Time goes by, time is running out, we have no time, time is money… Everybody talks about time. Nevertheless, nobody can define time without using…time! Time is undefinable: not so much because a definition doesn’t exist, there are a lot, all equally valid in their respective fields, but properly because every possible definition is unavoidably incomplete. Sometimes we talk about an existing substance, independently by us, some others like a product of our consciousness which measures phenomena, some others again like a fluid flowing. Almost every ancient civilization, discuss time in their myths. Chronos is the most famous. The interesting thing is that every culture has the same type of approach: at the beginning, there was a world without time. Time joins later and is being described as the thing that triggers a process, an evolution and since that moment you can’t get back: this is how you recognize the birth of time. Ancient time: Aristotle and Augustine of Hippo One of the first ones to meditate on time was Aristotle. The greek philosopher of the Hellenistic period considered it the measure of change. According to him, time doesn’t exist but comes directly from events, which is directly linked to. If nothing changes and happens, times don’t flow. Some century later is the philosopher Augustine of Hippo to reserve wide space to time in his masterpiece: “ the confessions”. According to the Cristian thinker, time is a product of God, who, during the creation of the world had created the time as well. Augustine, after many reflections, comes to the conclusion that time is inner, a dimension of the soul. It’s the consciousness itself that expands, a psychological dimension of the human being that embraces the plurality of external experiences. The reflection and the philosophical speculation will give way to quantitative assessment only since the scientific revolution of the 17th century, when thanks to Galileo Galilei and to the others creators of the scientific method, time will become a measurable quantity. 1656 the Dutch physicist Christiaan Huygens invented the pendulum clock: for the first time, the speed of time could be measured. This was the first precision clock. However, if we think about it, a clock measures time by a movement across space: it converts a movement of a lancet in the space in a length. If there wasn’t any clock would the time exist as well? We‘d get old for sure, but in that case, we would be the clock. Is the movement itself across the space that seems to operate like a clock, by which we deduct the flow of time. So, does time exist, or it’s just an illusion? Just by now, we guess how space and time are separated entities, but in reality, deeply related. Absolute time: GALILEO AND NEWTON The Italian physicist Galileo Galilei, in the ‘17th century, creator of the scientific method, investigated time from a scientific point of view. He was one of the first to introduce time as a measurable quantity. According to him, time is absolute: all those who measure it, moving or still, watch it flowing with the same speed and times the same length for the same phenomena. Moreover, the simultaneity among two events is absolute as well, which means that every watcher, independently from its position or velocity, can assess the simultaneity among two events. The contemporaneity of events is therefore absolute. So, a passenger on a train traveling at a certain speed takes 1 hour to read a book, both seen by his travel mates on board, and seen by a watcher standstill on land. In other words, the lenght of an event is absolut. It will be Isaac Newton, years later, to formulate the idea of absolute time. As a matter of fact, according to his “Principia Mathematica Philosophiae Naturalis (1687), he defines a mathematic time, different from the time that triggers the movement and the sequence of events. This time is rather a uniform, unflappable recipient, in which phenomena occur, invariant… as time goes by. This absolute time is required by Newton’s laws of dynamics and gravitation, discussed in his work. Nowadays this can be considered self-evident. In reality, this is an incredible insight by a genius like Isaac newton. If, for example, our smartphone suddenly went out of order, we would never think: “My lord, the laws of electromagnetism broke down”. Alternatively, if our fridge suddenly didn’t cool down anymore, we would never doubt thermodynamic laws. We would rather look for a failure in our smartphone or in our refrigerator! This is because we spontaneously take for granted the laws of physics across time. TIME, IRREVERSIBILITY AND ENTROPY This conception of time, however, gave rise to a problem. All the laws discovered by Newton, from mechanics to gravity, seem not to distinguish between past and future: they likewise describe phenomena even if time is inverted. By substituting t with -t, in fact, nothing changes. According to Newton’s physics, as a matter of fact, phenomena are reversible only. They can equally unfold in one direction or in the opposite one, by rewinding the tape without any other change in the universe. However, our experience is made up of irreversible events only. Let a gas expanding freely within a container: it will tend to occupy all the available volume, spontaneously. We will never observe it coming back and spontaneously compressing. Now, let fall a drop of ink, into a bottle of water, it will spontaneously tend to mix with water, spreading across the bottle! We will never see it get back, separated from the water like it was prior to falling down in the bottle. In the end, let’s simply think about arson or a fire. Once you have the trigger, even if you want to turn it off, it is impossible to think of returning the environment exactly as it was before it was started. Natural phenomena, both the spontaneous ones and the triggered ones, are all irreversible: they have a lot of undesirable consequences that modify permanently the universe. You can’t get everything back to the way it was before: or rather, at best you can, but you necessarily have to do other actions impacting the surrounding environment. Why in reality, transformation shows a well-defined direction that distinguishes the past from the future? Were classic mechanics laws wrong? Or just incomplete? Was Newton missing something? The issue was clarified during the 19th century when physics introduced the concept of entropy (go and watch our dedicated video on entropy if you had missed it!). It’s entropy that destroys the symmetry of time, the equality among past and future. Time flows in the direction of the increase of entropy and disorder. We can therefore talk about the arrow of time, at least for physics phenomena: phenomena occur in the direction of increasing entropy and disorder. However, time is different from becoming: every present instant is unique, is irreversible, and undirectional. Newton was right: there is an absolute time and an inner time. Our inner time always points in the same direction as the thermodynamic time. In order to guess why we can relate our remembering process to a storage process of data. In order to save data in memory, a computer must convert its own memory from a more untidy state to a more tidy one and in order to do so, according to the law of entropy, it must use some energy, partly dissipated as heat. This heat will increase the entropy of the universe. As a matter of fact, when a pc works, it recorder information in memory and produces heat dissipated outside by a fan. For human beings works at the same time: in order to memory, we must necessarily increase the entropy of the universe. Furthermore, with the birth of modern cosmology and of the theory of the Big Bang, it became clear that the universe took place in a very far moment, and since then it’s been expanding in space and in time more and more ( we talk about it in our dedicated video on dark energy! Go and see it!) Thus, there is also a cosmologic arrow of time that is the direction of time in which the universe expands, and in this precise moment, this seems to point in the same direction of the other two: the universe is expanding in the direction in which its entropy increases, but is not exactly clear why yet. Actually, it’s deemed that the universe expands in the direction of an increase of entropy because this is the only configuration where we, as intelligent living beings, can ask this question: otherwise we even couldn’t. Special relativity AND SPACE-TIME Don’t go further beyond and let’s go back to the 19th century. This was a century of great discoveries which pave the way for the revolution of time and space at the beginning of the 20th century by Albert Einstein. Electrical and magnetic phenomena were investigated by new accurate experiments and in the end, it was clear that they both were aspects of the same entity: the electromagnetic field that was unified by the Scottish physicist James Clerk Maxwell in 1871. At first, strange things were observed: according to some experiments, some electromagnetic fields seemed to transform depending on the speed of the observer. In particular, a contraction of some magnetic fields in the direction of the motion had been observed. Furthermore, it soon became clear that light was also an electromagnetic phenomenon, but unlike sound, it could even propagate in a vacuum. Furthermore, straddling the 1886 and 1889, a series of experiments cleared up that light has a finite speed by measuring it with very accuracy and showing that it’s constant in every direction and by every watcher: any observer sees light traveling at the same speed, independently from its speed. Since speed is a space/time ratio it was immediately evident that a revolution was coming. Time and space will have to change in the transition from one reference system to another. This is precisely to keep the speed of light constant. Absolute time and space of Newton and Galileo, concepts, up to that moment, unquestionable, because self-evident and intuitive, suddenly turned out wrong and had to be replaced with a new revolutionary and counterintuitive idea. Albert Einstein, with special relativity in 1905, and then completing his work in 1915 with general relativity, introduced a new insight: this is space-time. He did it with the help of mathematics as Bernhard Riemann and his professor at the time of Zurich Polytechnic, Hermann Minkowski. Space-time unifies the 3 dimensions of space with the dimension of time in a single 4-dimensional entity: the space-time. In space-time doesn’t make sense anymore talking about absolute time: there is a relative time-correlated to the spatial coordinates. Moreover, simultaneity becomes relative: depending on the reference system from which two events are watched these can result simultaneous or not. This can be very weird: in relativity, the “now” is not universal anymore: it’s only the moment in which I’m in my time, in my reference system, but my present moment doesn’t match the same present moment in another reference system. Universal clock doesn’t exist, every observer has its own clock and time flows depending on its speed. In 1971 scientists performed the first experiment that assessed the relativity of time. 4 cesium atomic clocks were embarked on commercial jets that would go around the world. At the same time, similar atomic clocks worked on the ground. 50 hours later, on landing the time marked by the instruments showed a difference of a few billionths of a second, an imperceptible amount of time but in perfect agreement with the predictions of relativity. It was the first of many experimental confirmations made with real watches. Past and future are as real as present: “Now” in time is the counterpart of “ here” in space. The flow of time is just an illusion. “It is in the gaze of consciousness that the fleeting images of a space that changes over time, are formed”.