Time, a ubiquitous yet enigmatic phenomenon, governs every aspect of existence, from the biological rhythms of life to the dynamics of the cosmos. Ancient philosophers like Aristotle viewed time as a measure of change, tied to the motion of celestial spheres, while Sir Isaac Newton championed the notion of absolute time- a steady, universal flow independent of external factors. However, the advent of Albert Einstein’s theories of relativity in the early 20th century revolutionized this understanding, revealing time to be a malleable, context-dependent dimension intertwined with space. Time is a complex and multifaceted concept that has been defined and interpreted differently within the realms of philosophy and science. These definitions offer distinct yet complementary perspectives, reflecting the diverse ways in which humans perceive and measure this fundamental dimension of existence. Aristotle, in his work Physics, defined time as “the number of motion in respect to before and after.” This view links time intrinsically to motion and change, emphasizing its role as a relational measure rather than an independent entity. Immanuel Kant proposed that time is not an external reality but a form of intuition inherent to human perception. According to Kant, time and space are the lenses through which we perceive the world, not properties of the world itself.
In science, time is defined in precise and operational terms, often tied to measurable phenomena. It serves as a fundamental dimension in describing the universe’s physical processes.
Time as a Dimension
In classical mechanics, time is a scalar quantity, progressing linearly and uniformly, independent of the observer. This conception was foundational in Sir Isaac Newton’s framework, where time flows uniformly as an “absolute” entity.
Time in Relativity
Albert Einstein revolutionized the scientific definition of time through his theories of relativity:
- In Special Relativity, time is not absolute but relative, depending on the observer’s velocity. Time dilation demonstrates that time passes more slowly for objects moving at high speeds relative to a stationary observer.
- In General Relativity, time is intertwined with space, forming a four-dimensional spacetime continuum. Massive objects curve spacetime, affecting the passage of time- a phenomenon known as gravitational time dilation.
Time as a Measurable Quantity
In physics, time is quantified using intervals and standard units, such as seconds. The most precise measurement of time today is based on atomic clocks, which rely on the vibrations of cesium atoms.
Thermodynamic Time and Entropy
Time in thermodynamics is associated with the arrow of time, characterized by the increase of entropy, or disorder, in a closed system. This aligns with the second law of thermodynamics, marking time’s unidirectional flow.
Quantum Time
In quantum mechanics, time plays a peculiar role. It is often treated as a parameter rather than a dynamic variable, raising questions about whether it is continuous or quantized. Theories attempting to unify quantum mechanics and relativity, such as quantum gravity, explore the fundamental nature of time.
Time dilation
In 1971, physicists Hafele and Keating placed atomic clocks on airplanes flying eastward and westward. Clocks on the eastbound flight (moving faster relative to Earth’s rotation) ticked slower, while those on the westbound flight ticked faster. The results matched the predictions of both special and general relativity. This was the fundamental experiment that gave us a lot of information regarding the relativity of time. In general, there are various examples of time dilation. Some of these are.
Time dilation in special relativity
Time dilation is the phenomenon where time slows down for an object moving at a high velocity relative to a stationary observer. Imagine twin siblings, one of whom stays on Earth while the other travels on a spaceship at near-light speeds. Upon returning, the traveling twin would have aged less than the twin who stayed on Earth. This is because the moving twin’s clock runs slower due to their high velocity, illustrating time dilation. While the paradox itself is hypothetical, it is grounded in experimental evidence, such as the behavior of fast-moving particles in particle accelerators.
Gravitational time dilation in general relativity
Time runs slower in stronger gravitational fields, a phenomenon known as gravitational time dilation. GPS satellites orbit Earth at high altitudes, experiencing weaker gravity than clocks on Earth’s surface. Satellite clocks tick slightly faster compared to clocks on Earth. GPS systems incorporate these time differences to maintain accuracy in location calculations, demonstrating the practical application of time relativity.
Muon decay and time dilation
Muons are subatomic particles that form when cosmic rays collide with Earth’s atmosphere. They provide direct evidence of time dilation. Muons, moving close to the speed of light, have a much longer lifespan when observed from Earth compared to their intrinsic decay time. Time dilation slows the decay process as observed from Earth’s reference frame, allowing muons to reach the surface despite their brief existence.
Black holes and gravitational time dilation
Near the event horizon of a black hole, time slows significantly compared to distant observers. In the movie Interstellar, astronauts orbiting near a massive black hole experience hours, while years pass on Earth. The intense gravitational field near the black hole warps spacetime, causing time to move more slowly.
Particle accelerators
Particles accelerated to near-light speeds in accelerators exhibit longer lifetimes due to time dilation. Subatomic particles, like pions or kaons, traveling at relativistic speeds decay more slowly when measured in the lab frame. This is a fundamental principle used in high-energy physics experiments to study particles’ properties.
Astronomical observations
Light passing near massive objects, like galaxy clusters, takes longer to reach Earth compared to light traveling through “empty” space. The curvature of spacetime around massive objects slows the passage of time for the light in transit, illustrating gravitational time dilation.
Time in hindu cosmology
Hindu texts describe a cosmic framework where time operates on vastly different scales depending on the realm or dimension. Hindu cosmology divides time into recurring cycles called Yugas (ages), which are part of a larger cycle known as a Kalpa (day of Brahma).
Satya Yuga (Golden Age): 1.728 million years.
Treta Yuga: 1.296 million years.
Dvapara Yuga: 864,000 years.
Kali Yuga (Current Age): 432,000 years.
One complete cycle of the four Yugas equals a Mahayuga (4.32 million years). A thousand Mahayugas make up a single day of Brahma, followed by a night of equal duration. A single day of Brahma, the creator deity, equals 4.32 billion human years. Time for celestial beings, such as the Devas (gods), moves slower than for humans. For instance, one day in the realm of Indra, king of the gods, equals one year on Earth. This relativity of time across different realms resembles the concept of time dilation in Einstein’s relativity, where time flows differently for observers depending on their frame of reference.
Examples of relativity of time from hindu mythology
Hindu mythology is replete with stories illustrating the relativity of time, emphasizing the varying perceptions of its flow.
The story of king Kakudmi
King Kakudmi, accompanied by his daughter Revati, visits Lord Brahma to seek advice on a suitable husband for her. However, upon returning to Earth, Kakudmi finds that thousands of years have passed, and the world has changed entirely. Time moves much faster on Earth compared to Brahmaloka (the abode of Brahma), illustrating the relativity of time between dimensions.
The Mahabharata and the Chakravyuha
Abhimanyu, the son of Arjuna, learns the strategy to enter the Chakravyuha (a battle formation) while in his mother’s womb. However, before the explanation of exiting the formation is complete, the speaker is interrupted. This reflects the idea of memory and awareness spanning across different temporal states, hinting at non-linear perceptions of time.
The Ramayana: Hanuman’s Journey
When Hanuman leaps to the sun to find the herb Sanjeevani, the story subtly indicates the relativistic nature of his journey, where the speed and intensity of his actions transcend normal temporal experience.
Hindu mythology offers a nuanced and timeless perspective on the relativity of time, blending philosophy, spirituality, and cosmology. Stories like King Kakudmi’s journey to Brahmaloka and the cyclical nature of Yugas illustrate a deep understanding of time’s multifaceted nature, paralleling modern scientific discoveries. By exploring time as both a measurable reality and a metaphysical concept, Hinduism bridges the gap between the temporal and the eternal, providing a holistic framework for contemplating existence.
Time in quantum physics
Time is a cornerstone of both relativity theory and quantum physics, yet these two frameworks conceptualize time in fundamentally different ways. Reconciling these differences is one of the greatest challenges in modern theoretical physics. While Einstein’s relativity treats time as a flexible dimension intertwined with space, quantum mechanics views time as a fixed backdrop for probabilistic events. These differing treatments raise profound questions and challenges when attempting to unify the two into a comprehensive theory of quantum gravity. In quantum mechanics, time has a markedly different role:
Fixed and Absolute: Time is treated as an external parameter, not a dynamic variable. It provides a fixed stage on which quantum states evolve.
Schrödinger Equation: The evolution of a quantum system is described by the Schrödinger equation, where time appears as a background variable, similar to Newtonian mechanics.
Uncertainty Principle: While position and momentum, or energy and time, have inherent uncertainties, time itself is not treated as an operator or observable in standard quantum mechanics.
However, it should be noted here that general relativity and quantum mechanics have incompatible treatments of time. In relativity, time is a dynamic entity; in quantum mechanics, it is an immutable background. In attempts to quantize gravity, such as in the Wheeler-DeWitt equation, time disappears from the equations, creating the “problem of time.” Some quantum gravity approaches suggest the universe is fundamentally timeless, and time emerges as an effective concept in certain contexts.
Depictions of time in films
Movie, Interstellar (2014), directed by Christopher Nolan was based on gravitational time dilation and the relativity of time. Astronauts venture to a distant galaxy via a wormhole to find a habitable planet. The mission included visiting a planet near a massive black hole, where time moves slower due to gravitational time dilation. On this planet, one hour equals seven years on Earth, leading to emotional scenes where crew members grapple with the passage of time for their loved ones. The depiction of gravitational time dilation aligns with Einstein’s theory of general relativity. Physicist Kip Thorne, a consultant for the film, ensured the science was accurate, including visualizing the black hole (Gargantua). In this movie, time becomes a bridge between science and human connection, symbolized by love transcending temporal boundaries.
Arrival (2016), directed by Denis Villeneuve, was based on non-linear time and linguistic relativity. The story follows linguist Louise Banks as she learns to communicate with an alien species (Heptapods). The aliens perceive time non-linearly, a perspective Louise adopts as she learns their language. Her newfound understanding allows her to see her life’s events, including past and future, as a cohesive whole. The concept of movies was inspired by the Sapir-Whorf hypothesis, which suggests that language shapes thought and perception. It challenges linear perceptions of time, presenting it as a holistic, interconnected phenomenon.
The movie Tenet (2020), directed by Christopher Nolan, was based on temporal inversion of time. Characters manipulate the flow of time by inverting their entropy, enabling them to move backward through time. The film challenges linear storytelling, presenting events from multiple temporal perspectives. Although the concept of entropy inversion is speculative, the film builds on physics principles like the arrow of time. The film uses complex timelines to engage the audience, making time itself a character in the story.
The arrow of time and entropy
The arrow of time refers to the one-way direction or asymmetry of time, distinguishing the past from the future. In our everyday experience, time flows forward—we remember the past but cannot predict the future with certainty. This phenomenon is deeply linked to the concept of entropy, a measure of disorder in a system, as described in the second law of thermodynamics. Understanding the arrow of time involves exploring its relationship with entropy, as well as its implications in physics and philosophy.
Coined by physicist Arthur Eddington in 1927, the arrow of time signifies the unidirectional flow of time, observed in various physical processes and phenomena. While fundamental laws of physics, such as Newtonian mechanics, Maxwell’s equations, and even quantum mechanics, are time-symmetric (they work equally well if time is reversed), the arrow of time emerges from macroscopic phenomena governed by the second law of thermodynamics.
Entropy is a measure of the number of possible microscopic configurations of a system’s components that correspond to its macroscopic state. Higher entropy corresponds to greater disorder. Entropy tends to increase in an isolated system, as systems evolve from less probable (ordered) states to more probable (disordered) ones. The second law of thermodynamics states that in any thermodynamic process, the total entropy of an isolated system will either increase or remain constant; it never decreases. This principle establishes the arrow of time, as processes like heat flow (from hot to cold) or the mixing of substances are irreversible in practice.
Time remains a profound enigma at the intersection of quantum physics and relativity. While relativity views time as a malleable dimension woven into the fabric of spacetime, quantum physics treats it as a static parameter. Bridging these contrasting views is key to developing a unified theory of quantum gravity. Emerging theories and experiments continue to push the boundaries of our understanding, offering glimpses into the true nature of time and the universe.
