What is time? Such a familiar concept seems to require no explanation. However, modern physics has to dispel many of our beliefs, and time is, to a large extent, an illusion. The debate about the true meaning of time, even about its very existence at a fundamental level, is today more lively than ever and is connected to the deepest questions of theoretical physics.
Einstein and spacetime
At the beginning of the twentieth century, Albert Einstein revolutionized our perception of space and time with the theory of special (1905) and general (1915) relativity. According to them, time depends on the speed of the observer and the gravitational field to which he is subjected, phenomena that have been experimentally proven.
Time Passes More Slowly in Your Head Than in Your Feet
Let’s say your house clock strikes 10 when you go for a walk and 11 when you return. Common sense tells you that an hour has passed for you and that the watch in your pocket (if it’s working properly) will also have run an hour. But, strictly speaking, this is wrong. Because it has moved, the time that has passed for you and your watch is slightly less than the time you left at home. The difference is so small that for practical purposes it is zero. But when this type of experiment is done using atomic clocks, it is found that the time that has passed is different (and the difference is exactly what Einstein’s theory predicts).
The same is true for gravity: the stronger the gravitational field, the slower time passes. When you stand, time passes faster in your head than in your feet. This has also been proven experimentally.
There is no universal time. It is not possible to say that “at a given moment” reality is determinative, consisting of events occurring simultaneously in different places. For another observer, the events that make up “the reality of the present” are different.
The Flow of Time for Physics
The most powerful psychological insight about time is that, unlike space, it flows. Past events have already happened. They existed, but they no longer exist. And the future has not yet happened. Only the present has real existence.
However, there is nothing in the equations of physics that tells us that time flows in this way. These laws describe events at different times (that is, they are characterized by different values of the time coordinate), but they do not tell us that time flows from the past to the future.
As incredible as it may seem, the passage of time is probably an illusion. A very powerful illusion, of course.
What is the reason for this collective illusion?
Suppose time were flowing more slowly. How would you observe it? In this game you should not be fooled into imagining that time is running slower for everyone except us. We are part of the universe and, therefore, our brain processes (and subsequent thoughts) will also slow down, in sync with all the clocks. So the answer is that we would not notice it at all. We would perceive everything exactly the same.
This would be the case even if the “flow of time” were reversed. At every moment our thoughts would be the same, and consequently so would our “memories” and our perception of time.
The arrow of time
However, time seems to pass, and it does so in a specific direction and not in the opposite direction. This is what is called the “arrow of time”. When we stir milk into a cup of coffee, the two mix, but the reverse never happens. When an egg falls to the ground, it breaks, but it never happens that the pieces spontaneously recombine and the egg jumps into our hands. The reason for these seemingly irreversible processes lies in the second law of thermodynamics, according to which “entropy always tends to increase”.
In colloquial terms, entropy is something like the disorder of a physical system. In fact, the second law is not a physical law, but pure statistics. There are far more disordered states than ordered ones, and therefore evolution always tends towards disordered systems.
Take a deck of 20 red cards and put 20 black cards on top and shuffle it. You will quickly lose this special arrangement. But no matter how much I shuffle, in practice it will never return to the original configuration, although in principle it would be possible. It is these irreversible processes that create an arrow of time, which distinguishes the past (minor entropy) from the future (major entropy).
Order in the Big Bang
The universe at the time of the Big Bang had very low entropy, that is, very little disorder. No one knows the reason for this crucial event (although there are interesting models to explain it), but thanks to this, entropy was able to increase and the arrow of time that we know was created.
Since that distant moment, entropy has only developed through irreversible processes, such as those mentioned (breaking eggs, mixing liquids) and others of greater magnitude (stars burning hydrogen).
Irreversible processes produce a strong sense of cause and effect, in this time series. However, in the equations of physics we do not find this distinction, since all processes are truly reversible.
The stone in the pond
Imagine throwing a stone into a pond, causing the typical concentric waves on the surface. In principle, the process could take place in reverse: with a random fluctuation of the water surface, perfectly concentric waves would form that would advance to the center where a vortex would be produced that would lift the stone from the bottom of the pond and drop it into his hand, then leaving the surface at perfect rest. Like a movie in reverse.
This is not forbidden by the laws of physics, but it is extremely unlikely. The reason lies in the second law of thermodynamics: processes always go from lower to higher entropy. Both the state of the stone in our hand and the calm lake have much less entropy than the stone at the bottom of the lake, and the water is slightly heated by the impact of the stone.
If we are shown a snapshot of circular waves on the surface of a lake, we will associate them (say they have been “caused”) to some state of lower entropy (for example, the fall of an object). The opposite (a random fluctuation of the water) is extremely unlikely.
Therefore, we will place the cause in the past, in the realm of low entropy. This is why we talk about causes and effects, where the former precede the latter.
And it is also why records and memories of the past are formed. If a stone falls into mud instead of water, the waves that are created “freeze”, providing a record of the “cause” that created them, which, as we said, is always an event in the past.
The world is full of traces of the past of this kind: craters on the Moon, fossils, human constructions, etc. For the same reason, we associate the records of our brain (our memories) with past events that have “caused” them. And this is what produces the psychological sensation of traveling from the past to the future: from the past (low entropy) we have abundant records and memories, while the future (high entropy) is uncertain. The perception is that the past “has already happened”, but the future “has not happened yet”, even though the laws of physics do not provide exactly this interpretation.
The Nature of Time
While most physicists would agree with the above, the truth is that we still don’t know the full nature of time. And we won’t until the theory of general relativity is reconciled with quantum mechanics, the two pillars of modern physics.
For now, we can enjoy the idea that the passage of time (desired or not) is simply an illusion.
