You'll recall that early on in our little adventure in time here, I made the point that temporal paradoxes were, by definition, impossible. I concluded that if a concept of time travel could not be conceived that didn't run into these paradoxes, then you would be forced to conclude that time travel itself is impossible. Well, the inverse of this is also true. If someone were to suddenly appear out of the blue and toss you the keys to a time machine, then you would be forced to conclude that temporal paradoxes are apparently not a problem. You may not be able to wrap your mind around how that could be possible, but nevertheless, you would be holding the proof in your hands. The constant factor is that paradoxes can not happen in reality. At this point, the simple solution would be to declare time travel impossible. Being presented with the reality of time machine would force you to cast about for another explanation, but the principle would remain the same. Well, it appears that there's a very old and familiar phenomenon that raises the issue of a temporal paradox, and you don't even have to bring a time machine into the equation. All you need is a call to a psychic hot line, and the power to believe. Whooo Ooooo.
The paradox in question is called the predestination paradox. I mentioned briefly, while discussing the ontological paradox, that the two were close cousins. In fact, many of these paradox scenarios involve a combination of the two. The ontological paradox is where a thing causes itself to exist, whether that thing is a physical object or merely a piece of information. The predestination paradox is where an event causes itself. The ontological paradox is like a domino coming into existence out of nowhere, while the predestination paradox is like a domino knocking over the same ring of dominoes that knocked it over in the first place. It's a bit more subtle and harder to see, because there isn't a tangible object to focus the imagination on like there is with the ontological paradox. But once you see it, it's a lot easier to understand how one could happen in a time travel scenario.
Let's say, for instance, that when you were little, your grandfather told you a story about a time traveler that appeared out of thin air in his yard one day when he was a kid. As you grew older, you doubted the literal truth of this story, but it still stuck with you and held a special place in your imagination. It inspired you to dedicate your life to discovering how to travel in time. Finally, one day, you succeed. You travel back to the day that grandfather told you about, hoping to shake hands with the fellow time traveler that inspired you, only to arrive and find no one else there. It's just you and little grandpa. Then it dawns on you. You were the time traveler that appeared to your grandfather that day. But...then that means that the event that inspired you to travel in time was caused by the event that inspired you to travel in time. You begin to see how cause and effect become like a run of dominoes looping back on themselves? Like the pocket watch, the event is self-generated.
Like I said, it's a little harder to see. Try this one: Let's say that your dog runs out and gets hit by a car. You're very upset about this, but you just happened to have a time machine and you believe it grants you ability to change the past. You believe that you can go back and save your dog. So you hop in your time machine and travel back to that fateful day. You speed to the scene, hoping to get there before the accident happens again. You remember the exact time. You look down at your watch. Oh no, it's only seconds away! You look up just in time to see the dog run out in front of your car. It's too late. Again, you have the event causing itself. Your hitting the dog is what makes you go back and hit the dog. (Sorry about that. Feel free to take a few seconds to go pet your dog if you have one.)
In most of these predestination scenarios there usually seems to be an element of ignorance and good intentions gone awry. You don't know that trying to save the dog is what actually resulted in its death. You don't know that your grandfather was talking about you. And so on. Only in completing the circle of events does the twisting turn of fate dawn on you, dun dun dunnnn! It might be easy to conclude that the predestination paradox is limited to such cases. The problem is actually far more insidious. You'll recall that I said that the grandfather paradox could apply to any successful attempt to change the time line. Well, the predestination paradox could possibly apply to any interaction with the past whatsoever.
Causality is a funny thing. Every event from the Big Bang up until this present moment is intimately interconnected. Everything that happens is caused by something else that happens which in turn was caused by something before that. It's a vast and intricate network of interlocking chains. It's too big and complex for any one person to see how it all fits together. Once you grant the premise that a time traveler was always present in the past, that things always happened the way they did, then any action that the time traveler takes, no matter how small, becomes a pre-existing part of this network. There's really no way of telling how their actions might have affected the intricate chain of events that led them to be there to be there to affect the chain of events.
Suppose you travel back to 1955. You're walking down a street and you briefly brush against a woman's shoulder. She stops and looks back, you apologize, and she continues on her way. It's a small, momentary incident, lasting only seconds. But that brief delay causes the woman to be a few seconds late getting home, and she misses the woman her husband has been having an affair with leaving the house. Their marriage endures, instead of ending that day. The husband keeps his assets instead of dividing them in the divorce. He has those assets invested in stocks that increase in value. He leaves a fortune to his grandson who starts his own company. The company funds various research projects, including your ground-breaking time travel device. So again, you caused the chain of events that caused you to be there to cause those chain of events. And that's an easy one. There's always a chance that the traveler's actions could somehow be connected to what eventually leads him there, no matter how long and complicated the chain. Once you move cause and effect out of its chronological place in the network, the problem is nearly inevitable.
So where does this psychic hot line come into all this? Well, so far we've only discussed ways that a traveler returning to the past can create a self-caused event. The problem can also work in the other direction. In fact, the name "predestination paradox" implies a future destiny that fulfills itself. If you learn of a future event and you knowledge of that event is what causes it to happen, then you're faced with a predestination paradox. It's an identical situation. Let's say that instead of owning a time machine, a psychic tells you that at 3:15 tomorrow your dog is going to get hit by a car. So, tomorrow you leave work early, and rush home trying to stop this tragedy from happening. You're almost home. You look at your watch. It's almost 3:15. You look up just in time to see the dog run out in front of your car. It's the exact same thing, except this time the psychic is the time machine, receiving information from the future. (Go pet the dog again.)
You've probably heard of the term "self-fulfilling prophesy". Taken literally, this is basically the same thing as the predestination paradox. Fictional accounts of this particular time paradox are far older than The Terminator or 12 Monkeys; they go all the way back to Ancient Greece. In the famous story of Oedipus, Oedipus is warned by an oracle that he will one day kill his father and marry his mother. Horrified by this idea, he flees home and gets as far away from his parents as he can. Well, it turns out that those weren't his real parents. They live in the kingdom that he flees do. He kills a man on the road, his father. Then he marries his widow, which turns out to be his mother. Rather than post their escapades on the internet, he stabs his eyes out instead. (It was a different time.) So the oracles prophesy is what causes the very thing that the oracle prophesied. Predestination paradox.
So, you see, this issue of paradoxes don't just raise the question of whether time travel is possible. They also raise the question of whether any kind intuition of the future is even possible. This recent study seems to suggest that it may be possible. So where does that leave us? What do you think? What do you believe?
(This post is also available in extra cheesy version.)
(This post is also available in extra cheesy version.)
