Home > Philosophy > David Lewis and The Paradoxes of Time-Travel

David Lewis and The Paradoxes of Time-Travel

Although quite unknown among laypeople and even lay philosophers, David Lewis is one of the most prominent philosophers in recent philosophical history. His influence is so widespread in so many philosophical fields that the Stanford Encyclopedia of Philosophy said of him, “It is hard to think of a philosopher since Hume who has contributed so much to so many fields.” His daring and imaginative mind led him to some of the most ingenious, logically consistent, yet undeniably counter-intuitive ideas of his generation. Here is his view on a very popular field of science fiction: time-travel. For Lewis, time-travel is entirely logically possible.

External vs. Personal Time

Lewis makes a distinction between external and personal time. External time is what Lewis calls “time itself” or what one might call the time on a timeline. The time between the year 1900 and the year 2000 is, externally, an interval of one hundred years. On the other hand, personal time is the time that one experiences personally (or the time one feels). Or as Lewis states, it is the time that is recorded on one’s wristwatch. Now, in ordinary cases, both external and personal time are the same. But such is not the case in instances of time-travel. Suppose that I travel one hundred years into the future or into the past. In terms of external time, I will be one hundred years removed from where I once was. But in terms of personal time, perhaps only thirty minutes have passed (my biological clock or my wristwatch have only experienced or recorded thirty minutes and not one hundred years). Such is also the case with instances of time travel and special relativity: if one travels fast enough, one may only experience, say, one month pass despite the fact that ten years have passed on earth.

Causal Reversals and Causal Loops

This distinction allows Lewis to clearly state how causal reversals are possible. Causal reversals become possible when the orders of personal and external time disagree. In other words, in cases of traveling back in time, personal time continues to progress from earlier stages to later ones while external time progresses from later stages to earlier ones. Causal reversals then are ultimately instances when, rather than earlier stages causing later stages, the reverse is true: later stages cause earlier ones. Thus, I may travel back in time and show a young Copernicus that the sun is the center of the solar system, which leads to his proposal of the heliocentric theory. Or perhaps I travel back in time with my contemporary clothes, which leads to a revolution in fashion in, say, the eighteenth century. In both cases, a future person with his future knowledge (or accessories) has caused something to happen in the past.

The possibility of causal reversals leads to the possibility of causal loops. Causal loops, as Lewis puts it, are closed causal chains in which some causal links are normal—they run from earlier states to later ones—while some are reversed. To put it another way, they are instances where the causal origin of a series of events can be equally located both in the past and in the future. Thus, perhaps the only reason why I can show Copernicus that the sun is the center of the solar system is because he proposed such a theory in the past and I am merely regurgitating what he once said. Or, on the contrary, perhaps the only reason he could propose his theory in the first place is because I, a future person, traveled into the past and told him. How then do we explain how the Copernican theory originated? Where did it come from? I was aware of it because Copernicus proposed it long ago; he was aware of it because I told him. The answer for Lewis is that there is no answer. The loop as a whole is not explicable despite the fact that the parts are.

Can One Change the Past: The Case of Tim and His Grandfather?

There is a paradox in discussions on time-travel known as the Grandfather paradox. The paradox asks the question: can a person (call him Tim) travel back in time and kill his grandfather? For the paradox to work, one must assume Tim travels back to a time before his grandfather had any children, namely, Tim’s father. So let’s assume that Tim travels back to his grandfather’s teen years in the 1920’s. Can Tim kill his grandfather? The paradox arises when one considers that it would seem impossible for Tim to kill his grandfather because of the fact that Tim’s existence is entirely contingent upon his grandfather surviving long enough to have children. But it also seems entirely possible for Tim to travel back in time, buy a gun, and shoot his grandfather. After all, if his grandfather is in clear view and Tim has his finger on the trigger, could he not quite easily pull the trigger?

In this situation, Lewis concludes that there is a sense in which Tim both can and cannot kill grandfather. This is because the word “can” equivocates. He can kill grandfather in the sense that there is a set of facts that are all compatible with him getting the job done. He has the motive to kill; he has the right sort of weapon; he has the proper training; his environment is of the proper sort; and so on. He can kill his grandfather just as easily as anyone else can. There is nothing about him being a time traveler that makes his gun or his training incapable of killing someone such as his grandfather.

The problem, however, is that the events of the past—the events that Tim is now witnessing—have already happened. His grandfather has already lived a full life in which he bore children who, in turn, bore further children (including Tim). The fact that Tim is even alive to contemplate killing his grandfather means that Tim cannot kill grandfather. It is logically impossible for him to do so despite the fact that he has what it takes because it is logically impossible to change the past. Thus, although there are various facts that are compatible with Tim killing grandfather, there is a more inclusive set of facts in which killing grandfather is not compatible (namely, that he was not killed).

According to Lewis, something must happen that prevents Tim from killing his grandfather in light of the very fact that his grandfather did not die. Tim gets cold feet, the gun jams, Tim is stopped by a policeman, the bullet hits his grandfather but is not lethal, and so on.

Suppose that the bullet hits grandfather and sends him to the hospital. Further suppose that it is here that grandfather meets his future spouse (she is a nurse). This would, again, be an example of what Lewis calls a causal loop: Tim appears to have set in motion a causal chain that will lead to his own birth. For Lewis, no matter how bizarre this may seem, it is entirely possible.

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