What Is Time? Does it Even Exist?

In the late 1600s, when Newton published his book Philosophiæ Naturalis Principia Mathematica, he stated that time is absolute and is the same for everyone, no matter where they are or how they are moving. However, in the early 1900s, Einstein’s theory of relativity showed that time is not absolute. Instead, time can pass at different rates depending on an observer’s motion and the strength of gravity around them.

Since then, our understanding of time has been completely transformed. Yet we still do not fully understand what time actually is—is it a fundamental part of the universe? Or could it emerge from deeper physical processes?

Einstein and the Relativity of Time

Einstein’s theory of special relativity tells us that time passes for everyone. In addition, it doesn’t always pass at the same rate for people in different situations, like those traveling close to the speed of light. Later, his general theory of relativity showed that gravity also affects time. For example, an observer near a massive object, such as a black hole, would experience time passing more slowly. This would be compared with someone farther away from the gravitational field.

Einstein resolved the malleability of time by combining it with space to define space-time. This space-time can bend but behaves in consistent, predictable ways. His theory seemed to confirm that time is woven into the very fabric of the universe. However, while relativity describes time as part of spacetime, it does not fully answer whether spacetime itself is a fundamental feature of reality. Alternatively, it could be something that emerges from deeper physics.

But there’s a big question it didn’t fully resolve: why is it we can move through space in any direction, but through time in only one? No matter what we do, the past is always, stubbornly, behind us. This is called the arrow of time.

Arrow of time

Why is it that we cannot bend or twist time? Why is it that time has a positive vector and we can only remember our past but not the future?

Entropy and the Direction of Time

In physics, this could be described by the Second Law of Thermodynamics, which says that systems will gain disorder, or entropy, over time.

The Second Law of Thermodynamics states that the total entropy of an isolated system tends to increase over time. Entropy is often described as disorder, but more precisely it represents the number of possible microscopic arrangements that a system can have. Thus, this tendency toward increasing entropy may be what gives time its apparent direction.

For example, if you leave your room with a pile of papers stacked together neatly and re-enter it after some time, then you might not be shocked to see those piles of papers losing their order and spilled all over the place. But if you leave the same room with those papers knocked all over and re-enter it to encounter a pile of neatly stacked papers, then surely you will be taken aback.

This is entropy—stating that objects, atoms, or matter tend to become more disordered with time, even if local exceptions can happen due to random fluctuations.

Although entropy generally increases, local decreases in entropy can happen. For example, stars, planets, and living organisms can form complex structures while the overall entropy of the universe continues to rise.

The Beginning of the Universe and Time’s Arrow

Recent studies in cosmology have even linked this arrow of time to the very beginning of the universe. The Big Bang started with extremely low entropy, and ever since then, disorder has been increasing. The early universe was remarkably smooth and uniform, yet its gravitational entropy was unusually low. Understanding why the universe began in such a special low-entropy state remains one of the biggest unanswered questions in cosmology.

Some researchers think that this special initial condition might be the key to understanding why time flows forward.

Is time merely an illusion?

So could it be possible that time doesn’t exist, at the most fundamental level? Could time just be some sort of illusion generated by the limitations of how we perceive the universe? We don’t yet know, but maybe that’s the wrong way of thinking about it.

Some approaches in modern physics explore whether time might not be a fundamental ingredient of the universe. Instead, it could be something that emerges from deeper physical processes. However, this remains an open question, and most physicists continue to treat time as a real and meaningful part of our physical descriptions of the universe.

Could Time Be an Emergent Property?

Instead of asking if time exists as a fundamental property, maybe it could exist as an emergent one. Emergent properties are things that don’t exist in individual pieces of a system, but do exist for the system as a whole.

Each individual water molecule doesn’t have a tide, but the whole ocean does. A movie creates change through time by using a series of still images that appear to have a fluid, continuous change between them.

Flipping through the images fast enough, our brains perceive the passage of time from the sequence of still images. No individual frame of the movie changes or contains the passage of time, but it’s a property that comes out of how the pieces are strung together. The movement is real, yet also an illusion. Thus, could the physics of time somehow be a similar illusion?

Some physicists think so. Some physicists have explored this possibility through theories of quantum gravity, which attempt to combine Einstein’s theory of relativity with quantum mechanics. One of the challenges is that these two successful theories describe time in very different ways. For example, relativity treats time as part of a flexible spacetime, while quantum mechanics generally uses time as a fixed background against which events occur.

Different Views on Time

Researchers like Carlo Rovelli have proposed that time may be a relational concept—that is, it only makes sense when objects interact. In this view, time may emerge from relationships between physical systems rather than existing as a universal background that flows independently.

Others, like Julian Barbour, argue that time is just a measure of change and not something that flows at all. This remains a controversial interpretation, but it suggests that what we experience as the passage of time may come from the changing relationships between different states of the universe.

In some versions of quantum gravity, like loop quantum gravity or causal set theory, spacetime itself may emerge from more basic building blocks—without any built-in time.

Another idea related to this question is the “block universe” interpretation of relativity. In this view, past, present, and future all exist within the four-dimensional structure of spacetime, and the feeling that time is flowing may come from the way conscious observers experience events. This interpretation is debated and is not considered a confirmed description of reality.

What Experiments Tell Us About Time

Still, experimental evidence supports relativity’s predictions about time’s behavior. For example, atomic clocks on airplanes and satellites tick differently based on their motion and position in gravity. GPS systems even have to account for this time dilation to remain accurate.

These effects have been measured repeatedly and are essential for modern technologies such as satellite navigation. They show that while the deepest nature of time remains uncertain, the effects predicted by relativity are very real and measurable.

The Mystery of Time Remains

And while physicists continue to dig deeper, asking questions about whether time is real, emergent, or illusory, we’re far from a complete explanation.

Time remains one of the greatest mysteries in physics. Whether it is a fundamental feature of the universe or something that emerges from deeper laws of nature, understanding time may be essential to understanding the universe itself.

At least for the moment.

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