過去への旅

Time travel is an exciting idea for human beings whose lives are shut up in a short region of time. With the advent of Relativity, it was found possible to travel to the Future. But the travel to the Past is still considered to be impossible because of the causal anomaly brought about by it. The purpose of the present paper is to find our way out of this difficulty, and show the possibility of going back to the Past. Before entering into the main subject, the present writer discusses a question of whether or not itis significant to talk about 'the Travel to the Past.' His argument is as follows. When so-called A-series or B-series is considered as real time representation, time travel becomes meaningless. On the contrary, ordinary time representation which is the mixture of A-series and B-series makes it meaningful. Here the four topics below are analyzed ; (1) Wellsian time travel, (2) reinterpretation of posiron, (3) tachyon, (4) Godel's rotating universe. The analysis of these topics gives us some theoretical ground for the possibility of time travel. But at the same time serious paradoxes arise. The most dramatical one is as follows. Time traveler goes back to the time when he was a child, and shoots his younger self. Then what happens? Three ways for dealing with a situation of this sort will be proposed here; (1) Let us suppose that everything happens in conformity with the rest of the world, past or future. If the time traveler shoots his younger self, then it breaks the consistency with the future events. Therefore it is prohibited physically. This way is also formed on the assumption that at one time only one event can happen at one place. Especially the event at the time when the time traveler arrives has to happan so that his arrival is included there from the first. (2) The second way is that two different events can be caused at the same place at the same time. The first event is without the time traveler, and the second event with his appearance. These events build up parallel worlds. In this case he can kill his younger self without making any contradiction. Because the duplication of the world removes the contradiction. (3) The third way is to consider time as two-dimensional. As the position of the Present moves along the ordinary time dimension, the past instants themselves vary along the other dimension. So the instant when the time traveler comes back is different from the original one. Hence, he can do what he knows did not happen before.