Continued from last week
This process is subsequently recognised as gaining insight into the state of affairs in the world. If we consider time as a mental construct employed to understand the state of the world, and we have become deeply accustomed to the passage of time and the measurement of events over moments, then the acceptance of time’s existence has primarily arisen as a pragmatic choice by scientists due to its usefulness in empirical pursuits. Time, as a concept, has three qualities dimensions, each of which holds significance for us.
I. Time can be viewed as a coordinate, serving as a means to locate events in the temporal dimension and providing labels to moments within the universe.
II. Time serves as a measure of the duration between events; it is what clocks quantify.
III. Time extends from the past through the present and into the future; it acts as the medium through which we progress from one moment to the next.
A lattice of clocks ticks the time and time becomes a new coordinate.
Although the three properties mentioned above are presumably appeared as the same, they are completely different concepts. While these three properties may seem similar, they are, in fact, distinct concepts. The first concept regards time as a coordinate. For example, if someone wishes to arrange an event, they need to specify not only the three spatial coordinates but also the time coordinate. Having only knowledge of spatial coordinates would not suffice to rendezvous for a meeting with friends.
The crucial missing piece of information is the element of time. Without specifying all four coordinates (x, y, z, and t), organising a meeting becomes impossible. This interpretation of time arises from the spatialisation of time, where time is used to label configurations with coordinates. Another significant aspect of the world’s state of affairs pertains to the concept of simultaneity in time.
Simultaneity is a relative concept, and all observers’ perspectives are equally valid; they possess equal rights to make claims about their observations. This means that what one observer perceives as ‘now’ can be chosen differently by another observer for whom things exist concurrently. However, the ‘now’ of the latter observer has a distinct correlation for assigning simultaneity. This allows for the division of the universe (x, y, z, and t) into various collections of different ‘nows,’ implying that everything coexists in the universe as a single block. One might humorously refer to this as ‘temporal gerrymandering.’ Therefore, the principles of special relativity and the relative nature of event simultaneity result in a form of ‘temporal gerrymandering’ within the block universe.
This way of conceptualising things has led to the assumption that all configurations exist as a singular entity, referred to as the block universe. In this framework, the past, present, and future coexist, and the labelling of objects on the time axis gives the impression that they extend across time. Viewing time as a coordinate allows us to comprehend that, for example, 2020 occurred before 2021, which, in turn, preceded 2022. Configurations transition from one to another in an ongoing process, with one moment seamlessly transforming into the next—a continuous evolution of objects forming what is known as a world line.
Arrow of time
The first property aids in our understanding of causation and the arrow of time. The concept of the arrow of time arises as a consequence of the timeless transformation of the universe when observed from an external perspective, as if one were a beholder situated outside of the block universe. A moment within one configuration, with fewer microstates, transitions into another moment within a new configuration containing a greater number of microstates. This gives rise to the arrow of time, which aligns with the famous direction of increasing entropy.
The second aspect of time is highly significant for understanding how we measure and perceive the passage of time. Additionally, it clarifies why we cannot think in an atemporal manner and underscores the necessity of the concept of time. Time not only assigns labels to events and situates them within distinct moments but also quantifies the ‘temporal separation’ between them.
Given that observers and beholders have grown accustomed to the concept of time and its progression, scientists endeavour to unravel the intricacies of the second property—how it operationally works. When it comes to the measurement of time, clocks are employed. What a clock essentially does is establish a common agreement with another clock. The act of measuring an observable involves comparing it to a predefined standard and assessing it in various contexts.
For instance, temperature is gauged using thermometers, and the entire process of establishing a globally recognised temperature scale relies on a fundamental principle known as the Zeroth Law of Thermodynamics. This law stipulates that if objects A and B are in thermodynamic equilibrium, and objects B and C are also in thermodynamic equilibrium, then it can be deduced that objects A and C are in equilibrium and share the same temperature.
The entire concept of measurement is founded on the idea that temperatures of objects A, B, and C are consistent across all possible temperature values. This transitive behaviour is crucial for temperature measurement in the universe; in other words, if A is greater than B and B is greater than C, then A must also be greater than C. Now, returning to the measurement of time, the key lies in synchronised repetition and comparison with other processes that occur repeatedly.
The relationship between synchronised repetitions of events within the universe gives rise to the concept of the passage of time. The accumulation of moments, one after another, signifies how an event has advanced over time and serves as a means of measuring its duration. Given that life has evolved to comprehend the synchronised repetitions of atoms and molecules, these synchronised repetitions mirror the progression of time in the rhythmic unfolding of cellular processes.
Consequently, as humans, we only have roughly 3 billion heartbeats at our disposal throughout our lifetime, representing a form of periodic repetition to be reckoned with. Heartbeats represent a form of periodic repetition, yet they do not meet the criteria of a reliable clock, as they can vary from moment to moment due to external influences. A good clock needs to maintain consistent synchronisation with other clocks, ensuring predictable repetition.
Regular vibrations
Quartz crystals, on the other hand, exhibit highly regular vibrations at a rate of 32,768 vibrations per second, even in the face of temperature and pressure fluctuations. Both living and non-living entities are composed of atoms and molecules that vibrate at precise and predictable frequencies, resulting in a diverse array of excellent clocks that remain in harmonious correlation with one another. The concept of time can be understood by examining events that occur simultaneously. Our goal is to comprehend how the notion of time originated.
By engaging in a straightforward thought experiment, we can elucidate this without any uncertainty. Let’s ponder the concept of time from the perspective of a photon. A photon is often described as a particle of light, travelling at a constant speed of 299,792,458 metres per second. It was initially postulated that the speed of light is both invariant and the maximum attainable speed for any moving object. Because the photon moves at this ultimate speed limit, it doesn’t receive any external information to keep track of the passage of time. This implies that from the photon’s own frame of reference, it would not perceive the passage of time.
Time functions as a means to quantify the duration between events. Moving from point A to point B alters the coordinate by 70 yards, even if the actual distance covered may be much greater. It’s important to recognise that in this context, time is being used as the coordinate, and the duration between moments is equated with the distance traveled.
In essence, the finite limit of the speed of light determines the ultimate capabilities of clocks marching in synchrony. Consequently, physicists have often speculated that time is a construct specific to life, as we are accustomed to living at lower speeds and within weaker gravitational fields. The flow of time can be influenced by either speeding up or slowing down this flow. But how would such changes in the rate of time be compared?
If time is slowed down, it renders us unaware, as all clocks slow down at the same rate. One second per second becomes another second per second. Therefore, the slowing or speeding up of the passage of time cannot be detected within a single system.
However, when considering three identical systems, each with a lattice of clocks, we would indeed observe discrepancies among the clocks that were initially synchronised. The postulate of the constant speed of light asserts that its speed remains consistent regardless of the motion of the observing systems. This postulate, in turn, eliminates the notion that time duration and distance are uniform for all observers.
Consequently, there is a genuine possibility for a lattice of clocks to measure time differently than another lattice of clocks. From an epistemological perspective, the total time elapsed along two distinct paths may indeed vary. The constancy of the speed of light allows for flexibility for both the duration of time and the measurement of distance to change according to the state of motion.
The distance covered while moving from one point to another on the ground does vary depending on the trajectory chosen to reach the destination, much like the time elapsed between two events can vary in duration. Taking into account the characteristics of time described above, the combination of three space coordinates and time creates what is known as spacetime, where each event can be located within this spacetime framework. In the spatial dimension alone, the shortest distance between two points is a straight line, whereas, in spacetime, the longest elapsed duration is described by the connection between two events.
The third characteristic of time is readily apparent and clear: time progresses from the past to the future. One crucial distinction between space and time, initially regarded as axes, is that all directions in space are equivalent. In other words, the laws of physics in the universe remain unchanged when spatial directions are inverted.
However, inverting the direction of time would fundamentally contradict the progression of all real processes. When posed with the question “What is time?” to a general audience, the response often carries the essence of the third characteristic. Time is often likened to a medium that moves from the past to the future. However, the ancient Greeks had a different perspective, describing the future as something approaching from behind them while the past receded before their eyes. We recognise the past as a collection of memories and experiences we remember, while the future remains speculative and uncertain.
Acceleration of the passage of time
If time were to be conceptualised as a material flowing like a fluid in a creek or river, this would suggest that the flow of time could potentially be altered. While we can easily comprehend the concept of a river’s flow speeding up, it is not as straightforward to contemplate the acceleration of the passage of time since we are all immersed in it. Inquiring about the slowing down or speeding up of time isn’t a sensible question because an observer within the flow of time wouldn’t perceive these changes.
This third characteristic also suggests that time is inherently linked to itself; it is a function of time. The river analogy of time can only be comprehended from a perspective outside of the time flow, an atemporal observer (though such an observer is not feasible). What can be discerned from this vantage point are correlated events arranged sequentially. Before we conclude with this third characteristic and its subtle implications, how can we justify the claim that our universe is 13.7 billion years old? To which specific clock does this concept of time passing refer?
Interestingly, the cosmic time of the universe we observe has been estimated to be approximately 13.7 billion years, and this timespan must be calculated based on a cosmic timekeeper. The gradual decrease in mass density offers a method for measuring the passage of time across the vast expanse of space, somewhat akin to counting the annual growth rings of perennial trees. If two trees happen to possess the same number of annual rings, they are of the same age. Similarly, in the universe, variations in mass density and the value of the inflaton field result in changes in the duration of time elapsed from its initial value.
Is it possible to alter our past, perhaps making a different choice for last night’s dinner? In the quantum realm, this concept is plausible—delayed choice experiments suggest that past actions that have already occurred could potentially be changed. Consequently, the quest to comprehend the nature of time remains an open-ended question, encompassing both ontological and epistemological dimensions, and continues to be a subject of debate as we seek to understand what time truly signifies for humanity.
Contemplating the true nature of time and the resulting insights leave us in awe of the mysteries surrounding our existence in the universe. The universe’s deeply concealed secrets, lurking behind us, cast a silhouette of an even more profound reality within our existence in the universe. These insights assist us in illuminating the inner workings of the cosmos.
Time serves as the agent for change, progressing from the past to the present and onward into the future. These three facets of time are intricately woven into our everyday experiences in the real world. Renowned American physicist John A. Wheeler encapsulated this concept by describing time as “nature’s method for ensuring that everything unfolds simultaneously.” When one arrangement of things transitions into another, we are unable to perceive all configurations simultaneously or in a single instant.
As I compose these words, my wall clock indicates that the hour hand and minute hand have converged at six and four, representing one configuration. However, when you read this, the arrangement of objects and the positions of the minute and hour hands will correspond to a different configuration.
Synchronisation
In essence, what we commonly refer to as measuring time is the process of aligning one clock with another, emphasizing the importance of synchronisation. Without this alignment, we are not truly measuring the essence of time. This underscores that measuring time relies on the operational confidence of recurring events. This line of thinking can be reasonably accepted, given the fact that the universe has presented us with the idea that elapsing time is the fundamental aspect of our world. In Newtonian physics, the universe is envisioned as consisting of a lattice of clocks, with their synchronicity being implicitly assumed to explain the behaviour of slowly moving objects. It appears that time is a prerequisite for defining the concept of time itself, although there is a potential strategy to liberate ourselves from the reliance on defining time as the change in the configuration of objects. This cyclic and self-referential definition of time emerges as a consequence of how time is perceived based on its inherent properties.
The attributes of time become more comprehensible when time is conceptualised in spatial terms. The existence of space is evident because we can observe various configurations of objects within it. In contrast, time remains somewhat enigmatic, as it acts as an agent of change that makes us conscious of events unfolding.
Interestingly, the concept of measuring time is intrinsically linked with our understanding of space. The properties of time mentioned above are rooted in the preconception of the existence of space that predates the existence of our own universe. It can be theorised that the independent existence of both space and time may need to be reconsidered when the speed of light remains constant, regardless of the frame from which it is being observed.
Hence, looking back, the measurement of time as it passes and progresses along a coordinate (where ‘x’ represents one spatial coordinate) is intricately tied to the notion of certain events occurring in synchrony with others. Consequently, space and time are operationally interconnected.
Einstein’s contributions have provided a clear understanding of the first two properties of time. However, the third property of time—the direction of time—remains a mystery. This is because the current flow of time and our limited awareness of the state of affairs in our subjective perception of the present are now viewed as unique occurrences.
As we conclude this column, offering a retrospective glimpse into humanity’s quest to comprehend the true nature of time, our ultimate objective is to construct a comprehensive model of reality that can successfully accommodate all these diverse aspects and notions of time. In summary, while we can accurately describe the manifestation of the aforementioned properties of time, the underlying reasons for these properties await a deeper understanding. This concept may not be entirely new, even for someone new to the topic; it involves the spatialisation of time as an additional dimension in which all events and occurrences are staged. This approach reflects the prevailing spirit of the 21st century in describing events within the spacetime continuum.
