For those of you that don't already know, today is Back to the Future Day: the day Marty visits in the Back to the Future film. In honor of it, I thought I would write a post on time travel.

If the past and God exist, time travel is possible. 

I gave a truthmaker argument that the past exists here.[1] The truthmaker argument says that if statements about the past can be false with the total present state of affairs existing, then the present state of affairs can't serve as a truthmaker for statements about the past. There are, however, true statements about the past. Hence, more than the present moment exists. By the same argument, future states of affairs would also be insufficient truthmakers for statements about the past.

It at least seems that statements about the past can be false with the present state of affairs existing. An explosion, for example, could have been caused by lightning, or by a chemical leakage, or by an electrical short circuit, or any number of other ways.

It may also be metaphysically possible that God—acting through Primary Causation for benevolent reasons—could have replaced or changed even an otherwise incompatible totality of past states of affairs such that it led to a present state of affairs (that's compatible with a different past). If this is metaphysically possible, then statements about the past can be false with the present state of affairs existing.

I think we're also going to have a hard time arguing that, if the past exists, God couldn't at least in principle Act by His Primary Causation to teleport a person to a point in the past (even if oddities may arise from it after the person's arrival there). So if my and others' arguments are successful, time travel is possible.

Here is a paradox from Robin Le Poidevin's Travels in Four Dimensions:

Tim is spending the summer holiday at his grandfather's house in rural Sussex. Bored one day, he wanders into his grandfather's library. On one of the more remote shelves, Tim discovers a dusty book with no title on its spine. Opening it, he sees it is a diary, written in a familiar hand. With a growing sense of wonder he realizes that one of the entries provides detailed instructions on how to build a time machine. Over the next few years, following the instructions to the last detail, Tim builds such a machine. It is finally completed, and he steps on board, and throws a switch. Instantly, he is transported back fifty years. Unfortunately, both the machine and the book are destroyed in the process. Tim writes down everything he can remember in a diary. He cannot rebuild the machine, however, because it requires technology that is not yet available. Reconciled to getting back to the twenty-first century by the traditional method of doing nothing and letting time carry one back, he marries and has a daughter. The family moves to a rambling mansion in rural Sussex. The diary is left to gather dust in the library. Years later, Tim's grandson, spending his summer holidays with his grandfather, discovers the diary.

The identity of Tim will be obvious, and this in itself is rather strange. But the question we are concerned with is this: where did the information on how to build a time machine come from? From the diary, of course, which itself was written by Tim. But where did he get the information from? From the very same diary! So the information has appeared from nowhere. At no stage has someone worked out for themselves how to build a time machine and passed on the information. The existence of this information is therefore utterly mysterious.

Here is another that supposes some of the future also exists:

Peter and Jane, both 20 years old, are out for a walk one day in 1999 when suddenly a time machine appears in front of them. Out steps a strangely familiar character who tells Jane he has an important mission for her. She must step into the machine and travel forward to the year 2019, taking with her a diary that the stranger hands to her. In that diary she must make a record of her trip. Obligingly, she does as she is asked and, on arrival, meets Peter, now aged 40. She tells Peter to travel back to 1999, taking with him the diary she now hands him, and record his trip in it. On arrival in 1999, he meets two 20-year-olds called Peter and Jane, out for a walk, and he tells Jane that he has an important mission for her.

This raises several questions: How many trips are made in total? What happens to Peter and Jane? When they have finished travelling, what are their ages? But the real tricky question is: how many entries are there in the diary when Jane first steps into the machine?

Heinlein's By His Bootstraps is about another. 

Some of you may find this essay by David Lewis useful. Happy Back to the Future Day.


[1]Truthmaking is an asymmetric, cross categorial relation in which the existence of one or more entities necessitates certain truths. One truthmaker can make true more than one proposition, and one proposition can be made true by more than one truthmaker.

Last edited by John West (10/21/2015 3:15 pm)