Friday, July 22, 2022

The arrow of time

In our universe the time moves only in one direction that is forward and the movement of the time in forward direction is called an arrow of time. The arrow of time refers to the way we always progressing in particular direction most of the events we experience are irreversible e.g eggs may break but they never spontaneously reform.
The arrow of time always moves in the direction of increase in entropy increase in disorder.

The increase of disorder or entropy with time is one example of what is called an arrow of time, something that distinguishes the past from the future, giving a direction to time. There are at least three different arrows of time. First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, the direction in which we remember the past but not the future. Finally, there is the cosmological arrow of time. This is the direction of time in which the universe is expanding rather than contracting.

You need to know that the no boundary condition for the universe, together with the weak anthropic principle, can explain why all three arrows point in the same direction- and moreover, why a well-defined arrow of time should exist at all.

Let us discuss first the thermodynamic arrow of time. The second law of thermodynamics results from the fact that there are always more disordered states than there are ordered states. For example, consider the pieces of jigsaw in a box. There is one, and only one, arrangement in which the pieces make a complete picture. On the other hand, there are very large number of arrangements in which the pieces are disordered and don't make a picture.

For psychological arrow of time, you would see broken cups gathering themselves together and jumping back on the table. However, any human who were observing the cups would be living in a universe in which disorder decreased with time. Such human beings would have a psychological arrow of time that was backwards. That is, they would remember events in the future, and not remember events in their past. When the cup was broken, they would remember it being in the table, but when it was on the floor, they would not remember it being on the floor.

For cosmological arrow of time, first you need to know that the conditions to be suitable for intelligent life exist only in expanding phase. The inflation in the early stages of the universe, which the no boundary proposal predicts, means that the universe must be expanding at very close to the critical rate at which it would just avoid re collapse. By then all the stars will have burned out and the protons and neutrons in them will probably have decayed into lighter particles and radiation. The universe would be in a state of almost complete disorder. In order to survive, human beings have to consume food, which is an ordered form of energy, and convert it into heat, which is a disordered form of energy. Thus, intelligent life could not exist in the contracting phase of the universe. The contracting phase will be unsuitable because it has no strong thermodynamic arrow of time. The intelligent beings can exist only in the expanding phase.

In fundamental physics, time has no preferred direction. A valid solution remains valid if you reverse the direction of time. It's called time-symmetry. There's no "before" and "after", so the concept of communication cannot be given a meaning in time symmetric systems. Moreover, there is nothing to be communicated: the state of a system at any moment in fundamental physics determines the state at every other moment: fundamental physics is entirely deterministic. It's causal in that sense but the terms cause and effect are not meaningful. Any information present at any moment in such a system is present at every moment of the system's existence.


So we know we need a new physical theory to explain the Universe, and that this theory might not feature time.

Suppose such a theory turns out to be correct. Would it follow that time does not exist? It's complicated, and it depends what we mean by exist.

Theories of physics don't include any tables, chairs, or people, and yet we still accept that tables, chairs, and people exist.

Why? Because we assume that such things exist at a higher level than the level described by physics. We say that tables, for example, "emerge" from an underlying physics of particles whizzing around the Universe.

But while we have a pretty good sense of how a table might be made out of fundamental particles, we have no idea how time might be "made out of" something more fundamental.
So unless we can come up with a good account of how time emerges, it is not clear we can simply assume time exists. Time might not exist at any level.

Our existence is not like that; we are affected only by our past; and cannot affect our past. In 1926 Arthur Eddington labeled this mysterious asymmetry The Arrow of Time and in 1927 identified it with the direction your entropy increases.

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