You might consider time to be such a fundamental feature of our existence that there really is nothing profound or meaningful that we can say about it. Maybe time, like space, simply is. Many have even argued time out of existence altogether. For example, if we were to split it up into three components: the past, the present and the future, then we can examine each in turn.
Time might be viewed as an essential aspect of our being, so much so that there seems to be little to add beyond a simple acceptance that it exists. Perhaps, like space, it just is. Some thinkers even contend that time ceases to be at all. A common approach is to break it down into three segments—past, present, and future—and to study each piece in turn.
Those who deny time’s reality argue that the past has already happened and disappeared; all that remains are records of those events—memories, books, photographs, or film—which can only be accessed in the present moment. But the events themselves no longer exist. In contrast, the future consists only of our predictions, hopes, and beliefs about what might occur, which again are states of the mind in the present. Regardless of whether the future is determined or not, it has yet to come into being.
That leaves us with the present moment itself; surely “now” is real. Yet although we perceive our present as a solid, continually evolving instant that converts the future into the past, it is in fact only an instant and lacks duration. The physical definition of “now” is not measured in minutes or seconds, according to the time-deniers; it is the boundary between what is to come and what has already happened. It has no thickness. The ever-changing present is nothing more than a dividing line between past and future and thus does not possess a tangible existence of its own.
Einstein famously stated that “the distinction between past, present and future is only a stubbornly persistent illusion.”
Thus, if none of the three components of time exist, then there is nothing left, and the whole concept of time would be illusory! This line of thought has been advanced by many thinkers over two and a half thousand years.
If this impression sounds like a clever trick, fear not: a number of philosophers of time, and physicists as well, embrace a diametrically opposite stance known as “eternalism.” This view holds that all three components—past, present and future—are equally real. This idea—often explored through the notion of the block universe—gained prominence after Einstein introduced the theory of relativity, in which time is regarded as the fourth dimension of spacetime. In this account, all moments in time coexist in the same way that every point in space does. Einstein is frequently quoted as saying that “the distinction between past, present and future is only a stubbornly persistent illusion.”
If you do not care for either extreme—that time does not exist or that all times exist—there is a middle ground known as philosophical “presentism,” which asserts that while the past has already occurred and the future is yet to happen, the present moment remains real. A related idea is the “growing block universe,” in which the past and the present exist while the future does not, but gradually comes into being.
So, you can see that all these possibilities are on the table. But which one is the correct one?
In fact, to give you a sense of the scale of the disagreement and the depth of contemplation on the nature of time, it may be that time is an “emergent property” of reality—comparable to temperature—that makes sense only in our everyday world but loses meaning when we probe the fundamental fabric of reality.
The earliest arguments against the existence of time go back to the pre-Socratic Greek philosopher Parmenides of Elea, who flourished in the fifth century BCE in what is now southern Italy. Only fragments of his writings survive, including a poem called On Nature, in which he outlines two views of reality. The one that matters here is the first, known as The Way of Truth, in which he denies the reality of change, arguing that change is impossible and that everything that exists is permanent and unchanging. Parmenides was no fool and could clearly perceive change around him, but like many ancient Greek thinkers he pursued a chain of deductive reasoning that led him into an inescapable corner. He was persuasive enough that his student, Zeno, whom we will discuss shortly, also argued that continuous change, or the “flow” of time, is merely an illusion.
Two thousand years later, Isaac Newton argued that time provides the backdrop for all change.
Heraclitus, another Greek thinker worth noting, was born around 500 BCE in the city of Ephesus (in present-day Turkey). Not much is known about his life, and he is described as a rather enigmatic and even unhappy figure. His ideas about time were paradoxical and cryptic, earning him the titles “The Philosopher of Riddles” and “Heraclitus the Obscure.” What matters here is that his view of time revolved around perpetual change: the world is always in flux, always “becoming” rather than “being.” The enduring image of a river of time flowing past us traces back to his writings.
There are other ways philosophers have imagined time’s flow. Instead of picturing time as a river running by, one can imagine ourselves in a boat carried along by that river, watching the world unfold. In this poetic metaphor we would face away from the direction of travel, with our backs to the future, watching the past recede while what is coming remains unseen.
Two thousand years after Heraclitus, Newton again argued that time provides the stage for all change. Since he treated time as absolutely real and external, he claimed that it endures and flows even in the absence of any change. Yet it seems we still rely on time to measure changes when they occur.
Nevertheless, even if time does not require change to exist, the opposite question—does change require time? It is easy to think of cases where something changes without necessarily changing across time. For example, atmospheric pressure shifts with altitude, the speed of a car changes when the driver accelerates, the clarity of a microscopic image changes with different lenses or focus, and my mood shifts when my team wins or loses. In physics, when we talk about changes over time, we usually mean quantities that vary dynamically, taking on different values at different moments.
The physicist Paul Davies rejects the idea of a flowing time with a sharp critique: “Attempts to graft on” a mysterious extra quality to endow time with a flowing motion—an addition reminiscent of a life force or vital elan—fail to capture what time truly is. Although we have the strong feeling that we move through time—or that time itself flows past us—this sensation of a flowing time appears to be purely subjective.
After all, what could actually be flowing? If time did flow, one would expect it to move at some rate, but against what would we measure that rate? Is time passing at the same rate as one second per second? What does such a notion even mean? And if our consciousness travels along a static time axis, how fast is it moving, and what external time would we use to define that rate? The very idea of a rate of change is itself embedded within time.
But the particle’s states at different times are merely snapshots at particular moments. There is still no flowing time.
What does modern science have to say about all this? It is one thing to argue that processes change along a temporal axis or that moments follow a definite order, and quite another to identify exactly where in our description of the universe the flow of time, as we experience it, arises. The honest answer is that contemporary science has nothing definitive to say about the flow of time. The flow we perceive in everyday life is not reflected in physical time. The closest physics comes to that notion is in Einstein’s relativity, where two observers in relative motion or under different gravitational influences disagree about how quickly another clock ticks relative to their own.
In broad terms, while time is a fundamental piece of our mathematical framework for describing nature, there is no hint in our laws or equations that it truly “flows.” In daily speech we speak of time passing, time arriving, or moments slipping away, but what we actually perceive is the passage of time. Physically, what exists is a sequential ordering of events: states of reality defined at successive moments along the time axis.
When we try to quantify how fast a process happens, we either tally events within a time interval—such as heartbeats per minute—or measure how much a system changes within that interval, like how much weight a newborn gains in a month. Time acts as the yardstick by which we gauge change.
Yet if we scrutinize the underlying equations of physics that describe this change, we see that time appears merely as a parameter: the symbol t, which can assume any value. For instance, a particle may have a particular position or velocity at a given time t. The dynamical equations then let us predict the particle’s behavior at other times, in the past or the future. In this sense, the particle’s state at different times are just snapshots, and there is still no flowing time.
It does seem as though the vivid sense of time streaming from past to future is an illusion, however compelling it feels. If science cannot fully account for this strong feeling of passing time and the shifting present, some physicists and philosophers suspect there is something missing in the laws that describe reality.
They may simply be right.
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From On Time. Used with the permission of the publisher, Princeton University Press. Copyright © 2026 by Jim Al-Khalili
