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Seven Paradoxes About Time Travel

Time travel has fascinated thinkers and generated several works of science fiction, philosophical essays, and scientific theories.

However, this possibility has only begun to be considered since the end of the 19th century, when physics defined time as one dimension plus a four-dimensional structure called space-time.

Before this conception of time, the dominant point of view in the West was the one proposed by Aristotle, as we will see later in this entry. But, before discussing this philosopher, let’s take a closer look at the study of time and what approaches classical thinkers made.

1. Anaximander and time travel

For Anaximander, if the universe has always existed (as was thought in Ancient Greece), there should exist a past of infinite duration.

If there is a past that has an infinite extent, it could never reach a present moment. In this case, time would be an infinite passage without reaching the now. It would be like endlessly waiting.

Nevertheless, we are here and now. Something doesn’t fit.

Is it possible that we have reached the present? It can be because the past is not infinite. That is, the world had a beginning. We are in the present moment and continue moving forward into the future.

In a way, we are time travelers, moving at a speed of 1 second per second.

We will discuss further the fact that the universe had a beginning and how this generates difficulties.

2. Negation and time in Parmenides

Parmenides was intrigued that, if the universe has a beginning, how was it possible that the things of the world came into existence from non-existence? And conversely, how something that exists can not exist.

For him, a “thing that does not exist” is a contradiction because we can only think and talk about what exists. For Parmenides, nothing can go from existence to non-existence. Nor vice versa because everything that exists must have a beginning or end. Therefore, the universe would be infinite.

Furthermore, if everything has always existed and will always exist, then there is no change. For instance, the apple we see will always be an apple, or the rock will always be that rock. Change is impossible.

For Parmenides, the world would be something like a large sphere (a symbol of perfection and non-change), without abrupt alterations, without variations in density and movement.

Non-being, like time, is only in our thoughts. We can travel in time, but it is a journey we make with the mind because the world is one, immutable, and eternal.

3. The immutability of the past in Aristotle and time travel

Aristotle noticed mothers who found out months later that one of the warships that had their sons had sunk. Mothers prayed to the gods that their sons had not died. For Aristotle, prayers to cause retrospective changes in time were meaningless. He believed that we can only influence the future.

According to the Greek poet Agathon, not even the gods could change what had happened.

Therefore, for Aristotle, the past is fixed, but the future is open to change.

“The past is fixed.”

However, other philosophers, called fatalists, also deny the possibility of change in the future. Fatalists believe that what will happen will happen because “the future has already been written.”

“The future has already been written.”

Since H. G. Wells wrote “The Time Machine” in 1895, the constant concern of writers, philosophers, and scientists on how a time traveler could travel to the past and modify the course of events. For example, accidentally stepping on a butterfly could cause a great cascade of changes that project into the present and continue. Or if we returned in time, we could kill our grandfather when he was young, and that would make it impossible for us to have existed and kill our grandfather.

The modern conception of a four-dimensional space-time allows for this possibility. Aristotle’s model (unalterability of the past) or fatalism (neither the past nor the future are modifiable) deny it. For this reason, no one consistently proposed the real possibility of time travel as something possible or dangerous during the subsequent events. That is until the end of the 19th century and the beginning of the 20th.

Some defenders of current time travel consider it possible to travel to the past without modifying. For example, if we visit this concept as mere observers without the capacity to intervene or make it impossible for the gun that is going to shoot our grandfather to emit the fatal bullet. Another example would be compensatory mechanisms emerging in this process, like a banana peel on our trajectory when we approach our grandfather, which makes the final result an absence of changes in the past.

In contrast, the failure to fire a gun, or the presence of a banana peel in a place where it was not before, is a change that could also create chain reactions towards the future with unpredictable consequences.

4. The impossibility of time travel

According to Aristotle, it makes no sense to talk about time travel. The future does not exist yet. Therefore, we cannot travel from the present to the future. Nor can you travel from a future (which doesn’t yet exist) to a past.

Traveling to the past would promote changes and create infinite space-time paradoxes (as shown in many works of science fiction). Many current thinkers consider it impossible to travel in time because it would violate all the laws of logic. 

Without a doubt, the history of humanity is full of advances made by people who did things they did not know they could accomplish!

It’s best to continue investigating. Let’s look at some more paradoxes about time travel.

Time travel
5. Augustine of Hippo and time as a subjective experience

Augustine of Hippo wondered if God knew everything at all times, then why didn’t he have the ability to predict Eve tempting Adam with the apple from the tree of knowledge? Nor could God know that, by creating man, he was going to cause several depraved acts in the world. If God predicted man’s actions, why did he take the time to make man?

Before converting to Catholicism, Augustine belonged to C (followers of Manichaeism, a Persian religion). He joked about this thinker’s ideas and stated that if God created the world at a specific time, what was he doing before? Is God a lazy being? Besides being lazy, is he arbitrary? What sense does it make to create the world and time at one given moment and not at another?

He replied to these criticisms and alluded that God created time and the world at the same time. However, this does not imply that time depends on the world. For this thinker, time will continue even if there is no physical change. For example, this is what happens when we perceive prolonged silence.

For Augustine, what would not make sense would be to affirm that there is time without a mental change occurring. Time is in the mind. If we think of time as an independent phenomenon of the mind, we will fall into the measurement paradox.

“Time is in the mind.”

This Manichaeism follower stated that the present is the limit between the past and the future. This point, called present or now, has a duration. For example, if I hear a sentence that lasts two seconds, “my” present lasts that long. But that is not the present objective. The objective is an infinitely small moment and has no duration. We can access things at that minimum instant (objective present). However, any of our present perceptions last much longer. The time I listen to a song, contemplate a mountain or make a decision is “my present” (subjective present). That present is the one that makes sense to me.

On the other hand, the objective present (the presence of physical things) cannot be measured. The measurement of duration requires a subjective presence.

The past, therefore, is what I remember. The present is what I perceive. The future is what I anticipate. The rest has no existence (the physical past no longer exists, the objective present has an infinitesimal existence, and the physical future does not yet exist).

Additionally, any observer can vary their subjective present amplitude (I can consider the present to be the second it takes for a leaf to fall from a tree or what I am doing this afternoon).

According to this thinker, God would have an infinite perceptual breadth. Everything would be present for him. God did not wait to create the world. God did not know what Adam and Eve were going to do. God does not predict but only perceives what has been, is, and will be for us. For Him, it’s every act, everything taking place now.

We have a limited perspective of the world that travels in time through our perception and subjectivity.

“We travel in time.”

To Augustine, time is simply a limited way of perceiving something greater: eternity. This concept is beyond our human capabilities to understand. Only God is capable of having a complete vision of this eternity.

We can only make attempts (always very limited) to travel in time by projecting our imagination towards the past or the future, as science fiction films already do or as history books tell us.

6. The absence of time in Thomas Aquinas

Thomas Aquinas wondered where God was. His answer was “everywhere.”

If God is everywhere, he cannot travel. If he is in all eternity, that is, in all moments (as Augustine of Hippo defended), he isn’t limited to the past or future. God cannot move through time like we do.

What’s more, the passage of time limits all possibilities. As time passes, the things that are possible happen. A student finishes or doesn’t finish his doctorate. A 30-year-old woman is either a mother or not.

The Creator cannot change what we have done over time. His power is conditioned by time because God is outside of time. Time is our distorted perception of a much greater limitless reality in which God is found and is eternity.

Therefore, time as such does not exist. It is what we perceive as a small part of something greater (eternity) that we can only understand in a very partial way.

7. Is time dynamic or static?

The English idealist philosopher McTaggart (1866-1925) considered time and combining the two concepts.

The first is called Series A, which refers to the use of “past, present, and future.” However, there would be another series called Series B, which would refer to the concept of “before, simultaneous, and after.”

If we consider time from the perspective of Serie A, time would be dynamic: what is past will become present, and what is present will become future. This Series A is what guides our actions in daily life.

Even so, time should not change from one mind to the other (let’s remember how we can set different durations for events, and what is present for one person could already be passed for another).

Therefore, Series B is important to consider when talking about the weather. Series B is static. We can harmonize all the temporal affirmations we make using this series. If a noise is heard and a child is scared, one event (the noise) will always come before the other (the scare), and the scare will always come after the noise.

What we describe from series B will always be static (it will be a relationship that does not change over time), and what we describe from series A will be subjective (my past, present, or future will vary from my perspective).

We use the concepts of past, present, and future (series A), time, as we understand it, is a mere illusion of our senses. It’s not real. Time is an illusion.

We can only travel through time using our imagination (as we already do in the 20th and 21st centuries).

Time for McTaggart, as we use it in daily life, is fictional and not a faithful reflection of reality.

“Time is fiction.”

Humans make time with our watches, our calendars, and our imagination. The only thing that exists is a system of relationships between objects and events that are established through relationships of “before,” “after,” or “at the same time as” (called Series B), nothing more.

Summary and conclusions

From ancient times to the present, humans have questioned the nature of time and our possibilities of modifying it.

Similarly, modern science and religions consider that there could have been a beginning of time in the creation of our universe and in which we existed.

In most traditions, time would be a distorted vision of a much broader reality that we would partially perceive and call eternity.

Time would be a product of our minds. According to this perspective, we could travel in time by going to the past through our memories and travel to the future through our anticipatory thoughts.

It would only make sense to talk about a subjective present (which we could measure). The objective present would be that instant in which things exist just after the past and before moving into the future.

Although, the present (objective and subjective) is the only place we can act. He now has immense power. As physicist Richard A. Muller said, “now” can build a civilization or destroy it forever. The past helps us better understand the present. The future helps us anticipate and know how we should guide our actions at this moment and in this specific place of our existence.

Muller, A. R. (2016). Now. The Physics of Time. New York: W.W. Norton & Company.

Sorensen, R. (2003). ABrief History of the Paradox. Oxford: Oxford University Press.

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