Unraveling the Universe: Our Understanding of Time

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At first glance, time seems one of the most obvious truths of our reality. To humans, “time” has meant a variety of things: shadows languidly rotating around a sundial, sand relentlessly flowing down an hourglass, or the perpetual ticking of a grandfather clock. Time plays a uniquely pervasive role in our lives, governing every action that occurs anywhere in the universe. In fact, the word "time" holds such significance to us that it is the most used noun in the English language. However, when examined closely, our severe lack of understanding of time becomes apparent. In the 4th century BCE, Aristotle proclaimed, “Time is the most unknown of all unknown things”. This statement holds as true today as it did then, although notable advancements have been made since that time. 

In the context of cosmology, time began roughly 13.8 billion years ago with the Big Bang. Scientists hypothesize that before this point, the universe existed as a singularity—a hot, dense state of infinite density and gravity, where the laws of physics as we know them break down. The Big Bang initiated the expansion and cooling of space, which has been happening ever since. This event is widely considered the “birth” of the universe and, with it, the “birth” of time. One theory—the past hypothesis—postulates that the Big Bang produced a universe with incredibly low entropy (disorder). The idea that entropy naturally increases, with every interaction contributing to its increase, is declared in the Second Law of Thermodynamics. Everything that has ever occurred in time since the Big Bang has been pushing the universe towards maximum entropy, where the universe reaches thermodynamic equilibrium and energy is evenly dispersed and disordered, leading to a "heat death" where no further interactions take place. Time will likely continue forever, but when the universe reaches this point, the very concept of time effectively becomes meaningless, as nothing interesting will ever happen again.

In line with this, time appears to have a direction. On one hand, the past lies behind us and is immutable, accessible only by memory and documentation. By contrast, the future lies ahead and is not necessarily fixed or predetermined. Almost everything we experience seems irreversible. For example, when an egg is broken, it is common knowledge that the egg will never unbreak and fit inside its shell. This one-way direction of events is known as the arrow of time, exhibiting the apparent unavoidable “flow of time” into the future. This model portrays time as a movie, where only the present is real. Such a unidirectional flow is similarly characteristic of the way that entropy never decreases in a closed system. However, while time existing as an arrow intuitively makes sense, it does not necessarily have to be this way. In other words, with the exception of the Second Law of Thermodynamics, the laws of physics do not specify an arrow of time.

The scientific study of time began with the work of Galileo Galilei in the 16th Century and continued to the 17th Century with the work of Isaac Barrow and Sir Isaac Newton. They considered time to be one of the fundamental scalar quantities, putting it along the lines of mass and charge. Their non-relativistic or classical physics is a model in which time is universal and absolute, meaning they believe it to be the same for everyone everywhere in the universe. This model, known as absolute time, claims that time and space are independent of objective reality and not dependent on each other. In their view, time was external to the universe, so it must be measured independently of the universe. With today’s understanding, this model is not entirely accurate. However, the Newtonian version is nonetheless a fine approximation of how time behaves in the world we actually live in. Indeed, his model only falls short when dealing with speeds approaching the speed of light or in situations of exceptionally high gravity.

Our idea of time was revolutionized by Albert Einstein when he published his Theory of Relativity, completely replacing the notion of absolute time. This theory hinges on the fact that the speed of light is invariable and cannot be exceeded. He proposed that space and time must be flexible and relative to accommodate this absolute speed of light. In his view, time and space are merged into four-dimensional spacetime, rather than the three dimensions of space and a completely separate time dimension. An event serves as both a place and a time and is modeled by a particular point in spacetime. Therefore, the whole of spacetime can be thought of as a collection of infinite events. The entire history and future of a specific point can be represented by a line in spacetime, known as a world line. The past, present, and future accessible to an object at a particular time are shown as a three-dimensional light cone, which is limited by the speed of light. Through the absolute nature of light, Einstein showed that space and time are connected and dependent on each other.

Relativity gives rise to the block universe theory of time, also known as eternalism. Therefore, time does not “flow” or “pass” in the traditional sense. Both the past and the future are simply “there,” occupying a particular spot in four-dimensional spacetime. Just like we know parts of space exist even if we are not there to experience them, all of time is also constantly in existence, even if we cannot witness it. Time, therefore, just is. The perception of a “now” only comes as a result of human consciousness and the way our brains are wired. This frozen block universe suggests that all times coexist and that free will is an illusion. However, if time is a dimension, it vastly differs from three-dimensional space. People can choose where they move in space, but movement through time is inevitable. So, time should be thought of as the 4th dimension in the sense that every moment exists at once, but the fundamental difference is our ability to travel through it.

Building on this theory, some have speculated that perhaps the block universe itself is evolving. In this growing block universe, the past and present both exist, but the future does not. The universe is, therefore, a growing volume of spacetime. For many, this model illustrates the passing of time in a much more satisfactory way. It proposes that time indeed passes and that the future is open. However, this seems to conflict with the law-governed universe, where future states are dictated by past physical states. However, quantum mechanics—our most accurate and current understanding— may have an answer for this. This branch of physics mainly occurs at subatomic scales, as relativistic physics still works well for large objects. Quantum mechanics states that certain parts of the universe are quantized, meaning they are comprised of discrete, indivisible packets called quanta. The uncertainty principle, which states it is impossible to know the speed and position of a particle at the same time, means quantum particles are inherently random. This provides plausibility for the growing block universe, but it is ultimately more of a philosophical concept rather than a scientific theory.

All these theories present many possibilities for time travel, although they have never been demonstrated and may be impossible. The first theory uses Einstein’s theory of relativity, particularly time dilation. Time is relative and can differ depending on an observer’s speed. As someone approaches the speed of light, time for him would slow down compared to an observer at rest. Thus, if a traveler left Earth at the speed of light and returned, more time would have passed on the planet than for him. Hence, he would have effectively traveled into the future by fast-forwarding time. The second possibility about time dilation regards how gravity affects the fabric of spacetime. Large masses like planets and stars warp spacetime around them. In these strong gravitational fields, time passes more slowly (if you’ve watched it, this is the method used in Interstellar). Still, it’s important to note that these speculations are all hypothetical. While relativity presents these two options, other time models present vastly different possibilities.

Wormholes, also known as Einstein-Rosen bridges, are shortcuts through spacetime that connect distant locations in the universe, allowing faster-than-light travel between them. If one end of a wormhole were moving through space at near-light speed, time dilation would make time pass more slowly at that end compared to the other. This difference could theoretically allow someone to travel back or forward in time. However, the problem is that there is currently no evidence to suggest the existence of wormholes. Another fascinating solution resides in the concepts of superposition and entanglement of quantum mechanics. Superposition is the idea that a particle can exist in many different states simultaneously. Only when observed does it collapse down to its most probable state. Entanglement is the principle that two particles can become linked such that one particle's state instantly affects the other's state, no matter how far apart. Combined, they create the many worlds interpretation, which is responsible for the idea of parallel universes. Rather than collapsing into one reality, the universe splits into alternative versions of reality, all equally real. If we could access these parallel universes, some form of time travel may be possible.

These notions of time travel present many interesting paradoxes. The first paradox is the twin paradox, which arises from time travel based on relativity. In this scenario, an astronaut returns to Earth from a near-light-speed voyage in space. Back at home, he finds his twin many years older than him. Because of the time dilation effect, time in the spaceship passed much more slowly than time on Earth. Another is the grandfather paradox, where a hypothetical time traveler goes back in time and kills his grandfather. However, this would mean the time traveler himself would never be born when he was supposed to be. But if he were not born, he would not be able to travel through time and kill his grandfather, which means that he must have been born. The many worlds theory tries to clarify this, explaining that time travel would create a parallel universe that branches off, meaning the original time stream would not be affected. Nevertheless, these paradoxes showcase the many complexities and complications of meddling with time.

By nature, time is arguably the most significant force in our lives, gradually eroding everything indiscriminately. We may try to flee its grasp, but nothing can endure forever—only the end of the universe could bring its reign to an end. Perhaps its inevitability is what makes it innately complex. We should recognize our minimal knowledge about time, much less time travel. Aristotle’s words, spoken nearly two and a half millennia ago, still hold true today. But perhaps they won’t always be. As technology advances and we gain more knowledge about the universe, we may someday be able to truly unravel the mysteries of time. New advances are being made faster than ever, opening a whole new realm of possibilities that nobody today could ever dream of. It may take centuries, or it may even occur within our very lifetimes. Until then, we’ll keep moving into the future, one second at a time.