The question of whether we can logically affect the past can be addressed in two parts. Firstly, is it possible to cause an event that has already happened? If backward causation is logically possible we may be able to perform actions which will have an effect in the past. Secondly, does the process of time travel entail any insurmountable contradictions? If so, then it may be possible for a time traveler to affect the past without the need for backward causation. I wish to argue that whilst it is logically impossible for us to change the past in any way, we may affect it without entailing any serious contradictions.

The contradiction entailed in backward causation is clear. Our first claim is that action B is the cause of event A. Secondly, as intelligent agents we are free to choose whether to perform action B or not. And finally, when we choose whether to perform action B, event A has already occurred. Therefore event A has occurred whether or not we decide to perform action B. B therefore cannot be the cause of A, as in order to avoid a contradiction, at least one of our three precepts must be false. When formulated in these terms, the logical impossibility of backward causation is inescapable.

Dummett1 discusses a different way to affect the past without direct backward causation. The concept of retrospective prayer introduces the possibility of communication with an intelligent agent who can act at some time in the past. Because God is omniscient, He will know that at some time in the future, a believer will perform action B (prayer), and can therefore act before the occurrence of A (the death of a relative), in order to prevent it. This formulation is just as unsatisfactory as the previous one, however.

All we have achieved is an amendment the above formulation: action B (prayer) is now the cause of event C (God responding to the prayer), and C is the cause of A. We are unable to escape from the same contradiction, unless one of the precepts is false: Dummett’s example is that the supplicant may not know whether their relative is dead, and are therefore praying for something which they have as little knowledge of as the future. Assuming this precept is false, it may become logically possible to affect the past.

We perform action B in order to produce the effect A, which occurred in the past but we have no knowledge of. If we subsequently discover that the outcome of A was favorable, it entails no contradiction to claim that our action has affected the past. We can now make an important distinction between affecting the past and changing it. According to the precepts of our original formulation, the latter is logically impossible, but assuming our ability to communicate with an intelligent actor in the past, we may be able to affect past events. It may be argued that our actions are not actually affecting the past.

Although A occurred as we intended, we have no way to prove that B was the cause. A would have happened whether B or -B. The traditional reply to this is to draw parallels with the untenable position of fatalism. 2 This posits that our actions are irrelevant in determining future outcomes; what will be will be regardless of what we do. This is easily refuted when considering future events, as it discards notions of causation and our status as intelligent agents. When applied to past events, however, it is more problematic. The past has already happened, it is fixed.

Events have occurred independently of our knowledge of them. This does not hold true of the future. This only rules out backward causation, however, which we have already dismissed. If we can communicate with some agent in the past, it becomes simple fatalism, and does not prove damaging for the argument that we can affect the past in this way. Another way in which we could potentially affect the past is through time travel. There are no serious logical contradictions involved in time travel, and although several attempts have been made to dismiss it, none are fatal or insurmountable.

One possible problem is that of backward causation; as we have already shown, this is not possible. Time travel requires a journey to end before it has begun, however. It is clear that some form of personal time must exist for the time traveler, independently from external time. Cause continues to follow effect in the personal time of the traveler, in the normal way. He will continue to grow older, for example, although the actual date may be many years before his birth. Reverse causation cannot therefore rule out time travel. More problematical may be the potential existence of causal loops.

Although we have ruled out backward causation, the discrepancy between personal and external time may create something similar. Although cause D precedes effect E in personal time, it is perfectly possible for D to follow E in external time. This could potentially lead to contradictory situations. An individual could give blueprints for his time machine to his younger self, thus facilitating his journey3. Within the loop, this makes perfect sense, but the loop itself has no cause. There are two possible solutions here. The first is to accept that causal loops can exist.

Lewis gives the example that God and the universe have no explicable cause. 4 Alternatively, this problem may be overcome by an examination of whether a time traveler can affect the past. Our original formulation still holds true. Travelling into the past in order to affect it is essentially the same of communicating with some agent in the past with the same intention. It would be illogical to travel into the past with the intention of changing event A that has already occurred. In this situation event A would be the cause of the time travel (event B).

If event A did not occur, event B also did not occur. This contradiction rules out changing the past through time travel, just as comprehensively as it does by any other means. To borrow Lewis’ example, Tim cannot kill his Grandfather. It is still possible for Tim to affect the past, however. To extend the grandfather illustration, we know that Tom intends to kill Grandfather’s partner, but fails. Suppose that the reason that he fails is because of Tim’s earlier failed attempt on Grandfather’s life. Perhaps Tom heard the gunshot and panicked.

It can be clearly seen here that Tom has affected the past, although he has not changed it. This would appear to place constraints on the behaviour of time travelers. This is not necessarily the case, however. A time traveler has as much freedom to act as any agent in the time to which he traveled. Tom could have performed actions that would have led to a different future, but he didn’t. This may also be a plausible explanation of causal loops. A time traveler could have performed actions leading to the formation of a causal loop, they just chose not to.