The arrow or asymmetry of time continues to be one of the great mysteries of the Cosmos.
The name was given by Arthur Eddington, and refers to the experience we all have that time always flows in a unique direction, from past to future. It is the main feature of time.
Well, from the point of view of physics, nothing shows that this is so. On the contrary, physical laws do not distinguish between past, present and future. They are worth as much from past to future as from future to past, and nothing indicates that these laws are wrong. This is called symmetry and, with mysterious exceptions, it is the law that governs the entire Cosmos.
One of those exceptions is what makes us exist here and now (we will deal with it later). Another exception is the temporary asymmetry we call "arrow of time".
It is about discovering a law that explains that asymmetry. That is, an equation that explains that time always passes forward and is wrong if it goes backwards. Or, a law that explains that exceptional asymmetry within the general symmetry of the Cosmos. The latter seems less likely.
Until a century ago it was believed that time was absolute and always passed forward. As we experience it.
But then, precisely in Switzerland, the land of precision, Einstein formulated his theory of relativity. Time, as we live it, became only an illusion. Until today, no law proves that time flows. In fact, it can be deduced from special relativity that time does not flow.
In 1926 Schrödinger formulates his quantum probability wave equation. A particle can take different paths at once, and the same could happen over time. Physics would not distinguish and it would be just as correct if time went to the future or to the past. The particle is only defined in a reality when we observe it, and the same would happen over time. This is called “collapse of the probability wave.” But we do not know if it is applicable to time. We lack quantum knowledge to know. It was one of the proposed solutions.
Another solution takes us to the first moments of Cosmos. The arrow of time is compatible with the Big Bang and inflationary cosmology. It would have explanation if the universe had been highly ordered in its first moments.
As the universe expands and loses its initial order, time would always flow forward. It is the theory of low initial entropy and seems backed by the laws of thermodynamics. It seems the most convincing answer. But we still don't know much about the first moments of the universe to know. Today, the arrow of time remains unanswered.
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