Gravity, one of the four main fundamental forces in nature. It is one of the most commonly observed forces in nature and yet it is as still one of the least understood of all forces.

Gravity was brought into fame by this famous apple shown above when it garnered enough strength and weight to detach itself from the tree and land on Newton’s head. This triggered his brain cells into thinking what made the apple fall down and thus Newton penned the theory of gravity.

The formula of gravity as given by him was:

And although he made the theory of gravity in a real short while it took him

twenty years to publish it in his book.Why? Because he had to prove that the mass of an object lies at it’s centre and for that he had to develop calculus to such a point that this could be proved. So in a way that apple was responsible not only for the discovery of the theory of gravity but also the the development in the field of calculus.

But why did newton have to prove that the mass lies at it’s centre?

Take a look at the above picture. Now if you see the formula you would see that given no action of any external forces the two bodies would attract each other with the force that is inversely proportional to the square of the distance between them. But what is the exact distance between them. Because mass can be distributed over a large volume. So if the two bodies touched each other the force of gravity is supposed to be infinite right? Because the surface has mass and the mass is at zero distance from each other. Well that’s what the mass being concentrated at it’s centre means. The force of gravity is taken by calculating the distance between the centres of gravity of two continuous volumes of masses. That way no matter how close or how small or heavy the objects are they would never make the distance zero. Think about it!!

This theory was an absolute success as it was a unifying concept that not only explained the free fall of apples towards the earth but also the movement of the planets around the sun and the movement of galaxies and stars!

Of course the problem was that nobody including Newton actually understood what gravity was!They knew it was a force but the origin or the behaviour of this force still remains unknown.That’s when Einstein made his epoch making theory of special relativity and also the general theory of relativity.

Einstein made a simple observation. He already found that nothing can exceed the speed of light from all frames of reference via his theory of special relativity. Then he put up a thought experiment. He supposed that the sun would suddenly disappear. In that case the force of gravity keeping the earth in it’s orbit around the sun would just disappear too. But the light from the sun takes 8 minutes to reach the earth. So in that case we would still see the sun and yet we would not feel it’s gravity. That leads to the conclusion that the effect of gravity reached the earth faster than the speed of light! But nothing can travel faster than the speed of light. This apparent contradiction led him to modify the theory of gravity a bit. While there was nothing wrong with the magnitude of gravitational force it was the way the gravitational force lines acted that was changed.

Einstein said that gravity was the effect of bending the geometry of the space around it that led to the observation of a force acting.

Look at the above diagram. Suppose the the black line represents the path along which it is possible to move also called a dimension.

If we move both one will travel in a straight line while the other will travel in a bent line. If the lines become invisible it will seem as if the the object on the straight line is under no force but the object moving on the curved line is being pulled by a force that is changing it’s direction.

This is exactly what einstein called it. He said gravity bends space in such a way that it seems like a force.His theories were tested and proven multiple times and they were almost always found to be true. He was truly a remarkable genius since he explained a fundamental force through geometry!

Now fast forward to the present we have new theories called standard model, string theory etc. However they still find explaining gravity a mystery! Yes what is gravity has been found but what explains it’s origin is still a mystery! While the world might be rejoicing at finding the higgs boson the boson does not explain gravity. And though string theory does predict it , the theory itself is yet to be verified by the scientific community.

Of course nowadays newer more expensive equipment are being created which sought to find the elusive particle of gravity called gravitons. And hopefully we will fully gain an in-depth understanding of gravity one day. Maybe one day the unknown apple shall fall on someone’s head again and we shall be saved from the ignorance of that special force called gravity!