What is Gravity?

            You might have wondered why you fall down while playing or when you lose your balance. You might have sometimes broken a few bones of yours as well. Why do your elders stop you from standing too close to the edge of a building or a place very high? Why do you see the moon every night? Why do you carefully lift a tray carrying glasses and cups? The answers you get are all pointing to one concept; the concepts around which our universe is revolving, that concept is "Gravity". But what is gravity? . . . .

            A young man once sat under a tree when suddenly an apple fell over him. That was the event after which that young man, Newton, revolutionized the World of Science. Saying that the force which made the apple fall over him was the same which kept the moon moving around the earth and our Earth and the fellow planets in an orbit around the Sun, he completely changed our picture of the universe. He gave the law of gravitation and three famous equations on which today's classical mechanics is based on. We still use his equations and that's the reason why we landed on moon or even left the earth. But wait; the story of gravity does not end here. 

            There was a clerk, about two centuries later, who was not completely satisfied with Newton's ideas. Observing the light rays coming from the window of his office, that clerk continued to study about light. That genius, Albert Einstein, concluded that the speed at which light travels is the cosmic speed limit. Nothing, literally nothing, can outrun light. Light takes about 8 minutes to reach earth and that means we see the sun setting, we are actually looking at past. Now Newton suggested that if our Sun vanishes from its place, all the planets, making a tangent from their orbits, will leave their orbits instantaneously. "Instantaneously "was what Einstein did not consider correct. Since he had already proved that the speed of light was the cosmic speed limit, how could gravity act on a body instantaneously? So he proved Newton wrong. Though we acknowledge Newton for such great work, there was something he did now know. He told about gravity but did not state what caused it. Why does gravity act on anything? According to him every two massive objects attract each other. But that's not the complete answer. 

             Einstein had to resolve the situation he had created. He had produced his special theory of relativity but he wanted to add gravitation in his work as well. Finally he came up with a solution when he produced his general theory of relativity in 1915. Saying that gravity is not a force, he suggested that it was the consequence of a strange phenomenon. Our universe is a four dimensional fabric. The three dimensions are what we see around us; length, breadth and height. The fourth one is time. Together they are called Space-time. Now that's really strange. Saying that the fabric of space-time can be distorted and bent; any massive object like planets, stars bend that fabric. Resultantly, nearby objects come towards that massive object. 
             To understand it better, make four persons hold a piece of cloth from its corners and put a football in the centre. Take a tennis ball and roll it on the piece of cloth. You will notice that the tennis ball will move towards the football because it has bent to the cloth downwards. Einstein said that this was why our earth is revolving around the sun, the moon orbiting our earth and an apple falls down from a tree. 
              Einstein became a highly influential person after his work, rather a celebrity of his work. However, that does not suggest that no credit goes to Newton. He has his own place in the history of physics. But other than these two geniuses, there have been others as well who have made invaluable contribution to the field of physics. Today, physicists are trying hard to find a single theory which can explain everything in the universe, at every level, be it cosmic or quantum level. Perhaps the major conflict is between Einstein's General Relativity and Quantum Mechanics. Sooner or later, this will be resolved.
Now you know what gravity is so you better be careful next time when playing with it.