The year’s start is a special moment, the beginning of a new cycle, and although this period is somewhat subjective, it allows us to estimate and appreciate time as a continuous, constant and permanent flow. Yet time has profound implications if we associate it with aspects of physics.


Referring to time, this is a concept that in itself is difficult to explain and understand, but it is something that in our appreciation is associated with our existence.

In science, time is mainly understood as a measure, it is what a clock tells us, and it is considered an infinitely divisible flow, even when we measure it in a discrete form, in seconds. This factor is an integral part of classical physics, Newtonian or non-relativistic, in which time was considered as a consistent progression for everything and all, this form of time is what people in general experience as time flows.

The first radical change on the understanding of time came with the concept of relativity, at the beginning of the 20th century; where time is no longer considered an absolute, but is dependent on the conditions of the observer; and with the inclusion of phenomena such as time dilation, which can be caused by the rapid movement of an object or by the extreme action of gravity, a model that has become the norm in the world of physics and is indispensable in certain services that we use on a daily basis, such as the GPS (Global Positioning System), navigation services.

 The arrow of time.

But time, as a physical phenomenon has a property that we all understand very well, it is that it flows in one direction, we all know that time passes from a fixed and immutable past, which is accessible either as memories or records, towards an uncertain and indeterminate future which is not precisely fixed; and it is the present what we actually experience; this being only the limit where the past and the future meet. This flow in one direction is what is denominated as “The arrow of time”, where physical phenomena are irreversible, for example, it is quite simple to break a glass or a dish, but it is very difficult (or very unlikely), if not impossible that this glass or dish spontaneously reintegrate itself or “un-break” being just as it was before the breaking. It is practically inconceivable to consider that this progression can happen in another way. It is such progression through different moments what’s associated with the flow of time from the past to the future.

Arrow of time

Arrow of time (wikipedia.org)

The arrow of time is associated with two fundamental physical phenomena, the second law of thermodynamics and the concept of symmetries in physics.


Symmetries are more commonly to associated to aspects of aesthetics and beauty; but in physics symmetries have to do with the mathematical characteristics of a system, where its characteristics are conserved in time and space after applying a transformation. This is why they are associated to conservation laws, a discovery made by Emmy Noether, with the Theorem that bears her name, and that proclaims that each symmetry has an associated conservation law.

Emmy Noether

Emmy Noether (sciencenews.org)

Probably the simplest example of a symmetry that is intuitive for all of us is bilateral symmetry. Most human faces closely approximate to a bilateral symmetry; a change like a mark or mole on one side of the face breaks this symmetry. Other examples are those of the symmetry of a triskelion (shown below), where a rotation of 120 degrees preserves the same appearance, or the perfect symmetry of a sphere, where no matter what rotation is applied, the sphere still looks the same. These properties also occur in nature, such as the law of conservation of energy or conservation of electric charge.


Triskelion – Rotation Symmetry (wikipedia.org)

But what does all this have to do with time? Well, time is one of the cases in which, opposed to symmetries in nature, it is an asymmetry, where we cannot apply a temporal transformation and maintain the same characteristics, this is due to the natural tendency of the matter to maintain a state of minimum energy, an effect that is intimately related to the second law of thermodynamics.

Second law of thermodynamics

Why is time related to thermodynamics? If you are familiar with the second law, it states that “The total entropy level of an isolated system is always increasing”, this law was the result of the works of Sadi Carnot, Rudolf Clausius and Eduard Boltzmann, among others, in synthesis, describes that the entropy or “disorder” of a system is always increasing. And this implies that everything that surrounds us, and the universe itself tends to flow from a state of order to one of disorder. Another way to distinguish this law is in the exchange of temperature, where heat always flows from a warmer object (a state of order), to a colder one (a state of disorder), and never in reverse.

Entropy and disorderliness

Entropy and disorderliness (autodesk.com)

Thermodynamics and the implications of the second law are considerable. The universe is losing usable energy (or rather dispersing its energy), constantly. The universe had a finite beginning, we know that this happened around 13.8 billion years ago, the moment when it had zero-entropy or when it was in its maximum order state. And since the Big Bang, the universe has been, continuously losing energy; this will continue until the last trace of order disappears in the universe, and where what will remain is an infinite sea of ​​homogeneously distributed particles, where the energy will be totally dispersed and there will be no orderly structures in the universe such as planets, stars, galaxies or black holes, all this will disappear. At one time in this extreme future there will be no more changes, there will be no way to distinguish the difference between one moment and another, and we can deduce that what allows us to differentiate the present from the past will cease to exist, being everything exactly in the same state, as in a photograph, static and unaltered, basically marking the end of the existence of the universe, and it can be said that, therefore, the end of time.

This description is only a hypothesis, one of many of how the evolution and end of the universe will be, and one which has a long way to go, given that conservative estimates of how long this might take are in the order of 10110 years (Number represented by a one followed by one hundred-ten zeros), so this is very far in the future, if we consider that the universe has existed for 13.8 billion years (or numerically 13,800,000,000 years). So, for now time continues, in the form of a new year; so, happy 2019 year to all.

Saludos, Alex; ¡ScienceKindle!

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