ARROW OF TIME
March 29, 2022
Space-time began at the big bang singularity and would come to an end either at the big crunch singularity, or at a singularity inside a black hole. The most accepted theory is that it started with singularity and then begun to expand. But why is the universe expanding? Why the time we assume is in forward direction? Why don't we measure it in backward direction? These are some of the many questions to be answered yet.
"TIME", a very simple word, yet a complex topic for the people interested in science and for the people who think deeply about it. Before Einstein's "Special Theory of Relativity" everybody thought time is absolute. But after Einstein's Special Theory of Relativity we came to know that time is not absolute. Time is relative. This prediction was tested in 1962, using a pair of very accurate clocks mounted on the top and bottom of a water tower. The clock at the bottom, which was nearer the earth, was found to run slower, in exact agreement with the general relativity. In the theory of relativity there is a unique absolute time, but instead each individual has his own personal measure of time that depends on where he is and how he is moving.
In any closed system disorder, or entropy, always increases with time. In other words, it is a form of Murphy's law, things always tend to go wrong! 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. The arrow of time, also called time's arrow, is the concept positing the "one-way" direction or "asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and an unsolved general physics equation. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies and might be drawn upon a 4-dimensional relativistic map of the world ("a solid block of paper"). There are atleast three different arrows of time. First is the thermodynamic arrow of time, second is the psychological arrow of time and the last is the cosmological arrow of time.
THE THERMODYNAMIC ARROW OF TIME: The direction of time in which disorder or entropy increases. According to the 2nd law of thermodynamics, there are always more disordered states rather than there are ordered ones. As an example we can see that when something is broken it gets more disordered rather than ordered. There is one, and only one, arrangement in which the pieces of a picture makes it complete. Supppose a system starts out in one of the small number of ordered states. As time goes by, the system will evolve according to the laws of science and its states will change. And it is more probable that the randomness of the system will increase after a certain time. There will be more disordered states rather than ordered ones. Thus, disorder will tend to increase with time if the system obeys an initial condition of high order. Suppose, if God decided that the universe should finish up in a state of high order but it didn't matter what state it started in. At early times the universe would probably be in a disordered state. This would mean that randomness would decrease with time. You would see broken things gathering themselves together and jumping back on the place. If God is the creator of the universe then at the first place why did he started the universe the way it started. However, any human beings who were observing the cups would be living in a universe in which disorde decreased with time. Maybe, such beings would have a psychological arrow of time that was backwards. That is, they would remember events in future but not remember events in their past.
THE PSYCHOLOGICAL ARROW OF TIME: The direction of in which we feel time passes, the direction in which we remember the past but not the future. The direction of time in which our mind remembers the past is same as that in which disorder increases. Our subjective sense of the direction of time, the psychological arrow of time, is therefore determined within our brain by the thermodynamic arrow of time. Just as a computer, we must remember things in the order in which entropy increases. This makes the second law of thermodynamics almost trivial. Disorder increases with time because we measure time in the direction in which disorder increases.
THE COSMOLOGICAL ARROW OF TIME: The direction of time in which the universe is expanding rather than contracting. In classical theory of relativity we can't predict how the universe would have begun because all the laws of science would have broken down at the big bang singularity. The universe could have started out in a very smooth and ordered state. this would have led to well-defined thermodynamic and cosmological arrows of time. But it could equally well have started out in a very lumpy and disordered state. In that case, the universe would alredy in a state of great disorder and so the disorder could not increase with time, or it would decrease , in which case the thermodynamic arrow of time would point in the opposite direction to the cosmological arrow. Neither of these possibilities agrees with what we observe. When the curvature of space-time becomes large, quantum gravitational effects will become important and the classical theory will cease to be a good description of the universe. One has to use a quantum theory of gravity to understand how the universe begun. "The boundary condition ofthe universe is that it has no boundary".The universe would be completely self-contained and not affected by anything outside itself. It would neither be created nor be destroyed. It would just BE. The no boundary condition implied that disorder would continue to increase during contraction. The thermodynamic and psychological arrows of time would not reverse when the universe begins to recontract or inside black holes.
So, the arrow of time implies that disorder increases with time, no matter if the universe contracts or expands. But we don't know yet that either time is real or just an illusion. We just assume time for the refrence. But there is a lot to discover yet. A lot to know about what we know, because we can't be so sure about what we know. This can be real or can just be an illusion.
0 Comments