In this blog post, we will explore whether time really flows in a straight line and how its direction can change in the universe and evolution.
What is time? Augustine of Hippo said, “We know what time is when no one asks, but when we try to explain it, we can’t figure out what it is.” Augustine of Hippo’s words are a good point about the nature of time. It is difficult to know what the nature of time is, but what is clear is that time flows from the past to the future like an arrow. This flow is deeply ingrained in every moment of our lives, and human experience is constructed over time. The past we remember, the future we predict, and the present we live in all exist within the framework of time.
The directionality of time was only scientifically studied in the modern era, and there are two main perspectives on time: cosmological time and thermodynamic time. Cosmological time is a concept related to time that progresses in the direction of the expansion of the universe. Thermodynamic time is a concept related to time that progresses in the direction of increasing entropy, or disorder. These two perspectives provide an essential framework for understanding the concept of time, and each theory plays a unique role in explaining time.
The concept of cosmological time, or time that can be applied to the universe, was proposed through Isaac Newton’s law and Albert Einstein’s theory of relativity. According to Isaac Newton’s law, if the current state of an object, that is, the object’s position and velocity, is known, then its future or past state can be known. However, if this law is applied to the entire universe, it becomes impossible to tell whether the direction of time is moving toward the past or the future. In other words, even if time is flowing backward, the motion of an object appears to be in accordance with Isaac Newton’s law. This is called the symmetry of time. For example, the film of the planetary motion taken from a space probe can be turned in either direction, which fits Isaac Newton’s law. Therefore, Isaac Newton’s law alone cannot properly explain the direction of cosmological time, which is thought to be progressing in the direction of the current expansion of the universe.
Moreover, even Albert Einstein’s theory of relativity, which is said to be the best explanation of the expansion of the universe so far, cannot explain the directionality of time. Albert Einstein’s theory of relativity redefined the relationship between time and space and made a groundbreaking contribution to explaining how the universe works, but there are still some incomplete parts regarding the asymmetry of time. These limitations have led scientists to call for a new unified theory, which requires 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 a direction in which energy is dissipated and entropy increases. Nature moves towards the state of maximum disorder, just as a piece of pottery falls to the floor and breaks, or just as the smoke that rises from a room gradually disperses and spreads further outside when the window is opened. The time observed in these cases is irreversible, so it is called irreversible time. The direction of the progression of this natural phenomenon is the direction of thermodynamic time. This law explains the direction of time that we experience in the everyday world without deviating from reality.
Sometimes, the second law of thermodynamics seems problematic. This law seems to contradict the theory of evolution, which states that living organisms are born and evolve into organized systems. This is because the theory of evolution sees simple organisms evolving into more complex organisms, which is an increase in the degree of order. In response to this seemingly contradictory fact, Ilya Romanovich Prigogine explained that the theory of evolution and the second law of thermodynamics are compatible by showing that order can also come from disorder. In other words, nature does not only consist of processes that aim for a state of thermal equilibrium, or a state of maximum entropy, but also non-equilibrium phenomena that try to minimize entropy growth. In other words, the entire natural world is bound to progress toward a state of thermal equilibrium, but non-equilibrium states can occur in specific times and spaces.
For example, if you drop a drop of ink into water, the final state will be a light-colored equilibrium state, but if you observe the process, you can see the patterns and structures created as the ink spreads out. This is an example of an equilibrium state that appears temporarily in water. The theory of evolution also sees this as a phenomenon that occurs in the process of maintaining a state of non-equilibrium. In this way, the second law of thermodynamics can explain the directionality of everyday time without contradicting the theory of evolution. In addition, this aspect of the second law of thermodynamics suggests that the direction of time does not simply follow the increase in entropy, but that locally, order and complexity may increase. This is consistent with many natural phenomena that occur around us and plays an important role in understanding the complexity and evolution of life.
But what would happen if we applied the second law of thermodynamics to the entire universe? Eventually, the universe would progress from a low-entropy state to a high-entropy, disordered state. If this process of increasing entropy continues indefinitely, the universe would reach a state of maximum entropy, a state we call heat death, in which all available energy is completely dissipated and no further activity occurs. This is the final arrival point of time. However, this interpretation does not take into account the fact that the universal gravitation force acts during the expansion of the universe, so it is merely an assumption and does not accurately describe the time of the actual universe.
As such, the second law of thermodynamics can only explain the world as we know it, but it cannot properly explain the direction of time that applies to the entire universe. Similarly, the laws of Isaac Newton and the theory of relativity also cannot explain the direction of cosmological time. The concept of time is much more complex than what we experience in our daily lives, and more research and understanding is needed to understand its nature. In order to arrive at a true explanation of the directionality of time, we need a unified theory that can explain the directionality of time in our daily lives and the directionality of time that applies to the entire universe at the same time. Developing such a theory is a major challenge facing modern science.
