Is the Passage of Time an Illusion? The Block Universe Theory

The world we live in is full of mysteries. Although we already know a lot about the universe, many questions remain unanswered. What is time? What does it mean for time to pass? There are many ways to think about this, and one of the most interesting is the block universe. According to this theory, there's no universal "now", and all events - past, present, and future - simply exist in the spacetime continuum. In many ways, this view of reality fits well with Albert Einstein's theory of relativity. In a letter to his friend Michele Besso, Einstein once wrote: "The distinction between past, present and future is only a stubbornly persistent illusion."

What might such a reality look like? Take an old film strip as an example. It contains every frame of the film. When we look at the strip itself, however, we don't see the larger context. We see only a sequence of still images that, on their own, tell us very little. Only when the frames are played in the right order does meaning emerge. Individual images turn into motion and a complete story. But for that, we need a projector.

We can look at our lives in a similar way. If they were recorded on a film strip, all of its "frames" could simply exist - from birth to death. What we experience as the passage of time would then be less a feature of reality itself than a way of reading it. But this raises a question: what is the projector of our film?

Time According to the Theory of Relativity

At the end of the 19th century, physics was considered the most complete of all the sciences. Many believed that the most important laws governing the world had already been discovered, and that further progress would only involve refining the details. This changed with Albert Einstein (1879-1955). The special theory of relativity (1905) and the general theory of relativity (1915) turned our understanding of reality upside down.

One of the most important changes concerned the relationship between space and time. In the view of the world that had dominated since the time of Isaac Newton (1643-1727), these dimensions were treated as independent of one another. Space was something like a vast, motionless stage on which all events took place. Objects could move through it and interact with one another, but space itself remained unchanged. Time, similarly, was understood as something absolute, flowing in the same way everywhere and for everyone. Einstein challenged this picture. He showed that space and time are neither independent of one another nor absolute. Together, they form a single four-dimensional structure: spacetime.

Spacetime is flexible. It can be curved by mass and energy. This is how gravity works. It isn't a force pulling objects toward one another at a distance, but an effect of the geometry of spacetime itself. This curvature is what makes planets orbit stars, light bend near massive objects, and a ball we throw return to Earth's surface.

One consequence of this flexibility is time dilation. Time doesn't have to pass in the same way for all observers. Its passage depends, among other things, on velocity and the gravitational field. A clock near Earth's surface ticks slightly more slowly than an identical clock at a higher altitude.

Time, then, doesn't work the way our intuition suggests. Since it's part of flexible spacetime, it stops being a simple, universal background for events. Its passage depends on the frame of reference, and there's no universal "now". This shows that our everyday idea of time may be only a simplified image of something far more complex.

Human Perception of Reality

We don't actually see three-dimensional space directly. The image on the retina isn't even three-dimensional; rather, it's a two-dimensional projection of three-dimensional space. The retina does not capture depth. The brain reconstructs it from cues: perspective, shadow, motion, differences between the images from the left and right eye, and learned patterns. Our experience of space is therefore a kind of model produced by our brain.

Something similar applies to the fourth dimension: we don't see time, but we do have clues from which we infer its existence. Those clues are our memories. Memories of past events allow us to make decisions in the present. This way of understanding time allows us to function in a four-dimensional universe, but it doesn't have to reflect its structure accurately.

In such a model of the world, we can understand the passage of time as a projection of reality constructed by our own minds. We can distinguish more distant memories from more recent ones. This creates a logical sequence of events resembling a strip of film. Our brain plays this film, giving events meaning and making it possible for us to make decisions.

But where did this representation come from? If, according to the block universe theory, both past and future moments simply exist in four-dimensional reality, why do we remember the past but not the future? Where does the direction of events come from?

The Arrow of Time and Entropy

The British astrophysicist Arthur Eddington popularized the concept of the arrow of time in 1927. There are different kinds of arrows: thermodynamic, cosmological, psychological, and so on. Although each describes a slightly different aspect of reality, they all point in the same direction and show that reality is not symmetric in time: broken glass doesn't spontaneously put itself back together, and cold tea doesn't heat itself back up.

To understand this direction, we can return to something we learned at school: the basic laws of thermodynamics. Most people associate thermodynamics with the first law, the conservation of energy. Here, however, the more important one is the second law, which is connected to the concept of entropy. Entropy is a state function and, in simplified terms, a measure of the disorder of a system: the greater the disorder, the greater the entropy. According to the second law of thermodynamics, the entropy of an isolated system tends to increase, and in special cases remains unchanged.

Take a glass of water and pour in some juice. The liquids mix until they form a uniform solution. This happens spontaneously, without our intervention. The mixing process increases the disorder of the system, which means entropy increases.

When we pour the pieces of a new jigsaw puzzle onto a table, in theory we can hope that, as they fall, they will arrange themselves into the finished picture. The probability of such an event, however, is so small that in practice it simply doesn't happen. A far more likely scenario is that the pieces remain in a random, chaotic arrangement.

The second example is especially important here because it helps us understand not only what happens when entropy changes, but also why entropy ultimately follows from probability. A disordered state is statistically much more likely than an ordered one, because there are far more possible ways for the elements of a system to be arranged chaotically. As a result, reality naturally tends toward states of greater entropy.

The Link Between Entropy and Information: Maxwell's Demon

James Clerk Maxwell (1831-1879) gave physicists something to puzzle over for a long time. As if a thought experiment that seemed to bypass the second law of thermodynamics were not unsettling enough, Maxwell added a demon to it. In his experiment, he imagined a tiny creature able to observe individual gas molecules and, at the right moment, open or close a small door between two parts of a container so as to separate faster molecules from slower ones. As a result, the system would become more ordered without any apparent work being done, which would contradict the second law of thermodynamics. This hypothetical scenario later became known as the paradox of Maxwell's demon.

This paradox was not resolved until the second half of the 20th century. The key was to link entropy with information. Rolf Landauer played an important role here. He showed that logically irreversible operations on information stored in computer memory have a thermodynamic cost. The crucial case is data erasure: according to Landauer's principle, erasing one bit from memory is associated with the release of heat and an increase in the entropy of the surroundings.

Charles Bennett later applied these conclusions to Maxwell's demon. He pointed out that, to control the door, the demon would have to remember information about the molecules. For the demon to operate in cycles, however, its memory couldn't grow indefinitely. It would have to be cleared from time to time. This is where Landauer's thermodynamic cost comes in. The increase in entropy associated with erasing information offsets the apparent decrease in the gas's entropy. As a result, the second law of thermodynamics is preserved.

Why Do We Not Remember the Future?

Landauer's principle mainly applies to the erasure of information in digital systems. Can the same idea be extended to human memory? Stephen Hawking used this analogy in his book A Brief History of Time. The human brain is a very complicated organ, and we still don't fully understand how it works. We understand computers much better because, as a species, we created them ourselves. Both the brain and computer memory store information in physical systems, and both are subject to the same arrow of time. That may make the comparison useful enough.

In answering the question "why do we not remember the future?", Hawking showed the relationship between the thermodynamic and psychological arrows of time. In the thermodynamic sense, the passage of time is tied to the direction in which entropy increases. The psychological arrow of time, by contrast, is an internal, subjective feeling that arises from the existence of our memories.

In essence, his explanation follows the same logic as the solution to Maxwell's demon. The key point is that memory is not detached from thermodynamics: it is tied to physical processes that increase entropy. Landauer's principle describes one part of this cost: erasing memory means removing information. A memory module in a computer, however, can't function on its own. Other components are also needed, and together they form a functional device. Such a device uses electrical energy, and the operations performed in it generate heat.

Something similar happens in the human body. The processes taking place inside it require energy and increase entropy. This is why the psychological arrow of time depends on the thermodynamic one. Trying to reverse this relationship would mean swimming against the entropic current of the entire universe, which, as one might imagine, would be extremely difficult, if it were possible at all.

Summary

The block universe theory is only one attempt to answer the question of what time actually is. It doesn't provide a final solution to this puzzle, but it lets us look at reality from a slightly different perspective. It suggests that the way we perceive the world around us may differ from what the world is actually like.

Even if this picture of the world turned out to be true, it would change little in our everyday lives. The future, although it may in some sense already exist, would still be unknown to us. We couldn't simply see what comes next or skip the path we have to take. To find out what will happen tomorrow, we would still have to live through one moment after another, make decisions, and only then turn those events into new memories.

This theory nevertheless captures the imagination and leads to further questions. If the future already exists somewhere, what do our decisions mean? Are they part of a fixed structure, or are they something that truly changes the course of events? Is time travel possible? Does every moment we experience really pass away, or does it perhaps remain forever recorded in the four-dimensional structure of our universe?