Time, the Eternal Question: A Mystery Pervading the Universe and Life
The astonishing advancements in modern medicine have extended human life expectancy, once struggling to reach 50 years, to a natural '100-year era.' However, no matter how long human life becomes, within the vast cosmic time of 4.6 billion years since Earth's birth, humanity's existence is literally but a 'fleeting moment.' If we were to compress Earth's 4.6-billion-year history into a single year, the first human ancestors would have appeared around 1:20 PM on December 31st, and modern humans would have emerged in Africa around 11:40 PM on the same day. Agriculture began just one minute before midnight, and Columbus discovered the New World a mere three seconds before midnight. A human life of a hundred years amounts to less than one second on this cosmic clock.
This stark contrast highlights the immense gap between human finitude and cosmic infinitude, posing fundamental questions about the nature of time. Stephen Hawking's analogy of humanity as "advanced fungi that briefly flourished and then disappeared on Earth" makes us reconsider the brevity of our perceived time and the meaning of cosmic existence. This contemplation leads to the essential questions: "What exactly is time? Where does time come from, and where does it go?" Even the ancient philosopher Augustine confessed, "What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know," illustrating the profound difficulty in exploring the essence of time.
This awareness of human existence as an extremely finite moment within the infinite expanse of cosmic time evokes a deep sense of awe, coupled with the instinctive human fear that compels us to postpone death as much as possible. This existential tension becomes a fundamental driving force for humanity to continuously search for the meaning and place of its existence. This report aims to invite readers on a contemplative journey that transcends a mere enumeration of scientific facts about time, delving into these fundamental human conditions.
Part 1: The Measurement of Time and the Dance of the Universe: The Astronomical Clock
1.1 Celestial Motion and the Precision of Time: The Science of Leap Years, Intercalary Months, and Leap Seconds
Our everyday units of time originated from the concept of quantifying changes in space. A year is based on the time it takes for Earth to orbit the Sun once, a day on Earth's rotation, and a month on the Moon's orbit around Earth. These units are the product of humanity's efforts to observe natural phenomena and impose order upon them.
However, despite our time units being based on celestial motion, precise adjustments are necessary because these motions do not perfectly align with integer units. Earth's exact orbital period is 365 days, 5 hours, 48 minutes, and 46 seconds, or approximately 365.2422 days, which is slightly longer than 365 days. To compensate for this discrepancy, a 'leap year' is introduced every four years by adding February 29th, aligning the calendar with celestial time. Without such adjustments, over a long period, seasons and the calendar would drift, leading to situations like swimming in January or ice in August. Furthermore, to correct this error even more precisely, a complex rule of having 97 leap years every 400 years is applied, which sometimes results in a leap year occurring only once every eight years.
In the lunar calendar, since the Moon's exact orbital period (sidereal month ~27.32 days, synodic month ~29.5 days) is not an integer, an 'intercalary month' is inserted approximately seven times every 19 years to harmonize the seasons and dates. Thus, the measurement of time is not merely a reflection of natural phenomena but also involves human intervention and adjustment. Because natural cycles do not perfectly align with the neat integer units set by humans, we strive to perfect these imperfections through precise calculations and corrections. This demonstrates that time is not simply an absolute given from external sources, but a concept intricately woven from natural phenomena and human intellectual activity. It reveals that time possesses a dual nature: an objective physical quantity and a concept understood within an artificial human framework.
A day is based on Earth's rotation period, but because Earth also orbits the Sun while rotating, a 'solar day' (based on sunrise and sunset) is about 4 minutes longer than a 'sidereal day' (23 hours, 56 minutes, 4 seconds), resulting in 24 hours. In modern times, since 1967, the concept of an 'atomic clock' has been introduced, defining 1 second as 9,192,631,770 oscillations of a cesium atom, which has dramatically increased the precision of time measurement. Atomic clocks remain constant regardless of Earth's movements. However, Earth's rotation speed is gradually slowing down due to the Moon's tidal forces, requiring a 'leap second' to be added every few years to synchronize the atomic clock with actual celestial time.
The shift from celestial observation to atomic clocks signifies a move in the definition of time from natural phenomena to the controllable microscopic world, yet the continued need for alignment with macroscopic celestial motion suggests that time is an entity constantly redefined and refined through the interaction of nature and humanity. This reveals that time possesses a dual nature: an objective physical quantity and a concept understood within an artificial human framework.
1.2 The Moon's Recession and Earth's Rotation: Time's Evolution Shaped by Cosmic Interactions
The Moon's gravity exerts 'tidal forces' on Earth's oceans, and these forces create friction against Earth's rotation. This friction causes Earth's rotation speed to gradually slow down each year. According to the 'law of conservation of angular momentum,' as Earth's rotation slows, the Moon progressively recedes from Earth. Currently, the Moon is moving away from Earth at a rate of about 3.8 cm per year.
Calculations based on the current rate of lunar recession do not match the Moon's actual distance (approximately 380,000 km) from Earth since its formation 4.6 billion years ago. This implies that when the Moon was much closer to early Earth, its gravitational pull was stronger, leading to more intense tidal forces, and consequently, a much faster decrease in Earth's rotation speed and a more rapid recession of the Moon than at present. It is an intriguing inference that a day on early Earth might have been only about 6 hours, not the current 24 hours.
These interactions will continue into the future. It is predicted that in about 7.5 billion years, Earth's rotation period and the Moon's orbital period will synchronize, leading to a 'tidal locking' state where the Moon will always show only one face to Earth. This is similar to the current state of Pluto and its moon Charon, which always face each other.
Furthermore, phenomena observed on other moons in our solar system, such as Jupiter's moon Io, which exhibits massive volcanic activity due to intense tidal forces from Jupiter's gravity, or the possibility of life existing beneath the ice-covered ocean of Europa, demonstrate the infinite diversity of time and planetary interactions in the cosmos.
If a moon approaches a planet too closely and crosses the 'Roche limit,' the moon can be torn apart, forming a ring system around the planet, similar to Saturn's rings. It is suggested that early Earth might also have had such a ring composed of dust and small rocks.
These phenomena illustrate that the universe is not a static and isolated entity but a dynamic system constantly interacting and changing. The length of our basic unit of time, the 'day,' has dramatically changed throughout Earth's history and will continue to do so. If other celestial bodies experience different 'times' due to their gravitational interactions, then time itself is not a fixed external background. Rather, time can be understood as a result and attribute of continuous dynamic interactions within the universe.
This suggests that time is not merely measured by celestial mechanics but is shaped by it. This perspective foreshadows Einstein's concept of spacetime, where spacetime itself is warped and deformed by matter and energy. Time is no longer a passive background but a dynamic dimension actively participating in the evolution of the universe.
Part 2: Einstein's Spacetime: The World of Relative Time
2.1 Time is Not Absolute: The Revolution of Relativity Theory
Isaac Newton believed that time flowed uniformly everywhere, an 'absolute time.' In his work Principia, he stated, "Absolute, true, and mathematical time, of itself, and from its own nature, flows equably without relation to anything external," considering it to exist independently of the existence or change of objects.
Einstein believed that time was merely the 'order' in which events occurred.
The theory of relativity predicts two main time dilation effects. First, time flows more slowly in a rapidly moving inertial frame. Second, time flows more slowly in a strong gravitational field than in a weak one. This is because gravity itself warps spacetime.
The relativity of time implies that time is not a universal and absolute constant. If time is relative, it means that time is intrinsically intertwined with matter, energy, and motion, rather than simply being a backdrop for events. The concept of 'spacetime' emphasizes that space and time are an inseparable, integrated entity that dynamically changes.
2.2 Relativity in Everyday Life: GPS and the Evidence of Time Correction
GPS (Global Positioning System), an essential component of modern civilization, is the clearest example of Einstein's theory of relativity applied in real technology. More than 24 satellites orbiting approximately 20,000 km above Earth transmit their position and transmission time data to the ground. A GPS receiver on the ground calculates the distance to the satellite by multiplying the difference in transmission and reception times by the speed of light (approximately 300,000 km per second) and then determines its precise location by receiving data from at least three satellites.
GPS satellites move at a very high speed of 14,000 km/h, causing time to slow down by approximately 7 microseconds (7 millionths of a second) per day according to special relativity. Simultaneously, because the satellites are in space where gravity is weaker than on Earth, time speeds up by approximately 45 microseconds per day according to general relativity. Combining these two effects, time on the satellites runs approximately 38 microseconds (45-7=38) faster per day than on the ground. If this tiny time difference were not corrected, the GPS would show an error of a staggering 11 km in position within just one day, rendering it useless.
The fact that GPS systems must correct for relativistic time dilation to function accurately is a clear and practical demonstration of Einstein's theory. This is more than just a verification of a theory; it is a profound statement about the predictive power of fundamental physics. The fact that an abstract theory, developed over a century ago and seemingly detached from everyday experience, is now absolutely essential for ubiquitous modern technology, illustrates how theoretical breakthroughs can lead to immense practical consequences. It bridges the gap between abstract physics and concrete reality. Furthermore, if such counter-intuitive concepts work so demonstrably in reality, it suggests that our everyday perception of time may be fundamentally limited.
2.3 The Possibility of Time Travel: Journeys to the Future, Dreams of the Past
According to Einstein's theory of relativity, time travel to the future is theoretically possible by moving at speeds close to light or by staying near a strong gravitational field, which causes time to dilate. Indeed, Russian cosmonaut Sergei Avdeyev, who spent 748 days in orbit on the space station, traveled approximately 0.02 seconds into the future compared to Earth time.
A hypothetical scenario suggests that if a spaceship were to travel to a star 500 light-years away at near-light speed (99.995% of the speed of light) and return, only 10 years would pass on the spaceship, but 1000 years would have passed on Earth, effectively allowing a journey 1000 years into the future. The movie 'Interstellar' dramatically illustrates this time dilation effect, with the protagonist traveling into the future due to the strong gravity of a black hole.
However, time travel to the past is currently considered impossible by science. According to Einstein's special theory of relativity, no object can travel faster than light, which is a fundamental constraint preventing direct time travel to the past. Furthermore, even traveling at near-light speeds is currently impossible with modern technology, requiring a rocket using matter and antimatter 4000 times more powerful than the Saturn V rocket that launched Apollo 11. Antimatter is incredibly difficult to produce, costing approximately 100 trillion dollars per gram. Time travel using black hole gravity also remains largely in the realm of science fiction for now.
Theoretically, if a 'wormhole' were to connect two points in spacetime warped by immense gravity, time travel to the past might be possible. A wormhole is a shortcut connecting two distant points in spacetime, a concept proposed by Professor Kip Thorne, who served as a scientific advisor for the movie 'Interstellar.
The complex principle involves moving one entrance of the wormhole at near-light speed to induce time dilation, then entering through the other entrance and returning through the original, which would lead to a point in time earlier than the departure. However, maintaining an open wormhole would require exotic matter with negative energy density and negative gravity (mass less than zero), which is currently a hypothetical substance.
Time travel, especially to the past, stimulates scientific imagination but faces immense practical and theoretical limitations in reality. Discussions about wormholes and 'exotic matter' naturally lead to the forefront of physics, particularly in the field of quantum gravity. The 'ER=EPR' conjecture posits a connection between quantum entanglement and wormholes, suggesting that every pair of entangled particles might be connected by a microscopic wormhole.
The idea that spacetime can 'emerge' from quantum entanglement transforms the impossibility of time into a challenge for a unifying theory, opening up the possibility that the structure of time and space could be built from quantum information. Such research opens the door to revolutionary technologies like future gravitational wave communication, wormhole generation, and time manipulation.
Part 3: Questioning the Nature of Time: Philosophy, Quantum, and Perception
3.1 Does Time Exist?: Diverse Philosophical Perspectives
Time is defined as 'a concept for perceiving the change of things' and 'occurs in a clearly irreversible continuum from past, present, to future.' This emphasizes that time is an aspect of matter itself, inextricably linked to the sequentiality of material motion.
Views on Time by Ancient and Medieval Philosophers:
Plato: Time was merely an imperfect 'imitation' of the true essence, the Forms, representing the imperfection of the sensible world where true purity of essence could only be realized in the realm of Forms, free from change and motion.
Augustine: Explored numerous paradoxes and problems of time's nature, arguing that humans' ability to divide and understand time stems from the 'mind (soul)' that can extract finite time from eternity.
Aquinas: Introduced a unique view of time to explain how the world was created by God, stating that the world was created 'with' time, not 'from' time. God is a being beyond time and space, while humans are confined within them.
Views on Time by Modern Scientists:
Isaac Newton: Argued that time, along with space, was an 'absolute time' that flowed uniformly and unchangeably everywhere and always. He believed it existed independently of the existence or change of objects.
Leibniz: Opposed Newton's absolute view of time, arguing that time arises as a result of the 'order' and 'relations' in which things come into being. That is, time is not an independent entity but a concept derived from the relationships between events.
Modern Philosophical Debates: Presentism vs. Eternalism: With the advancement of modern physics, philosophical debates on the reality of time have intensified.
Presentism: Only the 'present' is ontologically meaningful; the past no longer exists, and the future has not yet arrived. Time flows as the present continuously moves into the future.
Eternalism: The past, present, and future are fundamentally indistinguishable, differing only from the observer's perspective, and all exist equally. The flow of time is considered an illusion or a delusion, explainable by the 'Minkowski spacetime' concept in Einstein's special theory of relativity, which views spacetime as a single 4-dimensional block. Quantum mechanics also tends to support eternalism.
The Arrow of Time: Entropy and the Second Law of Thermodynamics: The phenomenon of time always flowing in one direction is called the 'arrow of time,' and it is closely linked to the 'Second Law of Thermodynamics,' which states that 'entropy' (disorder) tends to increase.
Entropy is a matter of probability; randomness tends to scatter ordered things and mix classified ones, which is the fundamental reason why time has a direction.
These contradictory perspectives on the flow of time reveal a fndamental philosophical division: whether time is a fundamental aspect of reality or a construct of our minds or measurements. The 'arrow of time,' linked to entropy and the Second Law of Thermodynamics, provides a physical basis for the directionality of time in macroscopic systems, suggesting a real, irreversible flow. However, quantum mechanics challenges this at the microscopic level, even implying that future events can influence the past. This leads to a profound question: is what we perceive as "flow" a phenomenon arising from statistical mechanics (entropy), or is it a fundamental property that disappears at the quantum level? If flow is emergent, it is not truly fundamental but a consequence of increasing disorder, a statistical tendency rather than a strict law in all cases. This suggests that our perception of the flow of time may be a macroscopic illusion or a statistical reality, and not an absolute truth.
Table 1: Comparison of Major Philosophical/Scientific Perspectives on Time
3.2 Time in the Quantum World: The Realm of Uncertainty and Probability
While time in classical mechanics was treated as a classical background parameter external to the physical system itself, the concept of time becomes far more complex in quantum mechanics. In the standard formulation of quantum mechanics, time holds a special status and is not a 'measurable (observable)' quantity. That is, there is no direct measurement of 'what' time is in quantum mechanics.
Carlo Rovelli, a prominent scholar in quantum gravity, argues in his book The Order of Time that the flow of time we experience may not be a fundamental property of natural laws. He presents five characteristics of time: absoluteness, invariance, discreteness, irreversibility, and delay, and states that the concepts of past, present, and future are not universal and can vary depending on the observer's location.
At the forefront of modern physics, intriguing hypotheses are being proposed about a deep connection between quantum entanglement and the structure of spacetime. The 'ER=EPR' conjecture is the idea that quantum entanglement is connected by an Einstein-Rosen bridge, or wormhole, suggesting that entangled particle pairs might be linked by microscopic wormholes.
These studies confirm that quantum mechanics struggles to deal with time, even to the point of not considering time a directly observable quantity. Carlo Rovelli's assertion that "time does not flow" further deepens this inquiry. If time is an 'illusion' or 'emerges' from more fundamental quantum phenomena like quantum entanglement, then our entire experience of reality, structured into past, present, and future, has profound philosophical implications: it is based on something that is not a fundamental component of the universe.
This implies that our subjective reality may be fundamentally different from objective reality. Furthermore, it shows that the 'problem of time' in physics is one of the biggest challenges, implying that a unified theory of quantum gravity is essential to truly understand time. That is, the quest for the nature of time is directly linked to the biggest unsolved problems in physics, and it has the potential to fundamentally change our ontological understanding.
3.3 Time in Perception: Subjective Awareness in Humans and Animals
Just as a day can feel long or short to us, time perception is highly subjective. Various factors such as drugs, age, and individual personality influence time perception.
Not only humans but also animals possess the ability to perceive time. Dogs cannot read clocks, but they perceive time through natural cues such as circadian rhythms, changes in scent intensity, and 'episodic memory.'
For instance, experiments show rats approaching food precisely after 7 seconds when rewarded every 7 seconds on a treadmill, or chimpanzees preparing to throw stones at specific times based on past experiences of being teased by visitors. These behaviors are presented as clues to episodic memory.
The subjectivity of time perception observed in both humans and animals suggests that the 'flow of time' may be a biological or psychological construct rather than an objective, universal property. The complex processing in the brain, influenced by attention, emotion, and memory, means that our internal clock is not a perfect mirror reflecting external physical time.
This challenges the notion of a universally experienced linear flow of time, suggesting that our 'present' is a reality dynamically constructed according to our biological and cognitive frameworks. This perspective emphasizes that time is deeply intertwined with consciousness and perception, adding another layer to the multifaceted nature of time.
3.4 Cosmic Observation and the Past Connection: The Speed of Light and the Veil of Spacetime
Due to the finite speed of light (approximately 300,000 km per second), observing distant parts of the universe is equivalent to observing their distant past.
Turning the observer's direction around allows for even more intriguing imagination. If someone on the star Sirius were looking at you on Earth right now, they would be seeing your appearance from about 9 years ago.
If someone on the North Star were looking at Earth right now, they might be curiously watching Yi Seong-gye's Wihwado Retreat. If someone at the edge of the Andromeda Galaxy were looking at us right now, they would be observing the process of our ancestors evolving from Australopithecus to Homo habilis.
If someone in the NGC4845 galaxy, 65 million light-years from Earth, were observing Earth right now, they might be fascinated by the extinction of dinosaurs due to an asteroid impact.
Furthermore, if someone in the galaxy cluster 'SDSS J1038+4849,' located 4.6 billion light-years from Earth, were observing Earth right now, they might be enjoying the secret of planetary formation by watching the early Earth, which was just forming by absorbing dust and rocks around the Sun, which was born only 400 million years ago.
Thus, when we observe the distant universe, we are observing its distant past, and when they observe us from there, they are observing our past. Beings in different spaces at the same time can only see each other's past, not their present. This is because the distances are too vast for light or radio waves to transmit information instantly. In fact, strictly speaking, if you are talking to a friend face-to-face right now, you are seeing their appearance from a very, very tiny fraction of a second ago.
This means that due to the finite speed of light, the act of observing distant places inherently becomes the act of observing the past. This is not merely a technological limitation but a fundamental property of spacetime. It implies that space and time are inextricably linked, and our 'present' is not a universal, simultaneous slice of reality but a localized experience within a dynamic, interconnected cosmic fabric. As spatial distance increases, the time connected to that space also links to a more distant past.
Based on these characteristics of spacetime, the flow of time can be expressed through the following analogy: the past is the transparent realm where the veil has been lifted by time up to the present, and the future is the unseen realm veiled by a translucent curtain that time has yet to unveil, while the present is the surface of the veil currently being lifted.
This analogy beautifully integrates the objective reality of observation based on the speed of light, the subjective experience of time's unidirectional flow, and the mystery of the future. The past is where light has already reached us, 'lifting the veil,' while the future remains veiled because that light (information) has not yet reached our present location. This emphasizes that time is not an independent entity but an intrinsic dimension of the universe, shaping how we perceive and interact with reality.
Conclusion: The Mystery of Time, and Our Existence
The exploration of time is a wondrous journey that begins with the finite human lifespan and extends to the vast history of the universe and the uncertainty of the microscopic world. We have confirmed that time is measured and corrected through precise astronomical calculations, and that cosmic interactions like the Moon's tidal forces have altered the very flow of time.
Einstein's theory of relativity revealed that time is not absolute but a dimension of 4-dimensional spacetime that flows relatively depending on the observer's speed and gravity, and modern technologies like GPS provide practical evidence for this theory. Time travel to the future is theoretically possible, but time travel to the past remains in the realm of imagination due to the speed of light limit and the need for wormholes and exotic matter.
Philosophically, there has been a long-standing debate about whether time is real or a product of perception. Newton's absolute time, Leibniz's relational time, and modern presentism and eternalism offer diverse perspectives on the nature of time. In particular, the Second Law of Thermodynamics and the increase in entropy provide a physical explanation for the 'arrow of time,' which flows in only one direction. However, in the quantum world, time is not a measurable quantity and exists in the realm of uncertainty and probability.
Scholars like Carlo Rovelli argue that time may not be a fundamental property but an emergent phenomenon, and recent research on quantum entanglement and spacetime connections suggests the possibility that time and space could arise from a more fundamental network of quantum information.
In conclusion, time is a precisely measured astronomical phenomenon, a relative dimension warped by gravity and speed, a probabilistic concept in the quantum realm, and a subjective experience shaped by biological and conscious processes. Despite these complex characteristics, the fundamental question, "What is time?" remains an unsolved problem, especially at the quantum level.
This multifaceted understanding of time makes us realize that our human experience is but one facet of a much larger and more mysterious cosmic reality. We live in a present where the veil of time is lifted every moment, with the past existing as a transparent realm already unveiled, and the future as a translucent realm yet to be revealed. While the ultimate reality of time remains a question, it deepens our reflection on the value of the 'now'—the moment we experience as finite beings in this universe.
