Wolfgang Schwarz

Predictable Time Travelers

Albert is a time traveler. In 2015 he travels back to 1995. There he meets his younger self and tells him in great detail what he, the younger Albert, will do in the next 20 years: that he will quit smoking, be injured in a traffic accident at a certain date and location, that he will work very hard in a physics lab to build a time machine, and so on. All these predications come true.

Isn't that puzzling? For example, on the day of the predicted traffic accident, why did Albert, who knew about the prediction, not avoid getting to that particular location? Why does he always behave exactly as he was predicted to do? This is certainly not what ordinary people would do. If you claimed to know that I will raise my left hand in a minute and told me so, I would try not to raise my left hand. Does Albert never try to make the predictions false? Or does he, but always fails? That seems unbelievable. How can you try not to work hard in a physics lab but fail? In fact, we may assume that Albert is told by his older self that he will never even try to make the predictions false. Then he never tries and fails because he just never tries. How strange. And how stupid: Albert knows since 1995 that he will eventually travel back in time with a time machine. For he has already met his older self. So why does he work hard at the lab? Why not lie in bed and watch TV instead? No matter what you do, you can't change the past. So no matter what Albert does in 2003, he can't change the fact that in 1995, he arrived as a time traveler from the future. So he's a fool when he's working hard to make it happen (or rather, to make it have happened).

Well, the story was told in a misleading way. Here is another way to tell it.

In 1995, a strange machine with a man inside appeared somewhere on a field. Albert met this man, and listened to what he said. He claimed that he came from the future, and that he was in fact Albert's future self. He also told him in great detail what his life will be like in the next 20 years.

From here, the story could unfold in one of these ways:

1. It turns out that the other guy isn't really Albert's future self, and his predictions don't (all) come true.
2. The guy isn't Albert's future self, but his predictions are true nevertheless.
3. The guy is Albert's future self, but his predictions are false.
4. The guy is Albert's future self, and his predictions are true.

In the previous version of the story, I told you right at the beginning that (4) is what happened. But now that I'm telling you the story in chronological order, we don't know that yet. And more importantly, Albert doesn't know. For all he knows, any of (1) through (4) could be the case. So when he lies in bed contemplating not to work at all on building a time machine, he might argue as follows:

I really do want to become a time traveler. So I want either (3) or (4) to be real. If all the guy's predictions come true, (1) and (3) are ruled out, leaving (2) and (4). But (2) is very improbable: If that guy is not a time traveler then how could he possibly know what will happen in the next 20 years? So if the predictions come true, (4) is the most probable possibility. Which would be great. If on the other hand the predications turn out false, (2) and (4) are ruled out, leaving (1) and (3). But (3) is not really much more plausible than (1). So, to sum up, if I want to be a time traveler, I want the guy's predictions to be true. So I better try not to violate them.

And that's what he does. He never tries to falsify the predictions. He quits smoking, doesn't try to avoid the accident, and works hard at the lab.

But all the time, Albert is of course free to choose. After 15 years of following the predictions, he could very well be fed up with being so predictable and spontaneously do something unpredicted. What would then happen? Nothing special. The story of course can't be the same as the original story. Maybe Albert would still become a time traveler in 2015 and meet his younger self. But then he won't tell his younger self correctly what he is going to do. Maybe he intentionally lies, or maybe he misremembers what he actually did. Albert could also try to kill himself in 2005, and nothing would prevent his death. If he dies, the story again simply isn't the same as the original story. It is rather a story of kind (1).

Incidentally, I think that something like Albert's reasoning above can be used to justify research on time machines. Sure, if there were people in 1935 stepping out of strange machines, nothing we can do will change this. And if there were no such people, we can't change that either. Indeed, if we are absolutely sure that there are no such people, we can be sure that we won't succeed in traveling back to 1935. But we can't be quite sure about this. And if there were such people, we have to do research on time machines to make it happen that these people are later stages of ourselves, rather than swampman-like oddities of physics.