Is Time Even Real? Does It Even Flow?

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.

Those who deny the reality of time will say something like this: The past has happened and is gone; all we have left of it is the record of past events (in our memories, or in books, photographs, or on film), all of which can only ever form a part of the present moment (when we access those records). But the events themselves no longer exist. In contrast, all there is of the future is our predictions, hopes, and beliefs about what might happen, which are again states of our minds in the present. Regardless of whether the future is predetermined or not, it has yet to exist.

That leaves the present moment itself; surely “now” is real. But although we “feel” that our now is a solid, changing moment steadily sweeping through time, converting the future into the past, it is nonetheless just an instant and as such does not have any duration. The physical definition of “now” is not a minute long or a second long, the time deniers will argue; it is the edge of the shadow between light and dark, between what is to happen and what has happened. It has no thickness. The constantly changing present moment is no more than a dividing line between past and future and so does not have 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, leaving the whole of time as illusory! Many thinkers going back two and half thousand years have argued thus.

If this all sounds like a clever sleight-of-hand, don’t worry: A number of philosophers of time, and physicists for that matter, subscribe to a diametrically opposite view, known as “eternalism.” This holds that all three components—past, present and future—are equally real! This idea, which we will examine more closely later on, emerges from an idea called the block universe, which gained prominence after Einstein put forward his first theory of relativity, in which time is regarded as the fourth dimension of spacetime. In this picture all moments in time coexist in the same way that all points in space exist. Einstein famously stated that “the distinction between past, present and future is only a stubbornly persistent illusion.”

If you don’t fancy either of these radical views: that time doesn’t exist at all or that all times exist, then there is a halfway house known as philosophical “presentism,” which holds that while the past and future do not exist (one has happened and gone, and the other is yet to happen) the present moment is nevertheless real. A related idea is the “growing block universe” view, in which the past and the present exist while the future does not, but instead gradually comes into being.

So, you see all options are available on the menu. But which of them is the correct one?

In fact, just to make sure you are fully aware of the scale of the problem and level of disagreement among people who have thought long and hard about the nature of time, it might even be the case that time is what is called an “emergent property” of reality, a bit like the concept of temperature, that only makes sense in our everyday world of experience, but becomes meaningless when we zoom in on the fundamental concepts that underpin reality.

The earliest arguments for the non-existence of time go back to the pre-Socratic Greek philosopher, Parmenides of Elea, in southwest Italy, who flourished in the fifth century BCE. Only fragments of his work survive, including his greatest work: a poem called On Nature, in which he describes two views of reality. The view of interest here is the first one, called The Way of Truth, in which he denies the reality of change, arguing that change is impossible and that its very notion is incoherent; rather that everything which exists is permanent and unchanging. Parmenides was no fool and presumably could clearly see change all around him, but like many of the ancient Greek thinkers he had talked himself down the rabbit hole of deductive logic from which it was hard to escape. He was certainly persuasive enough that his more famous student, Zeno, whom we will meet shortly, also held that continuous change, or a “flow” of time, is just an illusion.

Two thousand years later, Isaac Newton argued that time provides the backdrop for all change.

Another Greek philosopher worth mentioning is Heraclitus, who was born around 500 BCE in the city of Ephesus, now in modern Turkey. Little is known about his life, but what we do know paints him as a complex character and, as far as we can tell, a rather miserable one to boot. His philosophical ideas were, like him, paradoxical and cryptic to the extent that he earned the epithets “The Philosopher of Riddles” and “Heraclitus the Obscure.” Of interest here, however, is that his view of time stands in stark contrast to that of Parmenides. For Heraclitus was obsessed with change and viewed the world as being constantly in flux, always “becoming” but never “being.” The popular notion of a river of time, flowing past us, can be traced back to him.

That is not the only way philosophers throughout history have thought about flowing time. For instance, rather than thinking of time flowing past us as we stand on the riverbank, we can imagine that we are in a boat carried along by this river watching the world unfold. In fact, in this poetic example we would need to be facing away from the direction of travel, with our backs to the future, watching the past recede away from us, but never seeing what is coming up.

Two thousand years later, Isaac Newton argued that time provides the backdrop for all change. And because he believed that time is absolute and external, he argued that it continues to exist and flow even when there is no change. But it seems we still need time if we are to measure change.

However, while time may not require change in order to exist, what about the other way round: Does change require time? Well, one can easily think of examples involving things that change, but which are not necessarily changing over time. Atmospheric pressure changes when you climb to higher altitudes, the speed of a car changes when you press your foot on the accelerator pedal, the clarity of an image under a microscope changes when you use different lenses or alter their focus, and my mood changes when my football team wins or loses a match. However, in physics when we talk about changes over time we are generally referring to those quantities that vary dynamically, such that they are different at different moments of time.

The physicist Paul Davies dismisses the notion of a flowing time thus: “Attempts to ‘graft on’ a mysterious extra quality to bestow a flowing motion upon time which is simply ‘there’ is reminiscent of attempts to explain the vital quality of living organisms by adding a ‘life force’ or ‘elan vital’ to otherwise lifeless matter.” While we have the strong sense that we are moving through time—or conversely that time flows past us—this sensation of flowing time seems to be an entirely subjective experience.

After all, what could actually be flowing? If time flows then that would suggest it should be flowing at some rate, but what would we measure that rate against? Does time go by at a rate of one second per second? What does that even mean? And if it is our consciousness moving along some sort of static time axis, then how fast is it moving, and what (external) measure of time do we need to define this rate? The very notion of a rate of change is itself embedded within time.

But these states of the particle at different times are themselves just snapshots at certain instances. There is still no flowing time.

What does modern science have to say about all this? It is one thing to argue that there is continuous change along the time axis, or that moments are organised in a certain temporal order, but quite another to pin down exactly where in our objective description of reality it is that time flows in the way we experience it. The answer is that modern science has nothing whatsoever to say about the flow of time! The flow we experience in manifest time is completely absent in physical time. The closest physics gets to the notion of flowing time is in the theory of relativity where two observers, either moving relative to each other or experiencing different strength gravitational fields, disagree on how ‘fast’ the other person’s clock is ticking relative to their own.

In general, while time is an integral part of our mathematical description of nature, nowhere in our laws or equations of physics is there any hint that it actually “flows.” In our everyday language we say that “time passes,” “the time will arrive,” “the moment has gone” and so on. But we only sense that time flows. All that actually exists according to physical time is an ordered sequence of events: states of reality defined at different points (moments) on the time axis.

Consider what we do when we wish to describe the speed of a certain process. We either count the number of events in a given interval of time, such as the number of heartbeats per minute, or the amount of change in an interval of time, such as how much weight a baby has put on in one month. Time is the measuring stick against which we quantify change.

But if we look deeper at the equations of physics that allow us to calculate this change, we see that time comes in merely as a parameter: the symbol, t, that can take on any value. For example, a particle has a certain position, or is moving at a certain velocity, at a given value of t. These dynamical equations of motion then allow us to compute what the particle is doing at any other time, whether in its past or its future. In this way, we can see how the particle’s state changes, in the sense that it is different at different times. But these states of the particle at different times are themselves just snapshots at certain instances. There is still no flowing time.

It certainly seems that the strong sense we have of time continuously flowing from past to future is indeed just an illusion, however real it might seem to us. But while science is unable to provide a satisfactory explanation for this strong sense we have of passing time, and of a changing present moment, some physicists and philosophers think this is because there is something missing in the laws of physics.

They may just turn out to 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