The mysterious force of Gravity

We deal with the gravity in our every day lives, and we know gravity as a force that makes an apple to fall down on the earth, a force which keeps us and everything on the surface of earth. It makes the moon to rotate around the earth and the earth to rotate around the sun. But the question is, what actually it is, and how does it really works?

Hundreds of years ago, Newton gave us the simple equations to calculate the gravitational force exerted by the objects on each other and we still teach those equations in high schools. We know that we only need two things  to calculate that force; the mass of those two objects and the distance b/w them. But how it is even possible that an object affects another abject thousands of miles away through an empty space and without anything happening b/w them?
Newton gave us the equations to deal with the gravity, to calculate the gravity but surprisingly, he himself didn't know how the gravity worked.

This caught the attention of The great Albert Einstein, he read the book of Newton containing all of his work, equations & experiments written by himself. He straightaway went to the section of gravity. And To the very surprise of Einstein, Newton said that I don't know how it works, what he himself had written was: "I leave it to the understanding of the reader". Most of the people including scientists who read that book, went through that section, ignoring that part, but Einstein had different ideas. He took upon himself; the task to describe the nature, working and origin of gravity. And after a long research of 10 years, he came up with the 'general theory of relativity'. In which, Einstein showed that our space wasn't actually an 'empty space', but it had its physical component too. He called it as 'the  fabric of space-time'. Before Einstein , it was thought that space was the stage set where all the drama happens, but Einstein showed that space too, was an actor in the drama. In order to understand this, lets make this 3-D concept it simple. Consider the fabric of space as two dimensional:

You can think of the fabric of space as a thin plastic sheet, because it actually behaves that way in 2D. Imagine that we place the bigger orange ball on this fabric; due to its mass, it will bend the fabric  over some area or cause it to warp the fabric around it in the way shown in the figure below: ( neglect the presence of smaller ball for a while)

Now, The taking the smaller ball into account, we can tell that that smaller ball has lower mass as compared to the bigger one, so common sense says that it will bend the fabric over a smaller area around it, as compared to the bigger ball. There is a certain area around each ball in which, when  another ball comes, it moves towards the first ball because the sheet around it is bent. And we can intuitively say that the larger ball will attract the smaller ball towards itself because it bends the sheet more than the smaller ball.

Imagine that you push  the smaller ball towards the larger ball until it enters into the area bent by the larger ball, we know that it would fall into the larger ball, but instead of falling straight into it, it curls around it in circular paths and the radius of its circular path gets smaller and smaller until it finally falls into the bigger ball. Until or unless the smaller ball has a considerable mass too in addition to velocity, so it could also bent some fabric of space around it in order to cope with the bigger ball. So that it could rotate around the bigger ball. It is very important to note that it will acquire a curved path not because it do so, but because the space in which it was moving got curved.

 Now consider the example again by thinking of the bigger ball as the Earth, and smaller ball as the moon. This is how the gravity works.

The gravity is actually caused by the curvature of space which is caused by the mass of the bodies. Or It would be better to say that when matter interacts with space, it causes the space around it to warp, and its effect on another body within that warped space is what we call gravity.  And more mass, means more area (In 2D) of curvature, which means the motion of some other body can get affected at a larger distance. We simply say that a certain body has more gravity. 

We know that it warps the space around it, and as space and time are dependent on each other, (we've already discussed this in previous posts) It directly effects the time around it. The three dimensional demonstration is given below in the figure and  I suppose you can understand it in 3-D now with great ease.

One of the implications of the gravity is that it slows down the time which directly means  that for a person living on the ground floor of a building on earth, time will pass more slowly as compared to a person living on the top floor of the building because there is more gravity on the ground floor. But the difference will be so much smaller that we'll not be able to experience it, probably billionth of a billionth of a billionth second.  And for a person living on Jupiter, time will pass more slowly, as compared to the one living on the earth, because Jupiter has a higher gravity as compared to that of the earth. But a giant body with super gravity, like the black hole, a minute in its gravitational field would probably be equivalent to a year or more on the earth.

Some physicists claim that gravity is not only attractive, it can be repulsive too. They argue that gravity behaves in a different way in the presence of immense amount of radiation or heat. Because by making this assumption, We can give a better explanation of the Big bang. In addition to this, If it's not this way, we cannot explain the fact that the distances b/w galaxies in this Universe are continuously increasing. Because intuition says that Gravity should at least maintain the distances b/w the bodies, if not bring the bodies closer.    

This was the model presented by Einstein and I don't want to give an impression  that newton's equations got ruled out, we still use them in our daily lives, as well as when we have to land our spacecraft on the moon, or when we want to accelerate the rocket within the gravitational field of the earth. Newton told that gravity is 'something' and it works this way, but he didn't tell us how. Einstein told us  how it works and what are its implications. But the basic and most fundamental question is that why it works this way? The answer to this "why?" is still unknown. It took us almost 100 years to provide an answer to "how?", so it'll probably take a lot of time to have an answer of "why?".

Furthermore, there's a drawback in Einstein's model; it fails to explain the attraction b/w particles at the quantum level. We use Quantum theory for that purpose. Presently, physicists are trying to  find a theory of something which they call as "Quantum gravity", which will probably explain the attraction b/w particles in quantum world plus the attraction b/w giant bodies like stars, planets and galaxies using the same set of equations. One thing is for sure, there's still a long way to go in our journey of understanding the universe.