Physical shapes that allow us to imagine how time travel might work.

Credit: Sean MacEntee

Deep down inside we all look back on our choices in the past and wonder, what if I did it differently. What if I had said yes instead of no? What would happen if I could do it over?

This question has worked its way into numerous fictional works, especially in the last 100 years or so. And now more than ever we see movies, books and video games whose plots revolve around the ability to travel through time. Some focus on changing the past to affect the future, while others are dreams and nightmares of what the future holds for us.

To understand time travel we need to have an understanding of time itself. The problem is we have very little understanding of time. Instinctively we make observations of time based on the way we experience it, The present is currently happening while the past is fixed, it has already happened and cannot be changed, and the future is not set in stone until it manifests itself as the present and then becomes a fixed past. Science has no way of explaining these tenses of time, units of time are used in the mathematics of most of the sciences, but it doesn’t differentiate time as past, present and future.

So does time even exist? I mean of course it exists in the sense that time passes and things move, change and happen. But does time exist, in the physical sense. Is there an invisible time-line that exists that we have no way to observe. Some refer to time as the fourth dimension, it is a dimension of existence that we have no physical ability to observe except in the form of the present.

If time does exist and we just cannot see it what form does it take? Lets look at some of the possibilities and how they might relate to the possibility of time travel.

Time is a straight line

Time is a single continuous streaming line that exists in the form of past, present and future. There are two ways to look at the straight line theory when talking about time travel, the difference between them is whether the future already exists or is it constantly being created at the speed of which the present is happening.

This theory of time presents major problems for time travel. In either version the past is fixed so time travel backward may be possible. If the future is only created at the speed of the present you cannot travel to the future for it does not yet exist. If the future has already been set and currently exists you can travel to the future as well. BUT, if all time already exists and is written on a single continuous line you cannot travel back or forward and hope to interact with anything in a different time than the present because that creates a paradox that undoes what is already written. If you write a sentence in ink on paper you cannot erase or change what has already been written. If you were to physically travel to a point on the timeline it would have already had to have happened for it to be possible.

The best we could hope for in this theory is that you find a way to observe the past without interaction,  which would only really be novel if the future is not yet written, because we can actually control our destinies with the knowledge acquired from observing the past. If the future is already written we will only be able to do what we are predestined to do.

If we theorize this single timeline can be rewritten, we can visualize a how this might work. Upon time travelling you reenter the timeline at a point in the past effectively bending the line and making a loop. But because time only exists as a single timeline all time between the time travel event and the point you travel to immediately cease existence. Basically at the moment of time travel the past moment becomes the present and the present you formally existed in will disappear, making the time travel event its own world ending paradox with the lone survivor(s) being the time traveller(s).

Ti isme many worlds

In this theory we allow the possibility of time travel because time exists in a straight line with branching paths. Time branches are created when time travel happens. Basically time exists as a single straight line just as in the last example, except at every event there are branching timelines, both are real and exist but do not interact. A famous example of this is the thought experiment Schrodinger's Cat. In the thought experiment we imagine there is a cat in a closed box, and we do not know if the cat is alive or dead. Because both possibilities exist the cat is both alive and dead. These realities represent branching paths, they exist simultaneously but they do not interact with each other.

In this model time travel creates a new path off of the time period you travel to.

Time is infinite simultaneous timelines

In this model if I haven’t already given you a migraine you will get one now. Time exists as infinite timelines, infinite outcomes and possibilities exist simultaneously. Because infinite outcomes exist time is already written but we choose the timeline we exist in in every present moment. Because every present moment preceeds a possible new or different future. Because of this true time travel is impossible, you never really travel in time but the present reality reflects a change in the time period in which you perceptually exist but in reality it is just an alternate present that exists in one of the infinite realities that exists.

Feel free to comment, the theory of time and time travel can be taken in so many directions and creates so many paradoxes i find it a fun puzzle to work out, imagining how it could all work and poking holes in each others theories is part of the fun.