. Works on philosophy of logic and language.

Augustin Rayo https://www.youtube.com/watch?v=M8oITAoaCr4

Grandfather Paradox: Three Possibilities Time travel is impossible Time travel is possible, and normal rules apply  Grandfather paradox Time travel is possible, but special rules apply  banana peel paradox

Possibility #1: Time Travel is Impossible Can philosophy decide such things? (cf. Infinite chain of causes.) Physics says it is possible.

Possibility #2: Grandfather Paradox Accepted Possible solution: branching timelines https://www.youtube.com/watch?v=Y6RjjaEy59I

Possibility #3: Banana Peels https://www.youtube.com/watch ?v=YZRq3XxCZXo

Argument Someone who has prepared carefully can kill their grandfather. Bruno cannot kill his grandfather, though he has prepared carefully.  Contradiction?

Lewis on Bananas What I can do depends on an assumption of background facts.

Lewis on Bananas A gibbon cannot speak Finnish. BACKGROUND FACTS: The vocal tract/ toungue/ lips/ etc. of a gibbon are incapable of making the sounds of the Finnish language.

Lewis on Bananas David Lewis can speak Finnish. BACKGROUND FACTS: The vocal tract/ toungue/ lips/ etc. of David Lewis are capable of making the sounds of the Finnish language.

Lewis on Bananas David Lewis can’t speak Finnish. BACKGROUND FACTS: Lewis knows no Finnish words or grammar.

Aside: Fatalism

Theological Fatalism Suppose God knows that you will eat a hotdog for breakfast tomorrow. Then since what is known is true, you will eat a hotdog for breakfast tomorrow. Therefore, you *must* eat a hotdog for breakfast tomorrow.

Lewis on Bananas What I can do depends on an assumption of background facts. If we assume BF: I have a range of options at the canteen; I’m capable of saying “I want a hotdog” and also “I don’t want a hotdog, I want….” – then I can fail to have a hotdog for breakfast.

Lewis on Bananas What I can do depends on an assumption of background facts. If we assume BF: I have a range of options at the canteen… but we also assume that I do in fact order a hotdog… then I can’t logically do otherwise.

Lewis on Bananas For Lewis, it is equally true that we can do what God knows we will not do, and that we can’t do it. It just depends on the background facts we are assuming.

Back to: Time Travel

Lewis on Bananas Worry: it seems Bruno both can and can’t kill his grandfather.