In the early nineteenth century people believed that light travelled through imaginary stationary medium called ether. It was believed that ether filled all space, and all movements could be measured absolutely with respect to it. It was also thought that the speed of light relative to a moving observer could be calculated in the same way as the relative speeds of any two moving objects. For example, just imagine two cars in the same direction: one going at a speed of 110 km/hr and the other at 80 km/hr. Passengers in the slower car would observe that the faster car is travelling at 30 km/hr.

Two American scientists, Michaelson and Morley, experimentally tried to measure the speed of earth through ether in 1887. But their result did not confirm the existence of the hypothetical medium ether. Later the explanation of negative results was offered by Albert Einstein. According to him, nothing like ether exists in the universe and the concept of absolute motion is meaningless. He also said that the speed of light is constant, no matter how fast the observer is moving. No material body can travel faster than light.

On the basis of his conclusions, Einstein formulated the Special Theory of Relativity in 1905. He showed that physical quantities like mass, length and time are also not absolute. They change as the bodies move. If a body moves with a large velocity, its mass increases and it becomes shorter. 

These changes are not noticeable when bodies move at ordinary speeds. They become significant only when they move at a very fast speed approaching the speed of light. Cars on the freeway do not become heavier and shorter as they go faster, but the effect is very small. On the other hand an electron travelling at 99 percent of the speed of light becomes seven times heavier than its mass when it is at rest.

According to Einstein, no body can actually attain the speed of light. If an object did, it would be infinitely heavy and have zero length. The increase in mass and decrease in length of a body as its speed increases, led Einstein to conclude that mass and energy are different aspects of the same thing, but are related to each other. This led him to the famous mass energy equation, where c is the velocity of light and m is the mass of the body. Nuclear energy is a direct consequence of this equation.

The second part of Einstein’s theory is called General Theory of Relativity and was published in 1915. This theory applies only to bodies moving with constant speeds. It deals with the accelerated motion of bodies. It deals mainly with the way the force of gravity works. He suggested that the force of gravity is a property of space and time. He showed that space becomes ‘curved’ by the presence of mass. The motion of stars and planets is controlled by this curvature of space. Light rays, too, are bent by the curvature of space around a body that has mass. Strong fields of gravity such as that of the sun or a star would bend light rays. The bending of light rays has been observed in an eclipse of the sun.

Einstein’s Theory of Relativity is one of the greatest achievements of the human mind. This theory gave birth to the production of nuclear power. Atom bomb and hydrogen bombs could be made only due to the conclusions based on Theory of Relativity.

Albert Einstein, one of the greatest scientists of the present century was born in Germany in 1879, and lived there and in Switzerland and the USA, where he died in 1955.