Time is ubiquitous: it governs our daily lives, marking our existence from birth to death. We measure time in seconds, minutes, hours, days or years, using man-made tools like clocks and calendars which reinforce the perception that it is tangible and objective. In fact, the most used noun in English is time (1). However, delving into the realms of science and philosophy, the true nature of time becomes illusionary. 

 

We can acknowledge our personal perception of time is inherently subjective. Our experiences of time vary depending on our surroundings, emotional state and physical state. For example, while time may seem to drag on when we're bored or anxious, it can pass quickly when we're having a good time.  

 

Although we imagine time to be objective, it could be merely an illusion resulting from the limitations of our perceptions and the conditions of our observation. Exploring these questions requires scientific perspectives, so let's delve into the enigmatic physics of time.

 

In three-dimensional space, physical spaces are fixed, meaning that we can revisit the same location repeatedly. For example, we may visit our favourite restaurant as many times as we wish. However, this is not the case with time. Time only moves forward, and we cannot go back to a previous moment; it belongs to the past and cannot be retrieved (2). This unidirectional nature of time is referred to as the arrow of time.

 

Time is believed to originate from the Big Bang, the event that marked the beginning of the universe (3). From that point, time has progressed towards the present, where you are currently reading this article, and it continues to move into the future. The second law of thermodynamics, known as entropy, plays a crucial role in representing the forward movement of this arrow of time (4).

 

Entropy refers to the state of disorder, uncertainty, or randomness in a system like a measure of the disorder present in the universe. At the moment of the Big Bang, the universe had low entropy, with matter and energy concentrated and organised. However, since that initial state, matter in the universe has been expanding and moving away from each other, leading to an increase in entropy and transforming the universe into a high entropy system.

 

The concepts of the arrow of time and entropy, guided by the second law of thermodynamics, allow for a distinction between the past and the future and play a pivotal role in the existence of life. Without entropy and the resulting change there would be no discernible difference between events that occurred 1000 years ago and events happening in the present. Furthermore, the progression of life from birth to death can be explained through the phenomenon of entropy, as governed by the second law.

 

However, on the quantum level, the behaviour of particles becomes more complex. Just as there is no inherent forward or backward direction in vast space, at the molecular level, the concept of entropy is not as apparent. While time appears to have a clear direction on the macroscopic level, when observing the particles that make up the universe, time can flow and operate in multiple directions. The laws of physics that govern these particles do not distinguish between the past and the future. They describe the behaviours of physical systems without differentiating between temporal directions.

 

The theory of general relativity, proposed by Albert Einstein, provides a fundamental framework for understanding the workings of spacetime (5). According to the theory of general relativity, the presence of mass or energy causes a distortion in the fabric of spacetime, which in turn affects the motion of other objects. For example, it describes gravity as the curvature of spacetime caused by the presence of mass and energy.  Essentially, spacetime can be thought of as a fluid that is influenced by both gravity and velocity. This theory has illuminated not just the behaviour of celestial bodies and the vast structure of the universe, but also enhanced our understanding of the intricate interplay between space, time, and matter.

 

Within Einstein’s theories, time dilation is a scientific phenomenon that can be explored through a thought experiment known as the twin paradox (6). It demonstrates how the perception of time can vary between two individuals who experience different levels of motion or gravitational forces. Time dilation is not limited to the twin paradox or space travel; it is a fundamental concept in understanding the relationship between time, motion, and gravity. It has been experimentally confirmed and plays a significant role in our understanding of the universe.

 

Imagine you, Twin A, are stationary on Earth while your sister, Twin B, is traveling in a rocket at a constant speed. Due to the sideways motion of the rocket, Twin B’s clock will appear slower to Twin A since her path through spacetime is longer due to the effects of special relativity and time dilation. Therefore, from Twin A’s perspective on Earth, time seems to pass slower on the moving rocket. 

 

However, from Twin B’s perspective, Twin A is the one in motion and therefore Twin A’s clock appears slower to her. Both frames of reference seem to indicate that the other's clock is slower, which seems contradictory. In reality, both observations are correct because the laws of physics remain the same in both frames of reference.

 

Now, the question arises: who is actually younger? According to each twin's viewpoint, the other twin is younger. However, in reality, only one twin can have aged less than the other. 

 

Fortunately, there is a resolution to this paradox. When Twin B turns around to return to Earth, she undergoes acceleration which means the usual laws no longer apply. As a result, Twin B will be younger than her Earth-bound sister, Twin A, upon returning to Earth due to the effects of acceleration. To explain this effect during the period of acceleration, we need to consider that general relativity causes time dilation in the presence of gravitational fields. Gravitational time dilation means that clocks run slower in stronger gravitational fields compared to clocks in weaker gravitational fields.

 

During the acceleration phase, when Twin B’s rocket is returning to Earth, her time now appears to go slower, while the clock on Earth appears to run faster. This phenomenon is similar to the extreme time dilation experienced near the edge of a black hole, known as an event horizon (7). From the observer’s frame of reference outside the black hole, time slows as an object approaches the event horizon, until it appears time has stopped. Hence an object falling into the black hole would appear to have stopped, completely frozen. 

 

Even though it governs our daily lives and despite our ability to measure it with great accuracy, there is no definitive answer to what time truly is. From the subjective experiences of our daily lives to the enigmatic physics of the Big Bang and black holes, the illusionary nature of time unveils an array of complexities, reminding us that this fundamental concept remains one of the most captivating mysteries of our existence. As famously stated by Einstein: "For us believing physicists, the distinction between past, present, and future is only a stubbornly persistent illusion” (8).