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In this material we will talk about a physical phenomenon that is very present in our daily life,, The concept of the arrow of time is popularly referred to the direction that it records and that runs without interruption from the past to the future, going through the present, with the important characteristic of its irreversibility, that is to say, that future and past, on the axis of the present, show among themselves a clear asymmetry (the past, which is immutable, is clearly distinguished from the uncertain future). Sometimes we ask ourselves where is it coming from in time, well I invite you to observe this interesting publication so that you clarify your doubts .....
The expression itself, the arrow of time, was coined in 1927 by the British astronomer Arthur Eddington, who used it to distinguish a direction in time in a four-dimensional relativistic universe, which, according to this author, can be determined by a study of the different systems of atoms, molecules and bodies.
History of the expression
In 1928, Eddington published his book The Nature of the Physical World, which helped to popularize the arrow of time. In it, the author wrote:
Let's draw an arrow of time arbitrarily. If by following its course we find more and more random elements in the state of the universe, in such a case the arrow is pointing to the future; if, on the other hand, the random element decreases, the arrow will point to the past. Here is the only distinction admitted by physics. This necessarily follows from our main argument: the introduction of randomness is the only thing that can not be undone. I will use the expression "arrow of time" to describe this unidirectional property of time that does not have its torque in space.
Eddington, therefore, points out three distinctive points of this arrow:
- It is vividly recognized by conscience.
- It is also required by reason, which informs us that a reversible arrow would be an absurdity (as we will see in the example of the glass).
- The arrow of time is not included in physical science, except in the study of the organization of certain phenomena.
Thus, according to Eddington, the arrow of time indicates the direction of the progressive increase of the random element. Following an ancient argument from thermodynamics, Eddington concludes that in regard to physical science, the arrow of time is an exclusive property of entropy.
Explanation
Macroscopic and microscopic plans
From the emergence of quantum mechanics, a century ago, it is believed that physical processes at the microscopic level are for the most part temporarily symmetrical, suggesting that the theoretical statements that describe them will be true if the direction of time is reversible. In the macroscopic plane, the opposite happens, since there is a clear direction in the arrow of time, from the past to the future (the glass tumbler that falls from the table breaks against the ground, without ever recomposing itself on the table) . The arrow of time, then, would be represented by anything that exhibited such temporal asymmetry. Or, in other words, that in the macroscopic plane, or visible, time always moves forward, while in the microscopic, or elementary particles, can do it equally backwards.
Symmetry and asymmetry
The symmetry of time can be understood by a simple analogy: if the time were perfectly symmetrical it would be possible to see a movie -which would have filmed real events- so that everything that was visualized in it seemed realistic, and the film was passed to front or back.
The existence of an arrow of the determined time would be observed easily when seeing the crystal glass recomposing itself on the table after broken and judging that it would not be a realistic scene. However, a filming of the planets of the solar system orbiting the sun backwards could be as realistic as forward, because in both cases they would seem to obey the physical laws.
Example of irreversibility
It must be considered a situation in which a large container is filled with two separate liquids, for example, a colored dye on one side and water on the other. Without any barrier between the two liquids, the mutual thrust between their molecules would result in a larger mix as time passed. In the same way, once the dye and water are mixed, one would never expect them to separate by themselves. A film of the mixing process would be realistic if, and only if, it were projected forward, but never if it were projected backwards.
If the container is observed at the beginning of the mixing process, only partially mixed liquids would be seen. It would be reasonable to conclude that, without the need for external verification, the liquid would reach this state because it would be more ordered in the past, when there was more separation of molecules, and would be more disordered or mixed, in the future.
Now imagine that the experiment is repeated, this time using only a few molecules, for example, ten molecules, in a very small container. By colliding with each other, it could happen that the molecules, by mere chance, segregate neatly from each other, with dye on one side and water on the other, which can be expected to happen from time to time, obeying the theory of quantum fluctuation, which provides for the possibility, whether small, that the molecules separate at some point in that way by themselves. However, considering a much higher number of molecules, this segregation is so unlikely that, for it to occur, on average, it could be expected that more time would have elapsed since the origin of the universe.
Thus, a film showing a large number of molecules separating by themselves, as described above, might seem unrealistic and one would be inclined to claim that the film had been projected backwards.
Types
Thermodynamic time arrow
This concept is foreseen in the Second Law of Thermodynamics, which holds that within an isolated system, entropy can only increase over time, and never decrease. Entropy can be conceived as the tendency to disorder of any organized system, or as a measure of that disorder, and in this way the Second Law implies that time is asymmetric with respect to the amount of order in an isolated system: custom-made that time passes, every system becomes more disordered.
The immediate consequence is that this asymmetry can serve empirically to distinguish between past and future.
Thermodynamics is not strictly applicable to all phenomena, since certain systems can fluctuate to states of lower entropy, according to the Poincaré conjecture. However, it serves to describe the general tendency in nature to greater entropy.
The time arrow of thermodynamics seems to be related to the following arrows of time, and presumably underlies all of them, with the exception of the arrow of weak weather.
Arrow of cosmological time
The arrow of cosmological time defines the direction of an expanding universe, or inflationary. This can be related to the arrow of thermodynamics, which, due to the previously described entropy, foresees a universe aimed at a thermal death (in English, Big Chill or Big Freeze) in which the amount of usable energy becomes insignificant.
The British physicist Stephen Hawking raises, in this sense, what would happen if the universe stopped expanding and began to contract for having overcome the critical gravitational limit, with an arrow of time inverted, in which gravity tended to collapse all in one Big Crunch (in Spanish great implosion or great crunch, contrary to the Big Bang). It concludes that the thermodynamic arrow would not reverse and the disorder would not begin to decrease. "People would not live their lives backwards, towards birth."
Likewise, Hawking continues, according to the anthropic principle, we can only currently be living in the expansive phase (and biological evolution) of the universe, since intelligent beings can only exist in this phase because the contractive phase would be inadequate for it, by not having a clear thermodynamic and psychological arrow of time (as a result of the great cooling and the low level of entropy that would have been reached).
If the arrow of cosmological time is related to the other arrows, in that case the future is, by definition, the direction in which the universe is growing. Thus, the universe expands rather than contracts, by definition.
For the physicist Roger Penrose, the expected unification of relativistic and quantum physics (in particular the quantum theory of gravity) will finally allow the deep understanding of the arrow of time.
Arrow of radiation time
Every physical wave, from radio waves to sound waves, or those that arise around a stone thrown into the water, expand outward from its source, although the wave equations contemplate the existence of both converging waves and radiating waves . This arrow has been inverted in carefully designed experiments that have originated convergent waves. The possibility of creating initial conditions to produce convergent waves is much lower than the probability of conditions that produce radiating waves. Normally, then, the radiant wave increases the entropy, while the convergent wave reduces it, opposing therefore the latter, under ordinary circumstances, to the Second Law of Thermodynamics.
Causal time arrow
The causes usually precede the effects. The future can be controlled, not the past. But the problem of using causality as an arrow of time is that, as the philosopher David Hume pointed out, the causal relationship can not be perceived by itself, since the observer is only able to perceive "the chain", the succession of the events, the cause and the effect, but not a link, so to speak, material or in some way recordable.
On the other hand, it is extremely difficult to provide a clear explanation of the real meaning of the terms cause and effect. It is clear that dropping the glass vessel is the cause and its breaking the effect, however, it could be that the asymmetry that the observer perceives in such a case is not really the arrow of causal time really, but of thermodynamics. If the thermodynamic arrow were inverted, then one might think that the pieces of glass were the cause and the glass recomposing the effect on the table.
Weak time arrow
Certain interactions in the subatomic plane imply that the weak nuclear force violates the conservation of parity and charge, but only very rarely. According to the CPT symmetry theorem (fundamental symmetry of physical laws in the environment of transformations involving the inversions of charge, parity and time simultaneously), this means that time could be irreversible, and therefore sets an arrow weather. These processes could be responsible for the creation of matter in the early universe.
This arrow is not related to any other by any known mechanism, which could suggest that our universe could be made of antimatter instead of matter. More likely, the definitions of matter and antimatter can be reversed.
This broken parity very rarely means that the arrow only very little points in one direction, standing apart from other arrows whose directions are much clearer.
Quantum time arrow
According to the Copenhagen interpretation of quantum mechanics, quantum evolution is governed by the Schrödinger equation, which is temporarily symmetric, and by the collapse of the wave function, which is irreversible over time. Since the mechanism of the wave function collapse is still obscure, it is not known how this arrow is linked to the others. While at the microscopic level the collapse seems to show no tendency to increase or decrease entropy, some scientists believe that there is a prejudice that uncovers the thermodynamic arrow on a macroscopic scale. According to the theory of quantum decoherence, and assuming that the collapse of the wave function is only apparent, this arrow of time is a consequence of the arrow of thermodynamic time.
Psychological time arrow
Psychological time is, in part, the catalog of the growing accumulation of data in memory from continuous fluctuations in perception. In other words, what we remember configures the past, while the future consists of those events that can not be remembered. The old method of comparing unique events to understand and generalize repeated events, such as the apparent movement of the sun, moon and stars, transferred to all celestial bodies, is a good model of this. The accumulation of memories in memory creates an arrow of mental time.
Another arrow is originated by the sensation that our perception is a continuous movement and exchange between the unknown (the future) and the known (the past). The anticipation of the unknown shapes the psychological future that always seems to be something that advances forward, but, like the reflection in the mirror, configures what has already been stored in memory, such as desires, dreams and hopes, which In fact, they always seem to be for the person beyond time.
The mental association between the past (behind) and the future (ahead) can be culturally conditioned, as shown by an investigation carried out in 2006 with the Aimara Indians, who, contrary to other peoples, perceive the future behind and the past by come.
The arrow of psychological time is reducible to thermodynamics if we see memory as the correlation between neurons (or computer bits) and the outside world. The volume of memory increases in that correlation, but always in the sense of the future, never of the past.
The passage of time is also clearly seen in the field of volition and action, which tend to positively affect for us the configuration of the future. And, of course, no one would ever try to affect the course of the past.
The aging process is, unfortunately, something more than the simple accumulation of memories, hence the human being has always had more illusion to go back in time than to travel to the future or invest the arrow of time.
According to the physicist and student of this problem Paul Davies, «it could be that some future work locates the brain processes responsible for our impression of the passage of time».
In any case, Albert Einstein always affirms bluntly: "Past, present and future are only illusions, although they are obstinate illusions."
The passage of time has dazzled and intrigued entire generations of philosophers, artists and poets, but for physical science, since the beginning of the twentieth century, "time, in its conceptual framework, does not pass, but simply is" (Paul Davies ).
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