Time. A concept that has driven the whole world and the economic, social, political, financial, and many more frameworks which support the corporate sector since periods immemorial. Regarded by many as one of the most valuable resources for human beings, the idea of time in layman’s language is trifling and quite simple to grasp, but viewed at a cosmological level, time seems to have more to it than just a measure of day and night, dates, months and years.
In our day-to-day lives, we always seem to run out of adequate time to finish assignments, fulfill deadlines, prepare presentations, and other corporate responsibilities, but what if we could go back in time? The idea of ‘going back in time’ may sound absurd and apt only for science fiction to many, but what if there were such parameters within our control, or at least within our knowledge, which could help us to reverse time? Well, cosmology has an answer.
Cosmology
In cosmology, time is not only treated as a simple measurable quantity like mass but as a different dimension of its own, with a particular direction of flow concerning an initial starting at t=0. Treating time as a dimension, there are possibilities of it being represented as positive, negative, and imaginary to enable the functionality of the laws of physics, as they do not distinguish between the past and the future.
Laws of Physics
The laws of physics remain unchanged under the combination of operations known as C, P and T. C refers to the interchanging of particles into antiparticles, and P means taking the mirror image i.e. considering the lateral images with the left and right swapping each other and T refers to changing the direction of motion of all particles. The laws of physics perform the same functions under the operations C and P as well as C, P, and T, thus, they must also function in an unchanged manner under the operation T. Yet, a big difference is evident between the forward and backward direction of time in ordinary life. If a tap is turned on, and the downward flow of water is recorded and then run by reversing the tape, it would be seen that the continuous flow gradually becomes thinner and thinner and becomes reduced to drops, which appear to move back into the tap’s mouth. If this tape is viewed by a person, he would say that it is in reverse as things do not ordinarily occur this way.
The reason why the dropped water into the sink does not return to the tap is given by the second law of thermodynamics, which states that entropy on creases with time. The water in the tap goes from an ordered state in the tap to an unordered state in the sink gradually with time. It is this change in entropy that gives time a direction of flow and is thus called, an arrow of time. There exist at least 3 arrows of time. First, the thermodynamic arrow of time, which is the direction in which the entropy increases. Second, is the psychological arrow, which is the direction in which we consider time to flow, in which we remember the past but not the future. Third is the cosmological arrow, in which direction the universe is expanding rather than contracting.
The second law of thermodynamics is based on the fact that there are many more unordered situations than ordered ones. If a system starts in a highly ordered condition, it would evolve according to the laws of physics and would have a high probability to acquire a condition of disorder, simply because there are a far greater number of unordered states. Thus, this thermodynamic arrow of time points in a direction where disorder increases.
To describe the idea of the psychological arrow, Dr. Stephen Hawking used the analogy of computer memory with the human brain, in his book The Theory of Everything. He states that computer memory has 2 states – a one and a zero, before the recording of a data item in the memory, it is in a disordered state of ones and zeroes, with equal probabilities for each. After the recording, it would be in one of the states i.e., in order. But, in doing so, an amount of energy is also released into the external environment, this released energy serves to increase the entropy of the universe. This entropy due to the release of heat is found to be greater than the ordered condition produced by the storage of data in the system and so, the overall disorder in the system increases. Thus, a conclusion is drawn that the psychological direction of the arrow is determined by the thermodynamic arrow.
The Cosmological Arrow
The cosmological arrow’s direction has been debated by many scientists. According to the general theory of relativity, the universe must have started as a singularity of infinite density. Such a condition may imply that it is following the other arrows and in its contracting phase, would likely move from a condition of high to low entropy. This would lead to all kinds of science fiction scenarios, for those who survived the expanding phase of the universe. It is a concept with a debatable scope and feasibility. Dr. Stephen Hawking tested the feasibility of the idea through their no-boundary theory of the universe proposed by him. His presumptions about the boundaries of the universe were found to be not by the cosmological arrow of time. Realizing his mistake, Hawking withdrew the theory, accepting the loopholes present in it. Another idea of the cosmological arrow of time can be gained by considering an astronaut to be moving towards the event horizon of a black hole and another in a spacecraft orbiting it. The astronaut in the spacecraft receives signals at intervals of 1 second each. The signals traveling at the speed of light would gradually slow down and the intervals would increase and the final signal before entering the event horizon would never reach the craft as it would be absorbed by the gravity of the black hole. It is tough to wrap this idea around one’s mind with ease and is still difficult to uncover for scientists and laymen alike.
TL;DR
Thus, I hope this writing gives a wider idea of the undiscovered ambiguity of the rather easy concept of time and provides a slightly vague but well-researched answer to the question about the reversal of time and the viewing of the past. Till the universe starts contracting and scientists uncover more of the surprises time has in store, we still have to rush before deadlines.
