Time is a controversial topic in both the physics and philosophy community. Many have dedicated their lives to understanding this everyday aspect of our world, yet we have only scratched the surface of this huge topic. Out in the field, there are numerous theories on the nature of time, yet most of them fall within a range of two theories. On one side, presentism believes that only the present exists and that the past and future are not “real.” On the other hand, the concept of eternalism demurs presentism. Eternalists believe all points in this universe's past, present, and future are real.
First, eternalism. To understand the concept, we can take this example: imagine my friend, Bob. To describe my friend Bob’s location, I could give his latitude and longitude (27.9881° N, 86.9250° E) and his altitude (29,029 feet above sea level). From these three spatial coordinates, we can deduce that Bob is on top of Mt. Everest (good job Bob!). However, we also need to give the time he was there, say 10:36 AM on June 6th, 2123. This gives us 4 coordinates for 4 dimensions. Einstein’s general theory of relativity (GR) states that the universe is comprised of 4-dimensional spacetime, where there are 3 space dimensions and one time dimensions. Just as all points of space exist, all points of time can be seen as "existing." The present as we experience it is just an arbitrary point in time. This worldview, sometimes referred to as the “block” universe or the Static Theory of Time, is the main foundation for the concept of eternalism. Yet, while logical, the idea leaves a question. If all moments in time exist, why do we experience the passing of time? Some Eternalists have turned to the hypothesis of “Temporal Parts”. The idea works like a film strip; as you walk across a room, each frame depicts you at that moment in time. If you play the film strips, it will show you walking across the room, when in reality, the moving pictures are made out of frames - Temporal Parts. Each one of those frames would show a temporal part with your exact location, your particular place in time. Eternalists then say that our perception of time is an illusion and does not align with the true temporal nature of the universe.
On the other hand, Presentism relates that our universe has only 3-dimensions – length, width, and height. It is the view that only the present exists, and is supposedly the “common-sense” theory. You, the reader, exist, the ISS exists, and Drayton Dormitory exists, but William Penn in the past and Martian colonies in the future do not. The universe is not static but is dynamic; hence, the Dynamic Theory of Time. Furthermore, Presentists say that treating time as a dimension similar to space is wrong. Time and space have very different characteristics. For example, we can move around in 3-D space but are forced to move in one direction in time. They also reject the notion of Temporal Parts, instead accepting our perception that time flows or passes. Although Eternalists have touted Einstein’s general theory of relativity as supporting their view, Presentists have published papers that showed that Presentism could also be integrated into perhaps unorthodox variations of General Relativity. In essence, presentists believe that non-present objects do not exist, that time is not a spatial dimension, and that the universe is dynamic and ever-changing.
Moving on, there are a range of theories floating in between the two sides. Some lean towards one side or the other and others combine characteristics of both. For example, the Growing Block Theory is somewhat similar to Presentism with an important twist. It states that only present and past objects exist, but future ones do not. Unlike Eternalism though, it does not involve any 4th temporal dimension. Another interesting hybrid idea is the Moving Spotlight Theory, which describes objects as placed along the temporal dimension of spacetime. The idea is that a sort of moving spotlight is constantly moving through the time dimension. All objects within that spotlight are real and present, but those that are not are equally real as well. Unlike Presentism, it implies that objects do not just appear and disappear and involves a temporal dimension, but it also contradicts the Eternalist belief by hinting that there is a dynamic change in this universe. Spotlighters believe that this theory best combines the strengths of eternalism (compatibility with GR) while explaining our perceived flow of time.
In summary, eternalism is the view that all objects exist within a 4-D universe, presentism says that only present objects exist, and hybrid theories may exist between the two. Currently, most physicists and philosophers lean towards Eternalist ideas, citing that Eternalism is the theory most compatible with relativity, yet it still struggles to fully explain the flow of time. In the end, theories will continue to replace one another as we reach closer to understanding the fundamental aspects of our universe.
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