# Direction of Time

## Charge, Parity and Time Reversal Symmetry

Charge, Parity and Time Reversal Symmetry is the basic symmetry of laws of physics under combination of Charge Conjugation (C) Parity Transformation (P) and Time Reversal (T).
The laws of physics that govern the behavior of matter under all normal situations are unchanged under the operation C and P on their own.

In other aspects the life of the other inhabitants of another planet or celestial body and he holds out his life hand, don’t shake it. They might be made of antimatter. You would disappear in a tremendous flash of light. If the laws of physics are unchanged by the combination C, P, T alone. Yet there is a big difference between forward and backward directions of time in ordinary life. Imagine a glass kept on the table falling off it and breaking in pieces on the floor. If you make a video of the same, you can tell whether it is being run forward or backward. If you run it backward, you will see the pieces suddenly gather together off the floor and again jump of the same table where it was kept earlier in the same way it was.< br/>But we know that it is not real it is just a video because this kind of action is never observed in our daily life.

## The Arrow of Time

It seems to us that the Time has direction, to be inherently directional: the past lies behind us and is fixed and unchanged, and accessible by memory or written documentation; on the other hand the future, lies ahead and is not necessarily fixed, and, although we can perhaps predict it to some extent, we have no exact evidence or proof of it. Most of the events we experience are irreversible as in the above example of a broken glass. It appears inconceivable to us that that this progression could go in any other direction. This one-way direction or asymmetry of time is often referred to as the arrow of time, and it is what gives us an impression of time passing, of our progressing through different moments. The arrow of time, then, is the uniform and unique direction associated with the apparent inevitable “flow of time” into the future.

The Arrow of Time, concept developed in 1927 by the British astronomer Arthur Eddington involving the “one-way direction” or “asymmetry” of time. This direction, according to Eddington, can be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world. What interested Eddington is that exactly the same arrow of time would apply to an alien race on the other side of the universe as applies to us. It is therefore nothing to do with our biology or psychology, but with the way the universe is. The arrow of time is not the same thing as time itself, but a feature of the universe and its contents and the way it has evolved.

## The Thermodynamic Arrow

The thermodynamic time asymmetry is one of the most salient and pursuant features of the physical universe. The flow of heat is always from hot to cold it is never reversed. The smell of perfumes spreads throughout its available volume, never the reverse. Car engines convert fuel energy into work and thermal energy, never the reverse. And so on. The science of thermodynamics is able to capture these generalizations as consequences of its claim that systems spontaneously evolve to future equilibrium states but do not spontaneously evolve away from equilibrium states. This generalization covers an amazing amount of macroscopic physics and is rightly celebrated as one of the great laws of physics.

## The Cosmological Arrow

Scientific cosmology is an empirical discipline whose objects of study are the large-scale properties of the universe. In this context, it is usual to call the direction of the expansion of the universe the “cosmological arrow of time”. However, there is no reason for privileging the radius of the universe for defining the arrow of time over other geometrical properties of the space-time. Traditional discussions about the arrow of time in general involve the concept of entropy. In the cosmological context, the direction past-to-future is usually related to the direction of the gradient of the entropy function of the universe. But entropy is a thermodynamic magnitude that is typically associated with subsystems of the universe: the entropy of the universe as a whole is a very controversial matter. Moreover, thermodynamics is a phenomenological theory. Geometrical properties of space-time provide a more fundamental and less controversial way of defining an arrow of time for the universe as a whole. We will call the arrow defined only on the basis of the geometrical properties of space-time, independently of any entropic considerations, the “cosmological arrow of time”.

## Quantum Mechanical Arrow

The familiar textbook quantum mechanics of laboratory measurements incorporates a quantum mechanical arrow of time — the direction in time in which state vector reduction operates. This arrow is usually assumed to coincide with the direction of the thermodynamic arrow of the quasiclassical realm of everyday experience. But in the more general context of cosmology we seek an explanation of all observed arrows, and the relations between them, in terms of the conditions that specify our particular universe. This paper investigates quantum mechanical and thermodynamic arrows in a time-neutral formulation of quantum mechanics for a number of model cosmologies in fixed background space-time. We find that a general universe may not have well defined arrows of either kind. When arrows are emergent they need not point in the same direction over the whole of space-time. Rather they may be local, pointing in different directions in different space-time regions. Local arrows can therefore be consistent with global time symmetry.

## Does the Time of Arrow reversed?

The laws of physics are mostly time-symmetric they work whether time runs backwards or forwards. However, we certainly perceive an “arrow of time” whenever we smash an egg or drop a wineglass – the egg and glass will not reassemble themselves. Linking this arrow of time defined by thermodynamics to the cosmological arrow of the expansion of the universe defines, for us, a forward direction.
Why does the arrow point that way? Could there be pockets in the universe where the arrow points in the opposite direction, where time seems, to us, to run backwards?

Lawrence Schulman of Clarkson University in New York has been tackling this question. He claims that it is possible to have opposite-running time regions and that some “small” degree of contact between them will not destroy the arrows of time. However, on the question of causality (whether someone in a time-forward region could warn someone in a time-backward region that an event is about to happen), Schulman admits that his calculations rely on some assumptions to avoid paradoxes.

How could regions of opposite-running time have formed? In the same way that electrons and positrons run in opposite time directions, Schulman suggests a cosmological picture with a timeline running between the Big Bang and the “Big Crunch” – the massive contraction marking the end of our universe. Then what would appear to us as time-reversed regions are just matter following the timeline in reverse.

Although current cosmological thinking disfavors the Big Crunch scenario, if it does happen, it is still certainly a very long time away. Therefore, if there are galaxies with backward-running time, following the timeline from Big Crunch to Big Bang, at this instant they are very old and hence not very luminous, although they are still exerting a gravitational pull – exactly the hallmark of dark matter.

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