# What Is Time And Why Does It Move In The Forward Direction?

This is a guest article by Sumeet, a student from Guru Nanak Dev University, pursuing Master's in Physics. She has a deep interest in cosmology and Theory of Relativity. Currently, she is working in the field of Atomic Physics.

Time, most physicists agree, is one of the most difficult properties of the universe to understand. It is believed that time had a beginning and indeed came into being with, The Big Bang, some 13.8 billion years ago and has been flowing forward ever since. Ever wondered why? There have been numerous explanations to this bizarre question ranging from static time, entropy to the expanding universe. Well, I don’t promise to provide a satisfactory answer but I’ll try to convince you with the prevailing theories.

### The Concept of Static Time

An English poet, Austin Dobson, once said “Time goes, you say? Ah no!, Alas, Time stays, we go.” This poetic line seems more of scientific. Dobson is contrasting our passage through time with the passage of time. Consider the time variable to be a straight line on which a point marked “Now” moves uniformly. The now emerges from past and succeeds to the future. Another poet, Santayana puts it more beautifully, saying, “The essence of nowness runs like fire along the fuse of time.” But we all know, if time was static, there would be no special theory of relativity.

### The Concept of Dynamic Time

Let’s now suppose, time is dynamic. If it made sense to say that time flows, then it would make sense to ask how fast it flows, which doesn't seem to be a sensible question. Some people reply that time flows at one second per second, but does this make any sense either?  A rate of seconds per second is not a rate at all in physical terms. It is a dimensionless quantity, rather than a rate of any sort. So this leaves us unsatisfied again.

### The Concept of Entropy

If we look at the laws of physics, from Newton to Einstein, Maxwell to Bohr, Dirac to Feynman, they appear to be time-symmetric. In other words, the equations that govern reality don’t have a preference for which way time flows. Yet the idea of backward flow of time seems counter-intuitive.
Also Read: What Is The Real Concept of Entropy And How To Understand It?

So what is it that controls the arrow of time? Most of the physicists hold a quantity named “Entropy” responsible for this so called arrow of time. Entropy defines disorder of the system. The second law of thermodynamics states that the entropy of a closed (self-contained) system can only increase or stay the same over time; it can never go down. In other words, over time, the entropy of the entire Universe must increase. It’s the only law of physics that appears to have a preferred direction for time. Now let’s assume this explanation to be true. Physics always says that every system tends to achieve equilibrium. So, if every entropic increase for one direction of time is equal to the entropic decrease in the opposite direction of time, The question arises why the system (here universe) was ever far from equilibrium? This explanation, therefore, leaves us puzzled with an even bigger question.

### Expanding Universe

Another explanation of the arrow of time could be the expanding universe.

Professor Richard Muller's idea is that as new space is created, so is new time, and this constant expansion is what we experience as the progression of time. "Every moment, the universe gets a little bigger, and there is a little more time, and it is this leading edge of time that we refer to as 'now'," says Muller. "The future does not yet exist … it is being created. 'Now' is at the boundary, the shock front, the new time that is coming from nothing, the leading edge of time." Therefore there is no possible way of travelling to future because it doesn’t exist yet and travelling backwards in time is impossible unless the space decreases. It is possible for the amount of space in the universe to decrease due to cataclysmic cosmic events like a black hole disappearing, but only on an immeasurably small scale. Muller claims that his theory can be proved with the help of gravitational waves. These waves were detected by LIGO sometime back, caused by two black holes merging into one. In doing so, they created new space and, according to Muller's calculations, the event should also have produced about one millisecond of new time.

At this point, we have theories to guide us, numerous theories. But which one to believe is hence the real question.