This article explores the nature and directionality of time, reviewing the different scientific theories that describe time, focusing on cosmological time and thermodynamic time. In doing so, it helps us understand the directionality of time and its complexity, and addresses the challenges facing modern science.
What is time? Augustine of Hippo once said. “We know what time is when no one asks us, but when we try to explain it, we lose sight of what it is.” Augustine of Hippo’s words capture the nature of time well. While it is difficult to know what the nature of time is, what is clear is that it flows like an arrow from the past to the future. This flow permeates every moment of our lives, and human experience is organized by the passage of time. The past we remember, the future we predict, and the present we live in all exist within the framework of time.
The direction of time has only been studied scientifically in modern times, and there are two main ways of looking at time: cosmological time and thermodynamic time. Cosmological time is the concept of time as it relates to the expansion of the universe. Thermodynamic time is the concept of time as entropy, or the amount of disorder, increases. These two perspectives provide an essential framework for how we understand the concept of time, and each theory plays a unique role in explaining time.
Cosmological time, or the concept of time as it applies to the universe, was presented through Isaac Newton’s laws and Albert Einstein’s theory of relativity. According to Isaac Newton’s law, if you know the current state of an object, that is, its position and velocity, you can know its future or past state. However, when this law is applied to the universe as a whole, it becomes impossible to know whether the direction of time is backward or forward. In other words, the motion of an object does not appear to violate Isaac Newton’s laws, even if we assume that time flows in reverse. This is called the symmetry of time. For example, a film of planetary motion taken from a space probe can be spun backwards or forwards in any direction and still fit within Isaac Newton’s laws. Therefore, Isaac Newton’s laws alone do not adequately explain the direction of cosmological time, which is currently thought to be moving in an expanding direction.
Furthermore, even Albert Einstein’s theory of relativity, which is considered to be the best explanation for the expansion of the universe, does not explain the direction of time. Albert Einstein’s theory of relativity redefined the relationship between time and space and was a breakthrough in explaining how the universe works, but it remains incomplete when it comes to the asymmetry of time. These limitations have led scientists to call for a new unifying theory, requiring a deeper understanding of how time works.
Thermodynamic time, on the other hand, is the time described by the second law of thermodynamics. According to the second law of thermodynamics, natural phenomena proceed in the direction of dissipating energy and increasing entropy. Nature moves toward a state of maximum disorder, such as when a piece of pottery falls to the floor and breaks, or when smoke rising in a room slowly disperses and spreads further outside when a window is opened. The time observed in these cases is irreversible, so we call it irreversible time. The direction of progression of these natural phenomena is the direction of thermodynamic time. This law describes the direction of time as we experience it in our everyday world without contradicting reality.
The second law of thermodynamics is sometimes thought to be problematic. It seems to contradict the theory of evolution, which states that life arises and evolves into ordered organizations. This is because the theory of evolution suggests that simple organisms evolve into more complex ones, which is an increase in the degree of order. In response to this seeming contradiction, Ilya Romanovich Prigogine explained that evolutionary theory and the second law of thermodynamics are compatible by showing that order can also emerge from disorder. In other words, there are not only processes in nature that are in thermal equilibrium, i.e., those that are oriented towards a state of maximum entropy, but also non-equilibrium phenomena that seek to minimize the increase in entropy. In other words, while the natural world as a whole must be moving toward thermal equilibrium, non-equilibrium can occur at specific points in time and space.
For example, if you drop a drop of ink into water, the final state is a pale color equilibrium, but if you observe the process, you can see the patterns and structures that the ink creates as it spreads. This is an example of a non-equilibrium state that emerges temporarily in water. The theory of evolution also recognizes that this is a process of non-equilibrium. In this way, the second law of thermodynamics is compatible with evolutionary theory without contradicting it, and it provides a good explanation for the directionality of everyday time. Furthermore, this aspect of the second law of thermodynamics suggests that the direction of time does not simply follow an increase in entropy, but can locally increase in order and complexity. This is consistent with many natural phenomena that occur all around us, and plays an important role in understanding the complexity and evolution of life.
But what happens when we extend this second law of thermodynamics to the universe as a whole? Eventually, the universe will progress from a low-entropy state to a high-entropy, disordered state. If this process of increasing entropy continues indefinitely, the universe will reach a state of maximum entropy, a state called thermal death, where all available energy is completely dissipated and no further activity occurs. This state of heat death would be the final destination of time. However, this interpretation does not take into account that the universal gravitational force is at work in the expansion of the universe, so it is only an assumption and does not accurately describe the actual time of the universe.
Similarly, the second law of thermodynamics only has explanatory power in our everyday world, but it does not explain the directionality of time as it applies to the universe as a whole. Similarly, neither Isaac Newton’s laws nor the theory of relativity described earlier describe the directionality of cosmological time. The concept of time is much more complex than our everyday experience, and its nature requires much more research and understanding. To arrive at a true explanation of the directionality of time, we need a unified theory that can simultaneously explain the directionality of everyday time and the directionality of time as it applies to the universe as a whole. Developing such a theory is the great challenge of modern science.
