Time Travelling

Time travel is one of the most attractive things which may happen not only in science fiction novels or films but also may take place in reality according to David Lewis, a prominent American philosopher of the 20th Century. His daring mind led him to one of the most logically consistent ideas of his generation. “Time travel, I maintain, is possible. The paradox of time travel are oddities, not impossibilities”.

To explain David Lewis’s view of casual loops we should touch upon such a notion as Causal Reversals that leads to the possibility of Causal Loops.

According to Dr.Alasdair Richmond’s video lectures on coursera.org, there are two types of time: external and personal. Causal reversals become possible when the orders of these kinds of time disagree. It means that in cases of travelling back in time, personal time continues to progress from earlier stages to later ones while external time progresses in its own direction as well. Thus, we may travel back in time and show, for example, the young Wright brothers drafts of the first aeroplanes, which leads to an impressive scientific breakthrough. Moreover, we may travel back to our own past and change something that alters our future completely. 

Suppose, as a child you are visited by some stranger who gives you a key from some safe and asks you to keep it till you have a chance to use it. Following the instructions, you keep the key for many years and as an adult you get into a difficult situation so that you must save the key from some criminals, thus you come to conclusion that you cannot trust anyone except yourself. Accordinally, you travel back in time and give the key to yourself as a child. However, where did the key originally come from?

In both cases, a future person with his bag of knowledge and devices obviously causes some changes in the past. Now let’s try to explain Lewis’s view of casual loops.

What are causal loops? As David Lewis claims, causal loops represent closed causal chains in which some causative links seem to be natural, as they follow from earlier states to later ones, while some links are reversed. “Each event on the loop has a causal explanation, being caused by events elsewhere on the loop”. Thus, if we return to our examples, the only reason why the Wright brothers accomplished their first successful flight was our visit, the visit of future people. How then did the idea of a flying machine come from? Why did it occur to the Wright brothers? The future people tell the brothers their own idea which was originated in the past of those future people. That is the causal loop. How can it possibly exist? The answer for David Lewis is that there is no answer. We should admit that a loop as a whole is not explicable in spite of the fact that the parts are. That seems out of the ordinary, most people would find it too difficult to accept such an idea. However, as the philosopher David Lewis pointed out “it is not more logically impossible…than it is for God, or for the universe as a whole, to exist uncaused”.

All in all, causal loops seem to be logically explained. The question remains open whether we are able to travel in time in practice, not just to explain the fact theoretically. As David Lewis once said “A possible world where time travel took place would be a most strange world, different in fundamental ways from the world we think is ours.”

Recources:

THE PARADOXES OF TIME TRAVEL by David Lewis. Princeton University, received September 4, 1975