In this blog post, we explore whether time truly flows only in a straight line and how its directionality can change within the universe and evolution.
What is time? Augustine of Hippo said, “When no one asks, I know what time is; but when I try to explain it, I no longer know.” Augustine of Hippo’s words aptly point out the nature of time. While its essence is difficult to grasp, it is clear that time flows like an arrow, from the past towards the future. This flow permeates every moment of our lives, and human experience is structured according to 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.
Scientific research into the directionality of time only began in modern times, with perspectives on time broadly falling into two categories: cosmological time and thermodynamic time. Cosmological time refers to the concept of time progressing in the direction of the universe’s expansion. Thermodynamic time is a concept related to time progressing in the direction of increasing entropy, or disorder. These two perspectives provide essential frameworks for understanding the concept of time, and each theory plays a unique role in explaining it.
Cosmological time, the concept of time applicable to the universe, was presented through Isaac Newton’s laws and Albert Einstein’s theory of relativity. According to Isaac Newton’s laws, knowing an object’s current state—its position and velocity—allows us to determine its future or past state. However, when applying these laws to the entire universe, it becomes impossible to determine whether the direction of time points toward the past or the future. In other words, even if time were assumed to flow backwards, the motion of objects would still appear to obey Isaac Newton’s laws. This is called the symmetry of time. For example, film footage of planetary motion captured by a space probe would play back perfectly, regardless of whether it was played forwards or backwards, and would still fit well with Isaac Newton’s laws. Therefore, Newton’s laws alone cannot adequately explain the directionality of cosmological time, which is thought to be progressing in the direction of the currently expanding universe.
Moreover, even Albert Einstein’s theory of relativity, known as the theory that best explains the expansion of the universe to date, fails to provide a proper explanation for the directionality of time. While Albert Einstein’s theory of relativity made groundbreaking contributions by redefining the relationship between time and space and explaining how the universe operates, it still leaves gaps regarding the asymmetry of time. This limitation has prompted scientists to seek a new unified theory and demands a deeper understanding of how time functions.
Meanwhile, thermodynamic time refers to time described by the Second Law of Thermodynamics. According to this law, natural phenomena proceed in the direction of energy dissipation and increasing entropy. Just as a ceramic vase shatters when dropped on the floor, or smoke rising in a room gradually disperses and spreads further outward when a window is opened, nature progresses toward maximum disorder. The time observed in these examples is irreversible, hence termed irreversible time. The direction of these natural phenomena is precisely the direction of thermodynamic time. This law explains the directionality of time we experience in our everyday world without contradicting reality.
At times, the Second Law of Thermodynamics may seem problematic. It appears to contradict the theory of evolution, which posits that life forms emerge and evolve into ordered organisms. This is because evolution views simpler life forms evolving into more complex ones, implying an increase in the degree of order. Regarding this apparent contradiction, Ilya Romanovich Prigogine demonstrated that order can emerge from disorder, thereby explaining how evolutionary theory and the Second Law of Thermodynamics can coexist. In other words, nature does not only contain processes aiming for thermal equilibrium—the state of maximum entropy—but can also exhibit non-equilibrium phenomena that minimize entropy increase. In other words, while the entire natural world undoubtedly progresses toward thermal equilibrium, non-equilibrium states can occur within specific spacetime regions.
For instance, when an ink droplet is dropped into water, the final state becomes a pale, uniform equilibrium. However, observing the process reveals the patterns and structures created as the ink spreads. This is precisely an example of a non-equilibrium state that temporarily emerges within the water. Evolutionary theory is also seen as a phenomenon corresponding to this process where non-equilibrium states persist. Explained this way, the Second Law of Thermodynamics can coexist without contradiction with evolutionary theory while effectively explaining the directionality of everyday time. Furthermore, this aspect of the second law of thermodynamics suggests that the directionality of time does not solely follow entropy increase; locally, order and complexity can increase. This aligns with various natural phenomena occurring around us and plays a crucial role in understanding the complexity of life and evolution.
But what happens if we extend this Second Law to the entire universe? Ultimately, the universe will progress from a state of low entropy to a state of high entropy and disorder. If this process of entropy increase continues indefinitely, the universe will reach a state of maximum entropy—a state called a heat death, where all usable energy is completely dissipated and no further activity occurs. This state of a heat death represents the ultimate endpoint of time. However, this interpretation fails to account for the gravitational force acting during the universe’s expansion process. Therefore, it remains merely a hypothesis and does not accurately describe the actual time of the universe.
Similarly, the Second Law of Thermodynamics holds explanatory power only within the everyday world and fails to adequately explain the directionality of time applicable to the entire universe. Likewise, Isaac Newton’s laws or the theory of relativity, as explained earlier, also cannot explain the directionality of cosmological time. The concept of time is far more complex than what we experience in daily life, requiring deeper research and understanding of its essence. To arrive at a true explanation of the directionality of time, a unified theory is needed that can simultaneously account for both the directionality of everyday time and the directionality of time applicable to the entire universe. Developing such a theory is a major challenge facing modern science.
