THEORIES OF TIME

What physics and philosophy have to say about this mysterious aspect of human experience and the cosmos

Today I had the privilege of watching the new Spider Man movie. I won’t spoil it for anybody, but I will say that it has some pretty interesting references to concepts in physics. This got me thinking about how much influence intellectuals have in society. Intellectual concepts and ideas are important and influence the public. One idea is the idea of time. Time has been studied by physicists and philosophers for centuries, yet it seems we are just beginning to grasp the understanding of it. And these theoretical speculations have influenced the arts (movies for example), as do many ideas. Intuitively, we have an idea of what time is. But when we think about it deeply and seek to define and explain it, things get confusing. Today I hope to introduce you to some of the major concepts in physics and philosophy regarding the nature of this strange “thing”, so familiar, yet so uncanny.

In the Confessions, philosopher and theologian Augustine of Hippo, on Book XI, chapters XII-XXVIII, speculates and asks questions about time. It is a difficult read, but leads us to some of the important questions about what time is. “At no time, therefore, had You not made anything, because You made time itself. And no times are co-eternal with You, because You remain for ever; but should these continue, they would not be times. For what is time? Who can easily and briefly explain it? Who even in thought can comprehend it, even to the pronouncing of a word concerning it? But what in speaking do we refer to more familiarly and knowingly than time? And certainly we understand when we speak of it; we understand also when we hear it spoken of by another. What, then, is time?” (Chapter XIV — 17). Augustine concludes that time was created by God, and God transcends it. He raises some important points, such as the existence of past events in the mind; after all, I don’t see my childhood self, but I can imagine myself as a child and have a memory of my childhood self. In philosophy there are two big ways of looking at time. The first way was endorsed by Aristotle, Leibniz, and Ernst Mach. It essentially says that time is simply a way of describing changes and events. Time is not really a “thing.” Leonardo da Vinci also seemed to hold this view when he wrote, “The instant does not have time; and time is made from the movement of the instant.” (Codex Arundel, 176r). The second view was endorsed by Plato, Einstein, and Newton. Time is a “thing.” Everyone moves relative to this absolute time. These two may be called the dynamic theory of time (Aristotle, Leibniz, Mach) versus the static theory of time (Newton, Einstein, Plato). The Stanford Encyclopedia of Philosophy summarized the main ideas of each view in their article on time (which I will link at the end of this essay):

The Static Theory of Time

1. The universe is spread out in four similar dimensions, which together make up a unified, four-dimensional manifold, appropriately called spacetime.
2. Any physical object that is located at different times has a different temporal part for each moment at which it is located.
3. There are no genuine and irreducible A-properties; all talk that appears to be about A-properties can be correctly analyzed in terms of B-relations. Likewise, the temporal facts about the world include facts about B-relations, but they do not include any facts about A-properties.
4. The correct ontology does not change over time, and it always includes objects from every region of spacetime.
5. Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time. Also, we can in principle eliminate verbal tenses like is, was, and will be from an ideal language.
6. There is no dynamic aspect to time; time does not pass.

The Dynamic Theory of Time

1. The universe is spread out in the three dimensions of physical space, and time, like modality, is a completely different kind of dimension from the spatial dimensions.
2. Any physical object that is located at different times is wholly present at each moment at which it is located.
3. There are genuine and irreducible A-properties, which cannot be correctly analyzed in terms of B-relations. The temporal facts about the world include ever-changing facts involving A-properties, including facts about which times are past, which time is present, and which times are future.
4. The correct ontology changes over time, and it is always true that only present objects exist.
5. Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time. Also, we cannot eliminate verbal tenses like is, was, and will be from an ideal language.
6. The passage of time is a real and mind-independent phenomenon.

PS: To better understand the above ideas, I would strongly recommend reading the entire article on the Stanford Encyclopedia of Philosophy. For example, when referring to A-properties and B-properties, this is a reference to A-Theory and B-Theory of time, which were explained earlier in the same article.

Both are two very different ways of viewing time. I personally am more inclined to agree with the static theory of time because of its rationality, the ordered structure it seems to give to the universe, as well as the modern theories of physics (which to me seem to agree with this view more). But this is a complicated subject, and I am open to disagreements. This is a very intriguing topic, and there is still much to be discovered. Time, like space, generally remains elusive to us. For example, the issue of the infinite divisibility of matter, space, and time. Suppose you get a moment of time, or (which is easier to picture), an object in space. Suppose you had a powerful machine that could cut it in half. Then you get one of the halves, and cut that in half. Suppose you keep doing that. In theory, it seems you could infinitely keep cutting it in half. Aside from all the technological limitations and the fact that there are objects that are physically indivisible, it seems that everything can be infinitely divided in halves. This seems very strange and difficult to understand. It reflects how much progress must be made in our understanding of the fabric of the cosmos.

Moving on though, I would like to explain some of the ideas in modern physics about time. The first one is the idea of motion through spacetime. This one is actually easier to understand than some of the other ones. The idea is that every object is moving not only through space, but also through time. It is easy to understand that we move through space. Every time I get up and walk, I am moving through space. But time? Yes, we move through time too. And actually we do not just move through space and time. We move through spacetime. Space and time are combined and related. The faster I move through space, the slower I move through time, and vice versa. This is why to an object moving very fast through space, time goes by more slowly. It is difficult to perceive this in everyday experience, but if you moved really fast (say, close to the speed of light) you would see this. It’s as if all of us had two cups. One called space, the other time. But the two are related (the analogy is imperfect, because it gives the idea of space and time, not spacetime). You have a set amount of water. If you pour more water into the space cup, less water is in the time cup. This principle is actually what explains why the speed of light is constant relative to any observer, but this is not something I will get into in this article. And an interesting question to ask is, how much water are you given? In other words, what is the speed at which you are moving through spacetime? It’s the speed of light. Now, you cannot, according to special relativity, move at the speed of light through space. But you can do that through spacetime. This concept leads to all sorts of interesting conclusions. What if you were to pour all the water in the space cup? In other words, what if you could move at the speed of light through space? Well, like I said, you cannot do that. But light can. And if that were to happen, which it does, time would stop. This is very amazing. Photons, the particles of light. What do they experience in time? Does time not exist for photons?

Another interesting concept of physics is the idea of the relativity of simultaneity. Imagine if you were to slice all the moments of the history of the universe into slices. Your “now” slice would be everything that, to you, is happening right now. Einstein’s theories tell us that people’s slices would look differently. Two people at rest relative to each other, have the same now slice. But if, for example, an alien on a distant galaxy starts moving in his planet, relative to a human billions of light years away, their now slices would look a lot differently. In fact, if the alien moved away from you, his “now” slice (everything that, to him, is happening right now on the universe) would include something that to the human happened many years ago. If the alien moved towards you, the opposite would happen, and his “now” slice would include things on earth that, to you, have not even happened yet. This is a point made by Brian Greene, on his book The Fabric of the Cosmos, pages 134–137. I will not go into too many details on this. I still hope to study this concept in more depth, God willing.

Anyhow, another idea of philosophy and modern physics regarding time is something called the arrow of time. It seems like there is a “flow” to time, a past, present and future. But what determines this flow? What distinguishes a past event from a future one? After all, if you shoot a gun, it seems like there is nothing in the laws of physics that prevents the opposite from happening, namely the bullet returning to the gun, in which case there would be no distinction between past and future. But why does that not happen in everyday life? Well, because of entropy. There are a thousand ways for something to be disordered, but only one way for it to be ordered. Therefore, it is statistically more likely for it to be disordered. This is the second law of thermodynamics. It says that the universe moves to greater disorder over time. If I blow up something, it is a lot more likely for its pieces to fall to the ground in different locations in a disordered manner, than for them to fall in such a way that the object is again ordered. At least for physical phenomena, it seems entropy is the arrow of time.

The last idea I would like to look at with regards to time is something which is sometimes called eternalism which modern theories of physics seem to support. It stands in contrast with presentism (only the present is real) and the growing block theory (the past and present are real) and basically says the universe just exists. Spacetime just exists. Just like Mars exists right now, even though you are not seeing it, so all of spacetime (including all moments of time) exist right now, even though they are not present. In other words, if you were to zoom out and see all of spacetime, you would not just see me and you, and all other people who are alive right now. You would see the U.S. president in 2055 and Alexander the Great. Think of it like a movie. The movie is there. All the parts of the movie simply exist. It’s not as though, one part exists, then ceases to exist to give way to the next. The whole movie exists, even though you observe it one part at a time. What implications does this have for free will? The great debate over fatalism and freedom. I do not personally think this eliminates the possibility of free will. Even if it is true that the future is already there, it is possible the reason the future is the way it is, is because of us humans, in which case we can be said to be free. The thing is that if the future is not already “there” it would be pretty difficult to imagine time travel to the future. After all, if the future is completely open, what do we travel to? What does this mean for us though? What are we? This comes to the debate of three-dimensionalism and four-dimensionalism. The latter states that you are a spacetime worm; you are the collection of all your “parts”, each moment in your life from start to finish, in spacetime. The former states that “you” are simply present wholly and completely at each moment. You are not just a collection of parts in spacetime. You are fully in one moment, then fully in the next. The Time article, which I have already quoted, defines the difference like this:

Three-Dimensionalism: Any physical object that is located at different times is wholly present at each moment at which it is located.

Four-Dimensionalism: Any physical object that is located at different times has a different temporal part for each moment at which it is located.

Before I conclude, I would like to briefly present another theory of time, which to me seems like a combination of the dynamic and static ones. It interested me a lot when I first read about it, so I feel compelled to share it. It is called the moving spotlight theory. It basically says that yes, every moment in history does exist right now (as in eternalism) . But only things happening right now are “present.” This is a new way to think about time. I will link a paper by Bradford Skow on the issue (I have not yet read it), which might interest some audiences.

These are some of the big ideas on time. There is a lot we still don’t know. Maybe even some of the ideas in modern physics need refining. Maybe some will even be shown to be false altogether. If the past and present simply exist in spacetime (or so the evidence seems to indicate), what does that mean? Augustine speculated on this when talking about how the past exists in our memory. There are many unanswered questions. How does God relate to time? That is a big one. The important thing is to be open and reasonable in this discussion. At the end, I will link some articles and books that greatly helped me write this, as well as some others which I have not yet read, but might interest the reader.

Some main ideas:

• Substantivalism and reductionism with respect to time
• Time dilation
• Arrow of time
• Relativity of Simultaneity

Bibliography

• Time — First published Mon Nov 25, 2002; substantive revision Tue Nov 24, 2020 (Stanford Encyclopedia of Philosophy) https://plato.stanford.edu/entries/time/
• The Fabric of the Cosmos: Space, Time, and the Texture of Reality — Brian Greene
• Leonardo da Vinci — Walter Isaacson

Further readings:

• Relativity and the Moving Spotlight — Bradford Skow https://web.mit.edu/bskow/www/research/timeinrelativity.pdf
• Relativity: The Special and General Theory — Albert Einstein
• Time and Eternity: Exploring God’s Relationship to Time — William Lane Craig
• The Physics and Philosophy of Time — Carlo Rovelli https://www.youtube.com/watch?v=-6rWqJhDv7M&t=2121s
• Confessions — St. Augustine of Hippo