Contrary to the commonsense

Do you know Time is not constant? Do you know clock ticks slower if the clock is moving at high speed? If time can slow down, is it possible that it can stop? If time can stop, is it possible that it can move in reverse direction?

Phew! We are talking about time travel here. Mind you, this is not science fiction

Sometimes there are events and thoughts that completely turn the long held beliefs on their head. One such sequence of thoughts emanated in year 1905 from a giant among the humans - Einstein.

What Einstein produced remains unmatched in its simplicity and brilliance. His main contributions can be summed up in two major theories – Special theory of Relativity and General theory of relativity.

In this article, I am just touching upon some of the interesting aspects / consequences of Special theory of relativity that relate to Time Delay.

Despite the importance Einstein’s theory are least understood out side the science fraternity, not because it is difficult, but because it goes against our “rational” thinking process and some hard ingrained observation that are taken as facts.

It all starts with one basic principal – Constancy of the speed of light. Embellishing it further, it reads, Constancy of the speed of light irrespective of who is measuring it.

Pay attention, as this lies the bedrock for all further arguments.

You and your friend are standing some distance apart; you throw a ball at your friend at 50 km per hour. If your friend is standing still, he will measure the speed of the ball to be 50 km per hour. If he starts running away from you at 50 km per hour, the ball wont’ hit him ever and he would measure the speed of the ball to be 0 km, in his frame of reference. In the scenario that he rushes towards the ball at 50 km per hour, he would measure the speed of the ball to be 100 km per hour, in his frame of reference.

However, it does not hold true for the speed of light (300,000,000 m /sec). If your friend were to shine a light at you, you would measure the speed of the light to be same irrespective of your speed, towards or away from your friend. In other words, if you run towards your friend at the speed of 200,000,000 m/sec, you will still measure the speed of light to be 300,000,000 m/sec. If you run away from your friend at the speed of 200,000,000 m/sec, you will still measure the speed of light to be 300,000,000 m/sec.

Here’s a thought experiment that Einstein did to postulate his Constancy of speed of light theory – It’s called vanishing image problem. We know we see our image in a mirror because the light leaves our face, hits the mirror and comes back, thus we see our selves in the mirror. Consider that you start moving at the speed of light, with mirror held in front of you, since your speed is same as the speed of light, the light won’t leave your face to hit the mirror, hence your image will disappear. This experiment is basically flawed. The speed of the light relative to the observer, in this case you, will remain constant, irrespective of the fact that you yourself are traveling at the speed of light. The light will leave your face at the same speed, hit the mirror and come back to you at the same speed.

This thought experiment was conclusively proven in field experiments by Albert Michelson. I’ll get into details of this experiment in another article. For the time being take it that it has been proven that the speed of light remains constant, irrespective of the frame of reference used to measure it.

Let’s see how does constancy of the speed of light explains time delay. We will use an easy to follow experiment -


There are two observers, Observer A in a moving train, moving at a speed close to the speed of light. Observer B standing on the platform, stationery relative to the moving train.

Observer A conducts the following experiment – She takes a light source, a flashlight, and points it at a mirror, which is vertically below the light source. The light reflects back from the mirror and shines on the light source itself. The distance of the light source and the mirror is such that it takes light 1 sec to do the round trip, as measured by the Observer A. Pay attention to the fact that for Observer A, the light is traveling between the light source and the mirror, and back, in a straight line – Straight down and straight up, as shown in Figure 1

Observer B is watching this standing still, relative to the moving train, on the platform.
Let’s see what has observer B recorded – For her, when the light leaves the source and before it hit the mirror, the mirror has moved ahead with the speed at which the train is moving. Hence to her, the light is traveling a slated path (Path A in the Figure 2). Again, after the light is reflected back by the mirror, it travels a slated path to hit the source (Path B in the Figure 2), which has moved ahead with the moving train. Essentially, the light to this observer has traveled along slanted paths.

Basic geometry tells us that the slanted paths that the light traverse, as observed by the Observer B, total up to be greater than the round trip distance covered by light as observed by Observer A.


We know a basic physics equation

Time = Distance / Speed

Interpreting the above equation in light of our observations, we conclude the following.

Given that the speed of light is constant, the time observed is directly proportional to the distance traveled. Since for B, distance traveled is more, the time observed is also more. Which means, for B, time is moving faster. For A, time is moving slower!! Time delays for the fast moving bodies.

So here’s the secret of travel to the future – Spend some time traveling at speeds close to that of light and you would have arrived in future!! If you want more time to work on your job, move at the speed of light and the time will expand!!

In fact if the time delays were not being factored in the calculation of the GPS systems; we’d be way off in our calculations and positioning. Someone wanting directions for Delhi would land up in Amritsar!!

Now the speculation part, if the time can slow, can it stop? Can it flow backwards? Perhaps not, since there is nothing that travels faster than light, or shall we say, we don’t know. Perhaps there is a phenomenon that makes it possible for something to travel faster than light and we have yet to figure it out!!