What is time? An illusion or a reality?

Neil deGrasse Tyson, an American astrophysicist and one of the best (if not the best) science popularizer today, said that "Time is nothing more than what makes us prisoners of the present". And we can't think of a better way to start this exciting journey than with this appointment that invites both scientific and philosophical reflection.

And it is that as much as it is one of the most obvious and influential things in human nature, time is one of the greatest mysteries that science has faced, faces and will face. We know that it is there, advancing relentlessly and determining our life. That 60 seconds is 1 minute. That 60 minutes is 1 hour. That 24 hours is a day. And so on.

But what happens when we dive into the more fundamental nature of time? What happens when we try to define what it is? Is it an illusion, a physical magnitude or another dimension? Can time really be measured or is it just a human invention? No one can answer these questions.

And surely, the mystery surrounding the physical nature of time is what makes it so amazing, both positively and negatively. Get ready for your head to explode, because today we will embark on an exciting journey to try discover what time is, analyzing whether it is an illusion or a physical reality and observing how science has been (and continues) changing the conception of its existence.


    The arrow of time: illusion or reality?

    On a normal occasion, we would start the article by defining what time is. But this is not a normal occasion. And it is that from now on we have to warn that physicists have no idea what time is. And if even the greatest geniuses do not know what it is, things will surely be complicated. Without the "surely", actually.

    But one of the best ways to start is by talking about a key concept for our journey: the arrow of time. Coined in 1927 by Arthur Eddington, a British astronomer, this term is a way of explaining what time is but without getting too complicated. And now we will understand why.

    What is the arrow of time?

    "The arrow of time" is a concept that refers to the direction that it registers and that runs without interruption from the past to the future. Time is linear. It began to advance at the time of the Big Bang (about 13.8 billion years ago) and will continue to advance until the death of the Universe.

    This term is based on the asymmetry between past and future to explain the irreversibility of time. The past is immutable and the future uncertain. And between the past and the future, lies the present, an even more complicated concept. Because the "now" is actually something subjective. By the time your brain thinks of "now", you have already left it behind.

    We are prisoners of the present but we are unable to live in the present. I don't know if I have explained myself. I do not think so. Well, let's move on. And now that we have introduced this subjectivity, it is time to answer the big question: is time an illusion or a reality?

    Well, a very good question, yes. Do you want a clear answer? We are sorry. And it is that we cannot determine the existence or not of something whose nature we do not understand. But let's think about it a bit. Is time a physical reality or a simple invention fruit of human experience?

    Is time a physical reality or a human illusion?

    We can measure time because we are based on cosmic movements. The rotation of the Earth determines how long a day lasts and the duration of an orbit around the Sun, how long a year lasts. And from here, totally based on our experience, we have defined how long a second, a minute, an hour lasts and so on. Subjective concepts about something based on movements.

    As Aristotle said 2,500 years ago, "time is the most unknown of the unknown." He was right. And is that, does this subjectivity imply that it is an illusion? We do not know. Thats the big problem. But we have to bear in mind that, despite the fact that we think of ourselves as amazing beings, we are nothing more than sacks of organic matter with a one-and-a-half kilo brain with five senses.

    Our human nature greatly limits what we are capable of perceiving. And perhaps, time is a purely human phenomenon. Something that is in our consciousness. In our mind. And the fact that we have not found a single physical law (although we will talk about entropy later) that mathematically demonstrates this inexorable advance into the future.

    But, that we have not found a physical law to explain it means that it is a human illusion? No. Perhaps what happens is that it does not exist as an individual piece, but rather emerges as a consequence of the “whole”. In other words, a single subatomic particle does not experience time. But a material system, yes.

    Not understood? Normal. But let's take an example. A movie is made up of frames, right? If we take each frame individually, we do not see the passage of time. There is no movement. But when we put them together and project them successively, time is perceived. With "time" as a physical concept, the same could be happening. "Might". That is, we do not know if it is an illusion or not. But that does not mean that we cannot immerse ourselves in the most exciting physics.

    General Relativity: is time the fourth dimension?

    Perhaps it has seemed strange to you that we have not talked about dimensions yet. Nothing happens. Here we are. And it is that in effect, time can be defined as the fourth dimension of the Universe. A conception that was born with Albert Einstein, the famous German physicist who, between 1915 and 1916, developed the well-known Theory of General Relativity.

    And in it, one of the things he proposed was that time was not something absolute as we had always believed (we had the conception that, whether it was an illusion or a physical reality, it was a universal phenomenon), but that it was relative. What does relative mean? Little by little.

    Until the arrival of Einstein and his theory, we believed that there were only three dimensions in the Universe. And by dimension we mean the degree of freedom that a body can take in space. We had the three spatial dimensions: length (we can move back and forth), width (we can move left and right), and height (we can move up and down).

    And with these three dimensions everything seemed to work. We move in three spatial dimensions and we are subject to the inexorable passage of time. But if time stops being absolute and becomes, as Einstein said, relative, things change. Because "relative" implies that it is modifiable. And that it is modifiable implies that there is freedom (albeit limited, as we shall see) to flow through it.

    And that there is a certain degree of freedom, what does it imply? Exactly. That we have to talk about time as one more dimension. To the three spatial dimensions, a temporal dimension must be added. And these four form a single fabric called space-time, which is absolute.. Space is relative and time is relative. Separately they are relative. But together, absolute.

    And this conception of time as a fourth dimension over which three-dimensional bodies can flow served to understand, for example, the phenomenon of gravity. But we are very limited when it comes to flowing through it. Normal. We are three-dimensional beings who can only advance in the fourth dimension.

    We will advance more or less quickly depending on our relative speed with respect to other bodies and on the intensity of the gravitational field to which we are exposed, but we are forced to go inexorably towards the future and being trapped (being prisoners) in a present that does not even exist. Everything has happened, happens and will happen at the same time, without any special moment that can be marked as present.

    And if your head has not yet exploded, think that if we were four-dimensional beings (four-dimensional), then we could see all the infinite three-dimensional variations that an object follows throughout all the time of the Universe. That is, we would not care about the arrow of time. We would move along the timeline as we wanted. And we don't even talk about the fact that there could be 11 dimensions in the Universe ...

    So is it done? Time is the fourth dimension, right? Point. Man, no. We are really giving alone a synonym. But we are not defining its nature. And although it is impossible to define it, we have to talk about one last concept: entropy. But first, let's stay with this phrase by Einstein: "time and space are ways of thinking, not conditions in which we live."

    Time and disorder: what does entropy tell us?

    Did it seem to you that time was being a complicated concept? Yes? Well calm, now we add an equally complicated one. Well not so much. But it does not fall short. We are talking about the famous (but little understood) entropy. The term that is incorrectly used to describe the physical law that pushes the Universe into disorder.

    Why is it wrong? Because entropy is neither force nor law. It is a consequence of statistics applied to the Universe. And although you have an article where we delve much more about it, we are going to try to understand, briefly, what it consists of and, above all, what is its relationship with time.

    Entropy is the mainstay of the second law of thermodynamics, which tells us that the amount of entropy in the Universe tends to increase over time.. But entropy is not a force. And it is not a magnitude that measures the degree of disorder in a system. It is, as we have said, a consequence of probability applied to thermodynamics.

    And it is that entropy is a consequence (it is not a force by itself) of two factors that occur in the Universe and at the macroscopic level: many particles forming the same system and randomness in it. These two conditions make the system evolve towards the state that arises after the most possible combinatorial.

    The tendency towards disorder does not occur because there is a force that pushes towards disorder, but because at a statistical level, what we understand as disorder is much more likely than order. The molecular order is so incredibly unlikely that it is technically impossible.

    Entropy is not a force, but rather a consequence of the macroscopic states that we observe at the macroscopic level are the result of the sum of more probable microstates. Nothing has been understood, already. Do not suffer. Let's look at an example.

    Is it possible that, suddenly, the molecules in a glass of water get just in the conformation so that, in full sun, an ice cube forms? If it's posible. But it is so infinitely unlikely that it just becomes impossible in the time frame of the Universe.

    The important thing is the relationship of entropy with time. And it is that surely the time is a manifestation of this inevitable tendency towards the disorder. We move forward in time because the Universe is condemned, by simple statistics, to flow towards a state of greater disorderAs everything tends towards disorder, time will always go forward.

    Not because it is impossible for it to flow backwards, but because the probability of this happening is so incredibly (but very incredibly) low that, simply in the entire history of the Universe, it can never happen. It's crazy, but there's not enough time in time for time to go backwards.

    Time is that inevitable journey from an orderly past to a messy future.. But is time a consequence of entropy or is entropy a consequence of time? We may never know. We may never understand what time is because it is either a simple human illusion or a physical reality that escapes our limited understanding. But we know it's there. And whatever it is, we play by their laws.

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