Time Travel; An Essay


For my major essay we had the choice of four questions, and I naturally picked the one on time travel. Not my best essay ever, and if I wasn't already over the word limit I'd expand on it a lot. However, here you go.


“What is the difference between one and two dimensional time travel? Is one dimensional time travel into the past logically possible? Does it involve conceptual impossibilities? Is it committed to a Parmenidean conception of time?"


Time travel is a concept that humans are drawn to, largely because of the perceived impossibilities within it. In this essay I argue that all the perceived problems are valid, but are not arguments against time travel.

One dimensional time travel is that by which actions in the past bring about events so that the future plays out the way that you remember it occurring.

Two dimensional time travel is that which changes the past, taking a possible world and bringing it into existence, creating branching timelines.

They are very distinct, in that one dimensional time travel does not involve disrupting the history as the time traveler knows it, whereas two dimensional time travel ‘creates’ a new alternate reality/timestream which can range anywhere from being almost identical to bearing little resemblance to the time traveler’s remembered reality.

In this respect, two dimensional time travel is less time travel in the traditional respect, and more space travel, and so this essay shall deal henceforth with one dimensional time travel.

I believe that one dimensional time travel is logically possible, because of the fact that there are no contradictions in our explanations of time travel, and because we can conceive of a coherent world where some of the people we interact with are time travelers from the future, and where we might in future go back to the past and interact there.

However, at first glance there appear to be a number of conceptual impossibilities with one dimensional time travel, and here I will discuss and resolve these, explaining why they are not, as they first appear, an argument against time travel.

Conceptual impossibilities within time travel range from those concerning personal identities to causal loops to the Grandfather Paradox, and I’ll present them as such.

As David Lewis talks about in his “The Paradoxes of Time Travel[1], there are a few concepts with personal identity that can be hard to grasp; in particular the idea of having two of the same person in the one place. The problem there arises because of the lack of clarity surrounding personal identity.

It’s hard enough to come to terms with how we know a person, let us say Mary, is the same person when she wakes up each day as when she went to sleep. Adding time travel into the equation only increases the complexity, and hence the vanishing/appearance of Mary at different points in time – how do we know Mary in 1992 is the same person as Mary in 1301?

At this point it seems sensible to define what we mean by ‘person-stage’; the state of a person at a particular point in their personal time.

E.g. Joe at 5 years is a different ‘person-stage’ to Joe at 10 years.

Assuming there is a causal dependency between them (i.e. something that links the two Mary’s together, time travel, for instance) for us to consider this, the question becomes that of personal identity, and as Lewis concludes[1] can be resolved with the memory theory. Mary in 1992 is a person-stage, and Mary in 1301 is a slightly longer person-stage, but as long as Mary in 1301 remembers being Mary in 1992, they are both person-stages of Mary.

Here two more definitions seem in order:
‘Personal time’ here refers to an individual’s time line, the order in which they experience events from birth to death.
‘External time’ here refers to an absolute time line, independent of observers, where events happen at a rate of 1 second per second in a continuous order.

All of this does not seem too hard to grasp, it is only when Mary travels back again to 1992, before she left and holds a conversation with her earlier-self that it becomes complicated. However, as shown above, Current-Mary is merely a continuation of Earlier-Mary, at a different time within Mary’s personal time line. Hence the two of them conversing occurs when Current-Mary and Earlier-Mary meet at the same time and place within in external time, and as it is not the case that they are the same person, it causes no paradoxes of identity of one person existing in two places.

Many perceived paradoxes boil down to questions of cause and effect, split into seeming cause-effect relationships that violate the need for cause to precede effect, and causal loops.

When the effect seems to precede the cause in external time, the situation is simply resolved by referring to personal time. As causes and effects for Mary must happen in that order, so they must happen that way in her personal time line. However if my personal time line involves travelling backwards in external time then there will naturally appear to be a discordance between the apparent priority in the cause-effect relationship.

It is only when the effect and the cause are precisely linked (by which I mean the effect becomes the source of the cause which causes the effect and so on) that a causal loop seems to have been created. For example:

Say that I am digging a hole in my back garden, when I discover a glass slipper and a note that dates it as being placed there by none other than myself 30 years ago. I invent a time machine, and travel 30 years into the past to bury the shoe in the place I find it.

This scenario begs the question, “where did the shoe come from?”

Lewis’ solution to this is to compare the inexplicable nature of the loop to the Big Bang Theory, and leave it at that[1]. Whilst this does validly point out existing flaws within our understanding of the universe, it does not, in my mind, provide an actual resolution to this seeming paradox.

My resolution to this would be to argue through the Novikov Self-Consistency Principle[3] that this situation will never occur.

The Novikov Self-Consistency Principle states that the Universe’s defense mechanism to any event likely to create a paradox is to assign the occurrence of the event a probability of zero.
I would hence argue that although causal loops are a conceptual impossibility, insofar as we cannot conceive of an explanation for a situation like the above ever happening because it never could happen.

The Grandfather Paradox is a more commonly known paradox, and, focusing purely on one dimensional time travel, I claim it isn’t actually a paradox. I claim this because in a one dimensional view it is not something that can ever occur, and I also claim that this has no affect on free will either.

It is not strange to accept that one dimensional time travel means the past cannot be changed. It is strange however for there to be actions you cannot perform. To my mind, that’s not a correct interpretation of the idea, as it conjures images of supernatural forces physically preventing you from acting, whereas the reality is much more straightforward; even as I in my own time can attempt actions without a guaranteed outcome, so too may a time traveler in the past attempt an action, certain of a favoured outcome, and be disappointed with the result.

This relates directly to the Grandfather Paradox in that the mere fact that you cannot succeed in a goal does not mean that you cannot attempt to. Just as it is possible for me to point a gun at someone, shoot it, and have them die, it is possible for me to point a gun at someone, shoot it, and not have them die.

A fixed outcome does not change the freedom of will or action behind trying, and in the present scenario the question of it doing so would not be even considered, let alone that of it being a paradox. Hence given their parallel nature, I claim the same for the past scenario.

All of these paradoxes, and the very nature of one dimensional time travel, require a fixed, eternal past. However a clear understanding of the differences between a Parmenidean conception of time and a Heraclitean one is necessary to determine whether one dimensional time travel is committed to the former.

A Parmenidean universe is here defined as one where every event is already determined. The past, present and future are all viewed equally as fixed, and traveling to different points does not change that.

A Heraclitean universe is here defined as one where the present and past* exist, and the future is currently ‘unreal’; there is no predetermined way for events to unfold, and hence no facts about the future.

Using the above definitions I will argue that one dimensional time travel is committed to a Parmenidean conception of time, as a self-consistent timeline which assumes a fixed past requires the traveler’s personal future to be known.

Taking a one dimensional conception of time travel, where Mary goes back in time from point X to point Y, and we know that the events at point Y occur in a particular way, we must also know that the future of Mary’s personal time line occurs in a particular way as well, as her actions have already happened and are already known.

As a Heraclitean universe does not include facts about the future (given it is unknown), this causes an inconsistency within it, and hence by contradiction one dimensional time travel is committed to a Parmenidean conception of time.

Concluding, I have shown above that the difference between 1D and 2D time travel lies in the distinction between bringing about events, and changing events; that one dimensional time travel is logically possible, as there is no contradiction in the explanation of it; that whilst there are apparent conceptual impossibilities, none of them are arguments against the possibility of time travel; and that one dimensional time travel is committed to a Parmenidean conception of time.


There is debate about the past existing, however if we view the present as containing the “present pastness” of events, as Prior did[2], then knowledge of the past is implicit within knowledge of the present.


1. Lewis, David;
“The Paradoxes of Time Travel”, sourced in Metaphysics; The Big Questions
2. Prior, A N;
“Some Free Thinking About Time”, sourced in Metaphysics; The Big Questions
3. Friedman, John; Michael Morris, Igor Novikov, Fernando Echeverria,
Gunnar Klinkhammer, Kip Thorne, Ulvi Yurtsever ;   "Cauchy problem in spacetimes with closed timelike curves", sourced in Physical Review