Time and Its Inseparable Role in The Fabric of The Universe

The notion of time pervades our everyday lives so strongly that we often take it for granted. Time is so familiar yet is one of the most puzzling subjects of modern science.

Historically, the first concept of time as it relates to theoretical physics was of absolute time. That is, time is assumed to be a fixed entity and all observers measure the same time. In 1676, by observing the motion of Jupiter's moons, Danish astronomer Ole Rømer discovered that light had a finite speed. The discovery lead to some interesting revelations about light's relationship to time and how when we look at an object, we are seeing it as it was in the past. Though the effect is noticable at astronomical distances, the speed of light is so great that there is little to no noticable effect in day to day life, and thus Isaac Newton's classical physics and the notion of absolute time were accurate enough to be taken as correct. The idea of absolute time matches our everyday experiences so strongly that it prevailed for many generations until the early 20th century when Albert Einstein developed his Special and General theories of relativity.

Einstein's theory took the speed of light to be constant regardless of the motion of an observer, which meant that time was in fact, not constant. The faster something is travelling, the slower time will pass for that object compared to a stationary or slower moving object. Though Einstein simply assumed the speed of light to be constant, it was empirically suggested in 1887 by an expirement performed by Albert Michelson and Edward Morley who intended to detect the presence of a luminiferous aether thought to be permeating "empty" space. The event is commonly known as the most famous failed experiment due the ironic disproval of aether's existence and subsequent suggestion of a constant speed of light.

While Einstein's special relativity predicted time dilation due to velocity, general relativity predicted a time dilation would also occur due to differences in gravitational field potentials. For instance, a object on the earth's surface would age slower than an object high in the earth's atmosphere. Both time dilation effects were proved in 1971 when Joseph Hefele and Richard Keating observed the differences in elapsed time between highly accurate cesium atomic clocks which were placed in airplanes and an identical stationary reference clock. Even today, satellite systems must take the effects of time dilation into account in order to provide accurate data. For example, satellites in the Global Positioning System constellation age faster than objects on the Earth's surface, gaining roughly 38 microseconds each day.

Einstein's theories of relativity revolutionised the way we understand time, but like the classical physics developed by Isaac Newton, a fundamental component of how we perceive time was still unexplained: the arrow of time. Einstein had showed that time was strongly entwined with the three spatial dimensions we are familiar with, but there were still differences. Whereas the spatial dimensions contained no arrow and an object can travel freely in any direction, the dimension of time only allows forward travel, with a clear separation between what we understand as the past and the future. Both Newton's theories of classical physics and Eintstein's theories of relativity are time-symmetric, meaning they don't distinguish between the past and future, and fail to explain why events always unfold in one direction, but never the reverse.

We see such occurences in our everyday lives. The classic example in much scientific literature is a cup falling off a table to smash on the floor. Though equally valid and possible according to Newton's and Einstein's theories, the broken pieces of the cup never gather themselves up and form a complete unbroken cup on the table. Deep, unanswered questions about the nature of time can stem from the most mundane everyday occurrences.

Theoretical physicist Stephen Hawking identifies three arrows of time. First is the thermodynamic arrow, discussed shortly. Second is the psychological arrow, the one we are most familiar with. It arises from our recognition of change and the way we develop memories of the past as distinct from the unknown future. Last is the cosmological arrow, related to the expansion of the universe, but not of much concern here. We know the universe is expanding, and current evidence suggests it will keep expanding for eternity.

The thermodynamic arrow of time relates to entropy, the disorder of a physical system, and the tendency of entropy to increase over time. This is the essence of the second law of thermodynamics. The concept of entropy employs statistical reasoning to explain the overall properties of complex systems where direct analysis of individual components (like atoms) is unwieldy. For any non-trivial system, there are an infinite number of possible configurations, but only two classes of configurations worth considering: ordered and disordered. Entropy increases over time simply because there are more disordered states that ordered ones, and the more ways something can happen, the greater the likelihood that it will happen. While pieces of a broken cup rearranging themselves into an unbroken cup is theoretically possible, it's extremely improbable.

From observing everyday events, we could argue that disorder isn't always increasing. Human activity can create order out of disorder, and even the human body itself relies on a high degree of order to function. However, for any artifically created order, there is always a larger increase in disorder, often in the form of heat production, and so there is a net entropic increase. It may seem that human beings are proficient at creating order out of chaos, but our actions consume resources and produce heat and so there is no violation of the second law of thermodynamics.

The second law of thermodynamics might explain why many events occur in a certain order, but the connection to the psychological arrow of time is less obvious. If the acquisition of memories is assumed to be some kind ordered configuration process of the brain (as with computer memory), then the brain's operation of forming memory must produce heat and increase entropy. This means the psychological arrow of time is really just an result of the thermodynamic arrow. Entropy increases with time because physiologically, we can only measure time and observe change in the direction of increasing entropy.

Further investigation of the relationship between time's arrow and entropy reveal some interesting facts about the very beginning of the universe. The laws of thermodynamics are derived from time-asymmetric theories, and therefore the reasoning that leads to the prediction of increasing entropy in the future can be applied with equal validity to the past. In other words, while it is likely that a system will have higher entropy in the future, it is also likely that the system previously had higher entropy. The entropic arrow would seem to point in both temporal directions, and the current lower entropy is just a random fluctuation in a system with otherwise maximum entropy.

However, our own everyday experience is in stark contrast to this conclusion. If we see an unbroken cup, our memory of the past is of an identical unbroken cup, not ceramic shards which have since coalesced into the whole cup we see in the present moment. One solution to this discrepancy between theoretical reasoning and experience is if the system, our universe, begins in a state of low entropy. This provides significant insight into the birth of the universe as a critical factor in the formation of time's arrow. After the big bang, the initial state of the universe must have been one of low entropy. As gravity caused clumps of mass to coalesce, disorder began to rise and has been increasing ever since.

There are still many unanswered questions regarding time and its inseparable role in the fabric of the universe. The struggle to understand time has yielded more questions than answers and research into quantum mechanics promises to yield even more questions. While a fundamental explanation for time is yet undiscovered, the scientific community has made significant progress in correcting some seemingly obvious but flawed assumptions about the nature of time, and uncovered some truths about the very nature of the universe. Had the universe not begun with low entropy, the temporal arrow as defined by increasing entropy would be bidirectional and human beings nor any other life form would exist to ponder the nature of time.