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Moving Clocks Run Slow: The Special Theory of Relativity

In 1905, one of the scientific world’s most significant concepts was born. The Special Theory of Relativity has been the key to understanding the universe and its working. Ever since Albert Einstein ingeniously put it forward, the theory finds itself being applied in almost every field.

Albert Einstein is doubtlessly one of the best scientists to have ever walked on earth. With all his contributions in the year 1905 – the ‘miracle’ year – and the General Theory of Relativity, he is considered to have sculpted our understanding of the universe more than ever. Without these, all of our developments would have been frozen in the 20th century.


The Special Theory of Relativity was born when Einstein thought about the uniformity of time. He figured that if the speed of light (299,792,458 m/s) is constant, then time and distance need not be constant. Let’s say we have a spaceship traveling at a particular speed in deep space. Someone inside the spaceship shines a laser beam at a distant space rock. What would be the speed of the laser beam? Would it be more than the speed of light, since the spaceship is already traveling with some speed?

The answer is no. Einstein stated that regardless of anything, the speed of light is the same. In the above scenario, if the speed of the laser beam is constant, then either the distance between the spaceship and the rock would be smaller than expected, or the time taken for the beam to hit the rock would be larger than expected.

According to the Special Theory of Relativity, both of these happen. Time dilates and length contracts for any moving object.


The term ‘time dilation’ could be very abstract and logic-defying. Time dilation refers to clocks running slower than they usually do – at rest. Let’s fit a clock to the spaceship we just saw. If the spaceship were at rest, the time taken for the beam to have hit the rock would have been, say, 10 seconds. When the spaceship was moving, the time would have been observed to be 8 seconds (this would mean that the spaceship was traveling near the speed of light, but we’re not going into that now). Since the speed of light is the same, the clock on the spaceship had to be running slower.

In a similar real-life experiment carried out by scientists, two extremely accurate atomic clocks were synchronized and one of them was put in an airplane. The clock was flown around the world in the airplane. When the clock returned to the laboratory, it was found that this clock had recorded a lesser time period than the one that did not move. Although the difference was very, very little, the theory was proved true in this experiment.


Time dilation becomes significant when moving objects move close to the speed of light. When you’re just walking around, or even flying in a supersonic jet, time dilation is negligible. However, without the concept of time dilation, your GPS will not work at all since it uses the special theory to locate you.

Also, astronauts in the International Space Station (ISS) have clocks that run slower than ours on earth. Since the space station moves at a high speed in orbit and has a lesser gravitational pull, people aboard experience time dilation.

The special and general theories of relativity together have paved way for a plethora of other theories and paradoxes. Time travel, wormholes, and other such ideas have been born of these established theories. These ideas make Einstein’s theories an overt point of study to many scientists.