Our understanding of time is quite linear. Past, present and future constitute the so-called «arrow of time» that advances imminently towards the future, it never stops. One theory affirms that time passes in two opposite but simultaneous directions and offers a new perception of the entropy of the Universe. But how did this start?
According to cosmological history, the beginning of the arrow of time occurred with the burst of the Big Bang, 13.8 billion years ago. From there the matter of the Universe gradually took shape. First the particles, then atoms and the first aggregations of mass that later gave way to the formation of the first huge stars, galaxies and later, planets.
But why, if the Universe expands in all directions, is time linear? It is the questioning that many have raised and that Julian Barbour came to challenge. Barbour is a popularizer of science very committed to his profession as a physicist, he has published various articles in the most prestigious journals in the field. He is also the author of The Janus Point: A New Theory of Time.
To put ourselves in context, we must understand that Janus is the god of Roman mythology whose representation is two faces facing opposite directions. Hence, Barbour used it to name his theory that time moves in two opposite directionsnot just one as we all believe.
His theory is that, at the beginning of time, caused by the explosion of the Big Bang, time did not begin to pass in one direction, but instead it could also take a simultaneous but opposite direction.
Thermodynamics and entropy
It is in the second law of thermodynamics where a concept widely used today appears, which is used to describe the processes of the universe and where the idea of the «arrow of time» arose; the entropy.
This law states that as energy is transferred and transformed, some of it is dissipated (wasted). For this reason, a system always evolves towards a more chaotic state and never the other way around.
This can be understood very well with the classic glass cup example If we have a glass cup, the chances of it breaking and dissipating into a thousand pieces are immense.
However, we know that once it flies into a thousand pieces, if we put them back together, the probability that they will return to what they were before on their own is totally zero. So the level of entropy reaches its maximum state when the greatest chaos reigns over the situation and it is a process irreversible.
Hence our thinking about time is linear, since The idea is that the more time passes, the greater the entropy will be.. In other words, the longer the glass cup remains, the greater the chances that it will evolve towards a chaotic state and end up breaking, and when this happens, there is no turning back.
But Barbour makes clear the main deficiency in the explanation of entropy. To understand it, you must first know that the laws of thermodynamics arose in the industrial revolution. They were described with cylinders and steam engines in mind, where energy and heat interacted in a delimited space.
the ice cube
The problem is that The Universe is not a delimited space, but is infinite. A clear example with which Barbour supports his theory of temporal bi-dimension is with an ice cube. If we put an ice cube inside a box, we will first have a very ordered cube with low entropy. But as the water melts and spills out of the box; entropy will increase.
Thus, the entropy will reach its maximum level when the water evaporates and its particles are distributed indistinguishably throughout the box. But what if the box is not a limitation? In limitless space, water particles could continue to travel and then join other particles to form more complex structures, which will grow in all directions of space and time.
This perception of entropy changes everything, it makes us think that what determines the passage of time is not the increase in entropy, but the increase in complexity, without limits of time or space.
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