Dmitry Chernikov’s Blog

Consider a person, Tim, who travels into the past and tries to kill his grandfather. David Lewis argues (Philosophy of Time, 134) that he can kill his grandfather, in the sense of “can” being “compossible with a narrow selection of facts.” Tim’s got a sniper rifle, he trained himself to use it, he studied his grandfather’s daily walking routes, he has positioned himself to get a clear view of grandfather, there is no wind, etc. And yet he can’t kill his grandfather, because the future is already determined. His grandfather will definitely survive. So the meaning of “can’t” is that Tim’s killing his grandfather is incompatible with a wider selection of facts, namely, the facts about the future course of events.

But think about what that would mean. If Tim happens to know what occurred independently of his actions, then he is bound to recreate the future that, from his point of view, has already happened and is perfectly fixed. Suppose now that he knows his behavior in the past, e.g., from future (his present) police records. It seems that he has no choice but to engage in this very behavior, including think the same thoughts, etc. Normally, we say that a person comes to know how he acts, because he chooses his actions. If he had chosen differently, he would have known a different thing. But not in this case. If the past already incorporates any time traveler’s actions, then Tim no longer has any choice. He must robotically repeat the actions recorded in the time from the past to which he traveled to the time from which he departed. Also, since he knows that his grandfather lived, it becomes futile for him to try to kill him. But giving in to fatalism may cause him to fail even to try. But what if history records that he did shoot yet missed?

In fact, Tim cannot even choose not to go into the past, if, e.g., he remembers his younger self meeting his older self! (It seems to be sophistry in this particular case to say that Tim can choose not to go into the past but will not.) Suppose he “chooses” to go out of a sense of responsibility, simply in order not to allow the paradox that would otherwise have been created to destroy the universe. But that will contradict the fact that when his younger self asked his older self why he traveled into the past, the latter truthfully explained that the reason was to test his time machine, that is, something completely different.

So, if Tim knows that if he shoots, he will miss and then be arrested, then again, he may refrain from shooting altogether, thus altering the past. And this Lewis says is impossible. We may argue further that it is then impossible for a time traveler to know the past. He must by necessity be ignorant about the events between his temporal destination and his present. Or if he tries to learn, he will be mislead. And if he does find out, then something will necessarily prevent him from going into the past, e.g., some kind of malfunction in the time machine. But that conclusion seems preposterous. Surely, for some time traveler, it possible to know the pertinent facts about the past.

Moreover: from the point of view of the past to which Tim arrives, the future is still uncertain and unmade. Yet he can inform someone about what will definitely happen in the future, thus ripping out free will not only from himself but from other people, as well. Or we must presuppose that whenever he tries to tell someone what that person will do, he is prevented by something from doing so. But that again elevates accidental happenings to an essential law, namely, that it is impossible to tell a person how his life will necessarily unfold. And if Tim knows that he will tell John about the future, and John reacts accordingly, then this fact must be incorporated into the time period between the Tim’s present and his past. But again, must Tim necessarily discuss the future with John? Must John react in the way he did to that information? No matter how deep you go, fatalism cannot be escaped.

Yet another problem is that a time loop will be repeated an infinite number of times, resulting in paradoxical situations. Thus, when Tim goes into the past the first time, he will, we can suppose, meet his younger self. When his younger self grows up and goes into the past for the “second” time, he will meet not only himself, but his first older self. During the third trip he will meet three of himself. So, how many selves will he meet altogether when he goes into the past? (It may be objected that the person will become the same older self every time he jumps into the past. But suppose that every time an uncorrectable error in the targeting subsystem of the time machine causes him to travel one second earlier every time and appear in a slightly different place. Will he not meet his previous older selves then?)

An additional complication is, where in space will the time traveler end up? If I travel back 2 seconds, will I end up in the region of space already occupied by my 2-seconds-younger body? That surely could not be pleasant. Similarly, by what mechanism will my body push out the air molecules out of the way as I materialize in the past?

Finally, a time loop will be forever, if I may use that word, relived, forever actual. At t₂ Tim travels into the past, t₁, living it, seeing the sights, sensing the smells, etc. So, there will seemingly be two timelines and two presents, as it were, the present that proceeds from t₂ and will be experienced within the personal timelines of Tim’s contemporaries, and the present that will proceed from t₁ and will be experienced within the personal timeline of Tim. As time approaches t₂, his younger self becomes older and gets the schematics of a time machine and a desire to travel into the past. At t₂ he again travels into the past, again reliving it from his older self’s point of view. Again he sees his younger self alive and real and struggling. He watches him grow up and obtain a time machine at t₂ and travel back to t₁. And so on ad infinitum. So, the time loop will go on forever and may even prevent any of the times future of t₂ from coming into being.

Here we assume that time traveling does not result in branching of the future. If it does, then time traveling may be possible. However, it would create multiple actual worlds. And this seems to be even more outrageous than the difficulties described above.