# Puzzles and Paradoxes of Time Travel

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Puzzles and Paradoxes of Time Travel

Conceptions of Time 1) Presentism Only the present exists. The past and the future do not exist. 2) Eternalism Time is a fourth dimension. All past, present and future times always exist, but we can only perceive the present. The universe is a solid 4D block which never changes, so that all points in time exist eternally. We travel along one dimension of time and we can only perceive the present. Analogy: you are travelling on a train. The world seems to change all around you, but actually the world is constant and you are moving through it. When you move through time, you have the illusion that things are changing.

Time travel to the future
Not paradoxical. We all travel through time at the rate of 1 second per second. Time travel to the future just means travelling through time at a faster rate. Possible according to relativity. If you travel at near light speed, time slows down for you relative to other people. Subjective experience of time travel to the future possible through hibernation or suspended animation. Not really time travel though.

Time travel to the past Only possible if the past exists to travel to, hence eternalism. To travel in time is to travel along the time dimension. Problems: Grandfather paradox, causal loop, incompatible with free will.

What if you kill your grandfather before your father is conceived? What if you killed yourself? Grandfather particle: what if you shoot a particle through a time machine such that it destroys itself before it enters the machine? Butterfly effect: any change in the past would result in countless changes in the present.

Causal Loops If something causes itself, the thing has no origin. This is called “a causal loop”. E.g. gold watch in “Somewhere in Time”. The cause doesn’t have to be direct. If A causes B and B causes A Then, A causes A Can be an object or information. E.g. a man receives plans for building a time machine from his future self. In the future, he sends the same information back to his past self. No one invented the time machine.

Causal Loops (cont.) Ultimately everything is a causal loop:
If the present is caused by the totality of the state of the universe sometime in the past, and some moment in the past is caused by my action in the present of travelling to the past, then everything that happens from then on is a causal loop. In other words, with time travel, some things in the present cause some things in the past. But all the things in the past together cause all the things in the present.

Coherent Time Travel Novato self-consistency principle:
You can’t change the past because whatever you did in the past, you’ve already done. Those actions are already part of what has made this present. Whatever happened, happened. A solution to the grandfather paradox. Problems: What stops the particle from destroying itself, or stops you killing yourself in the past? Somehow, the universe has to step in, making the particle miss or making your gun malfunction. Seems to require some weird, extra laws of physics. Causal loops still a problem (e.g. gold watch)

Time Travel and Free Will
According to the Novato self-consistency principle, you can only do what you’ve already done. More of a problem for free will than mere determinism, because: Under normal determinism, your actions can be the result of your conscious decisions (i.e. compatibilism). However, if you travel to the past, no matter how much you consciously want do something different, you can’t. E.g. you meet your past self. As your past self, you remember your future self saying, “Hi! I’m your future self”. Now, as your future self, you must say those words, even if you don’t want to (because you want to prove Novato wrong). What makes you say what you don’t want to say?

Other conceptions of time travel
1) Travel through existing alternative universes If universe A and universe B are identical up to time T1, but in universe A, I at time T2 travel to universe B at T1, then universe B differs from universe A after T1. Solves paradoxes You can change the past without changing your own past Problem: not technically time travel

2) Constant creation of new alternative universes
Can travel to your own past, but changing your original past causes a new alternative universe to branch out. Based on Hugh Everett’s many worlds interpretation of quantum physics: every time a quantum particle makes a undetermined move, one universe branches into two. Some people think that all free (undetermined) decisions we make cause the universe to branch into two alternative universes Does this save free will? No, because we are forced to choose both undetermined choices and to branch into two alternative selves. Problems: unbelievably complicated

Suggested Readings Davies,Paul (2002), How to Build a Time Machine, Chapter 4: “How to make sense of it all”, available on moodle (entire book also on reserve) Law, Stephen (2003), The Philosophy Files, Chapter 5: “Is time travel possible?”, available on moodle