Why is time a stubborn illusion

Gather ye rosebuds while ye may, / Old Time is still a-flying. " the common belief that time flies. Who could doubt it? The passage of time is arguably the most fundamental part of human perception; all in all, we experience it more intimately than the experience of space or mass. The passage of time has been compared to the flight of an arrow or a flowing river that inexorably carries us from the past to the future. Shakespeare wrote of the "carousel of time", his compatriot Andrew Marvell of the "winged chariot of time that rushes by".

As catchy as these images may be, they run into a deep and indissoluble contradiction. Nothing in known physics corresponds to the passage of time. On the contrary: the physicists emphasize that time does not flow at all; it just exists. Some philosophers believe that the very idea of ​​running away is nonsensical in itself, and that to speak of the flow of time is based on a misunderstanding. But how can something that is so fundamental to our experience of the world prove to be in error? Or is there a crucial quality of time that science has so far eluded?

The "now" as a matter of opinion

In everyday life we ​​divide time into the past, present and future. The grammatical structure of the language reflects this fundamental distinction. We only admit reality to the present moment. We think of the past as no longer existing, the future as something shadowy to come, something without fully formed details. In this simple picture, the "now" of our conscious perception slides steadily forward, transforming events that once lay in the unformed future into the concrete but fleeting reality of the present and banishing them from there to the fixed past.

This representation seems self-evident to us, but it contradicts modern physics. Albert Einstein wrote to a friend: "Past, present and future are only illusions, even if they are persistent". The amazing statement follows directly from Einstein's special theory of relativity, which denies the present moment any absolute, universal meaning. According to the theory, simultaneity is a relative term. Two events that take place at the same moment as observed from one reference system can occur at different times from another reference system.

There is no clear answer to the innocent question "What is happening on Mars right now?" The reason is that Earth and Mars are far apart - about twenty light minutes. Since no information can be transmitted faster than light, an earthly observer cannot say what is happening on Mars at the same moment. He has to derive the answer retrospectively after the light has traversed the path between the planets. The indirectly inferred past event will turn out differently depending on how fast the observer is moving.

For example, during a future manned mission to Mars, someone in the earthly control center could ask: "What is Commander Jones doing now on Base Alpha?" Looking at the large display board, his colleague would see that Mars is currently 12: It is 00 o'clock and answer: "Lunch". But an astronaut who is passing the earth at almost the speed of light at the same moment would - depending on his flight direction - say when looking at his watch that it is earlier or later than 12:00 on Mars and, depending on the case, reply: “Eat do "or" wash the dishes ".

How time doesn't go by

Such differences thwart any attempt to give special meaning to the present moment, for whose "now" does this moment refer to? If you and I move relative to each other, an event that for me still lies in the undecided future may already belong to the established past for you.

The most obvious conclusion is that both the past and the future are fixed once and for all. For this reason, physicists prefer to think of time differently: as a time card - analogous to a map - on which the past and future are completely and jointly fixed. This performance is sometimes called block time. This lacks any reference to a specific moment as the present or a process that systematically transforms future events into present and then into past events. In short, the time of physicists does not pass or flow.

Some philosophers came to the same conclusion when examining what we usually mean by the passage of time. They claim the idea is contradicting itself. After all, the concept of flow has to do with movement. It makes sense to talk about the movement of a physical object - such as an arrow through space - because one can observe how its location varies over time. But what significance can be assigned to the movement of time itself? Relatively what is it moving for? While the usual type of movement relates one physical process to another, the supposed flow of time relates time to itself. Even the simple question "How quickly does time go by?" Reveals the absurdity of this idea. The trivial answer "At a second per second" means nothing at all.

Although we have got used to talking about the passage of time in everyday life, this idea does not convey any additional information. For example, consider the following text: Alice was hoping for a white Christmas, but when the day came she was disappointed because it was just raining; but she was happy when it snowed the following day. "

Although this description is full of times and references to the passage of time, the exact same information is conveyed by simply adding dates to Alice's mental states - without the slightest reference to the passage of time or the changed world. The following pedantic and dry catalog of facts is sufficient:

- December 24th: Alice hopes for a white Christmas.

- December 25th: there is rain. Alice is disappointed.

- December 26th: there is snow. Alice is happy.

Nothing happens or changes in this description. There are simply states of the world on different days and associated mental states of Alice.

Ancient Greek thinkers such as Parmenides and Zeno argued in a similar way. A century ago the British philosopher John McTaggart tried to draw a clear distinction between two descriptions of the world: the so-called A-series represents the world in the form of events, the B-series formulates states of the world. Each of them seems to be an accurate description of reality, and yet they appear to contradict one another. The event "Alice is disappointed" was once in the future, then in the present, and finally in the past. But past, present and future are mutually exclusive categories - how can one and the same event belong to all three? McTaggart used the incompatibility of the A and B series as an argument for the unreality of time itself - perhaps an overly radical conclusion. Most physicists would conclude less dramatically: the flow of time is unreal, but time itself is as real as space.

The standing arrow of time

Great confusion in discussions about the transience of time arises in connection with the so-called arrow of time. To deny that time is flowing does not mean to claim that the terms "past" and "future" have no physical basis. The events in the world are undoubtedly a directed sequence. For example, a raw egg will burst if it falls on the ground, while the reverse process - a broken egg spontaneously assembles into an intact egg - has never been observed. This irreversibility is physically expressed in the Second Law of Thermodynamics, according to which in a closed system the entropy - roughly speaking, the extent of the disorder - tends to increase over time. An intact egg has a lower entropy than a destroyed one.

Because most of the processes in nature are such irreversible physical processes, the Second Law creates an obvious asymmetry between the past and the future. This gives the time axis a direction, the so-called time arrow, which points from the past into the future. But that does not mean that the arrow is moving towards the future - any more than a compass needle pointing north indicates that the compass is moving north. Both arrows symbolize an asymmetry, not a movement.

The arrow of time denotes an asymmetry in the world with respect to time, not an asymmetry or a flow of time itself. The labels "past" and "future" apply to directions in time, just as "up" and "down" apply to directions in space. On the other hand, speaking about "the past" or "the future" is, strictly speaking, just as meaningless as saying that an object rises into "the upward" or sinks into "the downward".

In order to illustrate the distinction between "past" or "future" on the one hand and "the past" or "future" on the other, we imagine a film that records how an egg falls to the ground and shatters. If the film runs backwards through the projector, every viewer immediately recognizes the unreality of the scene. Now we assume that someone cuts the film strip into individual images, shuffles them completely at random and stacks them on top of one another. It will certainly be easy for us to put the stack of pictures in the right order, with the broken egg on top and the intact one at the bottom. This vertical stack reflects the asymmetry expressed by the arrow of time, because before us lies an ordered sequence in the spatial vertical, which proves that the temporal asymmetry is in fact a property of the world states and not a peculiarity of time itself. The film doesn't even have to be shown in the cinema for the time arrow to be recognizable.

Since most of the physical and philosophical analyzes of the time are unable to discover any signs of drainage, we are faced with a riddle: where does the strong, omnipresent impression that the world is constantly in flux come from? Some researchers, in particular Ilya Prigogine, a Belgian Nobel Prize winner in chemistry who now teaches at the University of Texas at Austin, believe that the subtle physics of irreversible processes makes the flow of time an objective trait of the world. But I and others believe that it is a specific illusion.

Ultimately, we don't really watch the passage of time. What we actually experience is that later states of the world are different from earlier states that we still remember. The fact that we remember the past and not the future does not mean that time is passing, but that it is asymmetrical. Only a curious observer registers the flow of time. A clock measures the time between events much like a yardstick measures the distance between places; it does not measure the "speed" with which one moment follows another. That is why the flow of time is evidently not given objectively, but a subjective phenomenon.

Flow of time - a fraud?

This illusion literally cries out for an explanation, which is not to be sought in physics, but in disciplines such as psychology, neurophysiology and perhaps linguistics or cultural studies. Modern science has hardly addressed the question of how we perceive the passage of time - we can only speculate about the answer here. Perhaps the problem is related to how the brain is working. If we turn quickly in circles several times and suddenly stop, we get dizzy. Subjectively, we have the impression that the world rotates around us, although our eyes see one and the same section. The apparent movement of the environment is an illusion created by the rotation of the fluid in the inner ear. Maybe the flow of time is something similar.

There are two aspects of time asymmetry to which the false impression that time is flowing could be put down. The first is the thermodynamic distinction between the past and the future. As physicists have known for a few decades, the entropy of a system is closely related to its information content. Because of this, memory formation is a directed process - new memories add information and lower the brain's entropy. Perhaps we perceive this directionality as a flow of time.

A second possibility would be that our perception of the time stream has something to do with quantum mechanics. Since the beginning of this theory, physicists have known that time plays a unique role in it, very different from space. Above all, the special role of time makes it so difficult to unite quantum mechanics with general relativity. The Heisenberg's uncertainty relation, according to which nature is indeterministic in itself, means that the future - including the past, by the way - is open. This indeterminism is most evident on the atomic scale and states that the observable quantities that characterize a system vary from one moment to another with a certain degree of uncertainty.

For example, an electron hitting an atom can bounce off in many different directions, and it is usually not possible to accurately predict the outcome. The indeterminism of quantum physics means that there are many - possibly infinitely many - alternative futures or possible realities for a quantum state. Quantum mechanics provides relative probabilities for every observable result, but does not indicate which possible future will become a reality.

However, when a human observer takes a measurement, he gets a definite result; for example, he will find that the rebounded electron is moving in a certain direction. The measurement process filters out a single, unambiguous reality from a huge arsenal of possibilities. In the mind of the observer, the possible becomes the real, the open future into the fixed past - and that is exactly what we mean by the flow of time.

Physicists disagree on how this transition from many possible realities to a single reality takes place. Many believe that it has something to do with the observer's consciousness because the process of observation forces nature to make a decision. Some researchers, like Roger Penrose of Oxford University, claim that consciousness - and the sense of time elapsing - are related to quantum processes in the brain.

Although there is currently no evidence of a special "time organ" in the brain that would correspond to the role of the visual cortex in visual perception, research may in the future identify the brain processes that are responsible for our sense of time. One could imagine active ingredients that cancel out the subjective impression of flowing time. Indeed, some people claim that through meditation they can naturally attain such mental states.

What if science could actually expose the flow of time to be imaginary? Perhaps we would no longer worry about the future or mourn the past. The fear of death would become as irrelevant as the fear of childbirth. Expectation and nostalgia might disappear from human vocabulary. Above all, the sense of urgency that so often attaches to our actions and aspirations would pass. We would no longer be slaves to the urge to act now, for the present and the future would literally be things of the past.


How time came into the world. The creation of an illusion of order and chaos. From Henning Genz. Hanser, Munich 1996.

The Physical Basis of the Direction of Time. By H. Dieter Zeh. Springer, Heidelberg 2001.

About Time: Einstein’s Unfinished Revolution. From Paul Davies. Simon & Schuster, 1995.

The Physics of Time Asymmetry. From Paul Davies. University of California Press, 1974.

The unreality of time. By John Ellis McTaggart in: Mind, Vol. 17, p. 456 (1908).


- Our senses tell us that time is flowing: the past is fixed, the future is uncertain, and reality is experienced in the present. But various physical and philosophical arguments speak against it.

- The passage of time is probably an illusion.Perhaps consciousness includes thermodynamic or quantum physical processes that create the impression that we are experiencing one moment after the other.

The other dimension

The theologian Augustine wrote in the 5th century: “So what is time? If nobody asks me about it, I know; if I want to explain it to someone who is asking, I don't know. ”Because we experience time psychologically, physical definitions of time seem dry and inadequate to us. For the physicist, time is what clocks measure. Mathematically speaking, it is a one-dimensional space that is usually considered to be continuous. But it could also be quantized and consist of discrete "chronons" - like a film made of individual images.

Time can be treated as a fourth dimension, but it is not identical to the three dimensions of space. Both in everyday experience and in physical theory, time and space are treated differently. For example, a different formula applies to the calculation of distances in space-time than to normal spatial distances. The difference between space and time is closely related to the concept of causality; it prevents cause and effect from becoming hopelessly mixed up. On the other hand, many physicists believe that space and time lose their separate identity at the smallest intervals and periods of time.

The present is nothing special

In everyday understanding the present moment is unique: only now is the world really there. While the clock is ticking, this moment passes and another arises. We call this process the flow of time. For example, the moon takes a certain position on its orbit around the earth. A little later it no longer exists in this place; but it can be found elsewhere.

But naturalists tend to take the view that we cannot highlight a particular moment as present if everyone can claim that right. Objectively speaking, the past, present and future are equally real. The entire eternity exists in a four-dimensional block, which consists of time and the three dimensions of space.

Pre-print from the special issue "Zeit" (publication date April 11, 2003)

From: Spectrum of Science 2/2003, page 1
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This article is included in Spectrum of Science 2/2003