June 2023

The Endless March of Time: Why It Moves in a Single Direction

June 27, 2023
The Endless March of Time: Why It Moves in a Single Direction

Many of us have pondered about time and its unflinchingly forward nature. But if time were to flow in reverse instead, would that mean events precede their cause? This incessant march in one direction has caused many to experience (and suffer) regret, one of the most dreadful emotions humanity can experience. While man has long pondered about this aspect of time, physicists only recently discovered why it works the way it does.

The Beginning of Time

Albert Einstein believed in an unchanging and static universe that just exists. He also thinks that the universe had no beginning. However, he called this his biggest blunder since it is now believed that the universe has a beginning (as covered in the Big Bang theory) and is far from static since it constantly expands. Moreover, the night sky we see would not be dark without this expansion but instead illuminated by the countless stars in space.

That said, Einstein was correct about time’s flexible nature and how it shrinks and stretches for different objects with different acceleration rates. Time runs slower for objects with a faster acceleration than those with a slower acceleration. This is where the concept of time being relative was born.

However, Einstein was accurate in describing the malleable nature of time and its contraction or dilation depending on the acceleration rates of different objects. When objects experience higher acceleration, time passes slower for them compared to objects with slower acceleration. This fundamental observation gave rise to the concept of relative time.

The Second Law of Thermodynamics

This second law states that entropy, defined as the measure of disorder in a given system, can only increase in an isolated system. Thus, time’s “forward” direction is the direction in which entropy or disorder increases. An isolated system in the context of thermodynamics means a confined space that no outside force or energy can penetrate.

Think of the dirty dishes left in the sink after a meal. If they are left unwashed, they will simply pile up over time. This isolated system of an unattended sink will only see disorder increase unimpeded.

Extracting order from disorder comes at a cost in energy, i.e. working and cleaning up the dishes. But as per the first law of thermodynamics, nothing can spend energy or do work at 100% efficiency, and the body is destined to absorb some energy and dissipate it as heat. Similarly, cars do not burn fuel with total efficiency as the engine absorbs a fraction of the converted energy, causing it to heat up.

Thus, the order in closed or isolated systems comes at the cost of ever-increasing disorder in the environment. That said, since our ultimate surrounding is the known universe, it is an isolated system that cannot receive any help from the outside. So how does entropy play into our increasing measure of time?

Time as a Psychological Invention

Man’s measure of time suggests that it is a psychological invention. While this notion could be very likely, using disorder to measure time helps explain time’s objective and subjective origins. To better understand this, let us generalise that human memory and computer memory operate similarly. While this is not entirely wrong, if computers could remember the future, anyone could get rich on the stock market.

Now, let us compare a newborn’s grid of neurons and a computer’s transistors. Both register information in a certain configuration of neurons and transistors, respectively. But in order for the brain to achieve this, which is by activating a sequence of biochemical reactions, a bit of heat gets dispersed since we know transactions are never efficient. As a result, there is an increase in disorder in the universe, and more heat gets distributed as the newborn brain registers more events.

In short, it is more so that we are measuring time in the direction entropy is increasing instead of entropy increasing over time, which also explains why time’s psychological and thermodynamic arrows move in the same direction. But another question remains: Why should time’s forward movement be in sync with the universe’s expansion?

The Arrow of Time Dictated by Expansion

The reason for the arrow of time moving forward as dictated by the universe’s expansion is the same as that of the thermodynamic and psychological arrows of time: the more the universe expands, the more disorder increases. The universe is inclined to foster disorder simply because it is more favourable than order, i.e. the configuration of atoms associated with this concept is more numerous than those of orderliness.

But the fact that disorder increases with expansion revolves around the assumption that the universe came about from orderly beginnings, gradually slipping into non-uniformity. This is what scientists believe to be true at present, as validated by the Big Bang model. Then, what happens if the universe contracts?

Scientists speculate that in the distant future, the universe will collapse upon itself when gravity becomes dominant over the force of expansion. While this event is far off, it is worth considering its potential impact on time. Specifically, if the universe starts contracting, will there be a decrease in disorder and cause time to go backwards instead?

Stephen Hawking believed this to be the case until one of his students pointed out his mistake, which he admitted to, and thus recognised that disorder will continue to increase regardless of when the universe starts to contract. But during this phase, the disorder would be so massive that life would no longer cease to exist. Remember that when we extract or create ordered energy, we also disrupt or reduce order in our environment. So, what happens when this order in the surroundings becomes exhausted? Nothing would be left to extract.


By examining the influence of luck on plant growth, we can draw parallels between this concept and its application to the universe and humanity. Our understanding of the universe is shaped by the fact that we are part of it, a principle known as the anthropic principle.

Currently, we find ourselves in a phase of the universe characterised by expansion. In this expansion phase, time is measured in a way that aligns with increasing disorder and supports the emergence of intelligent life. Consequently, making the most of our time on Earth by delving into the enigmas of the universe is essential.

If you find this concept intriguing, Tuition Physics can help you gain a deep understanding of the fundamental principles of physics. Whether you aspire to become a renowned physicist or simply ace your physics class, our passionate physics tutors in Singapore are dedicated to helping you achieve your academic and long-term goals. Contact us today to learn more about our programmes and curriculum.

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