8 - 4 - Part four - Causal loops

Okay, so, so far we've covered Lewis' account of what time travel is. We've covered his attempt to diffusing the grandfather paradoxes. And we've covered Lewis' reasons for thinking that a time traveler could genuinely be said to have an impact on the past. Well there's another class of examples, familiar from fiction, which don't involve paradoxes, but nonetheless seem to pose a problem for intelligibility of backward time travel, and these are cases of so-called causal loops. A causal loop, for our purposes, is a chain of events that loops back in time, so that an event turns out to be among its own causes. And there have been some very ingenious science fiction stories on these themes. So for example, let's suppose that I get in my time machine, clutching a copy of the complete works of Shakespeare printed in 2012, and I travel back to 1588. And I make contact with the young struggling player Will Shackspear, as he was then calling himself. And I take him to one side and I say, Will, you want to get ahead in this drama business, don't you? Try writing this down. What a piece of work is a man. How noble in reason, how infinite in faculties. In form and moving how express and admirable! And I then give him one of Hamlet's famous speeches. And Shakespeare goes, yes, that's not bad. Is there more where that came from? I said, there's loads more. There's all of this. And I hand to Shakespeare a 2012 copy of his own complete works, and I let him copy them down. So Shakespeare copies down his own works, those copies are transmitted to players, to theater goers, to printers, all over the world, they're popular, they become part of the history of the world. And they are transmitted to a printing works in 2012, whence is derived the copy that I take back to 1588. So Shakespeare copies the plays from a 2012 printing of his works. The 2012 printing exists because of Shakespeare's act of copying in 1588. But who writes Hamlet? Where does the information come from? Where do all these beautifully poised bounced, intricately wrought speeches and scenes and characters come from? Where is the information generated? Lewis takes perhaps a simpler example. Lewis says, imagine that you are home one evening. And the phone rings and an oddly familiar voice says, don't say a word, write these instructions down and follow them to the letter. And the voice proceeds to recite, some instructions for how to construct an operator time machine, and you follow the instructions and you discover the time machine has deposited you in the recent past. And once in the recent past, you call your own phone number. An oddly familiar voice says hello. And you say into the phone, don't say a word, write these instructions down, and follow them to the letter. And you proceed to give your younger self the instructions that you remember, for how to build and operate a time machine. So the question is, given that this doesn't seem to be inconsistent, there's no paradox, there's no event being replaced, there's no live and dead grandfather problem. There's still something very strange about a causal loop. When you think, well, how does my older self know how to build the machine? Well, the older self knows how to build the machine because the older self remembers hearing information as the younger self, and the younger self remembers because, he was told by my older self. But where does the information come from, where's the entry point for the information into the loop? And Lewis says, Well there just isn't an entry point. In the case of the time machine, there is no answer to where the information comes from. In the Shakespeare case, there's no answer to the question to who wrote Hamlet, Hamlet wasn't written. Hamlet merely exists. Now this seems very counter-intuitive. It seems a good principle that knowledge and information are created through normal causal processes. It would be very strange indeed to stumble across a fully-formed, highly-informative text or machine, that has no causal origin. And Lewis says, yes, it would be very strange. But that's not to say that it's impossible. So Lewis' answer to the problem of causal loops is to say this. A causal loop is very strange. To see information apparently springing into existence from nothing, is very counter-intuitive. But, says Lewis, where does information come from in any case? Lewis says, it's one thing to ask, where an event comes from. It's another thing to ask when an entire chain of events comes from. It can be a good, useful, well formed question to say well why did this event occur? I mean suppose you wanted to know how you came to be born. You might appeal to earlier events, facts about you parents, how they met, facts about your grandparents, facts about human evolution. Facts about how the solar system evolved. Facts about how the universe evolved. So you might be able to trace the chain of causes and effects back and back, and back, and back, and back arbitrarily far. Loomis says there are only three possible chains of events. And in each case, the question where does the information come from is just as pressing. Suppose that the chain of causes and effects stretches back infinitely. Every event has a cause, which has a cause, which has a cause, which has a cause, forever. So for each event, you can appeal to an earlier event, and an earlier event still, and an earlier event still, back, and back, and back, but there's no end to the chain. And so there's no answer to the question, where does the whole chain come from? The chain itself has no origin, has no end point. So that's an infinite linear chain. Suppose on the other hand, and this is a prospect of many physicists take very seriously, there are causal chains that just appear from nowhere. Quantum physics, big *** cosmology takes pu-, takes very seriously the idea that the laws of physics allow events to occur without any prior causes. The Big ***, for example. Standard Big *** cosmology says that the Big *** isn't the first event in time, it's the beginning of time. As Stephen Hawking memorably put it, asking what's before the Big ***, what's earlier than the Big ***, is like asking what's to the north of the North Pole. There's just no good answer. The question doesn't make sense. The Big *** is governed by physical laws, but there are no earlier events. So if Big *** cosmology is true, every causal chain in the universe is linear, but finite. There comes a point beyond which you simply can't appeal to earlier events. So infinite linear chains and finite linear chains, both pose the same problems, where does the information come from? In the Hamlet case, and the telephone case, we've got a finite, non-linear chain, events that prove to be among their own causes. But again, the question where the whole chain comes from, it's not clear that there's a good answer to it. So Lewis says yes, causal loops, events that are among their own causes, information seemingly generated from nothing, very, very counter-intuitive. But a causal loop is no more or less problematic than any other kind of chain. The options are, infinite linear, finite linear or finite non-linear and in each case there maybe a good explanation why each event occurred, but there's no explanation for the chain of events as a whole.