Today we are going to talk about a very important and revolutionary concept that is THE GENERAL THEORY OF RELATIVITY.

This theory came into existence after 10 years of special theory of relativity (1905), and published by Albert Einstein in 1915.

This theory generalise the special theory of relativity and refines the Newton's laws of universal gravitation.

After coming this theory people's perspective about space and time has been changed completely. And this theory give a new vision to understand the spacetime geometry.

This theory gives a unified description of gravity as a geometrical properties of space and time.

This theory helps us to explain some cosmological phenomenon that is ,

* why small planets revolve around the big stars?

* Why everything in this universe is keep moving?

* Why mostly planets and stars are spherical in shape?

* Why does gravity create?

* Why does time become slow near the higher gravitating mass. Ie. Gravitational time dilation.

And gravitational lensing, gravitational redshifts of light, and most importantly, that why planets and stars rotates?

So let's talk about

# THE GENERAL THEORY OF RELATIVITY.

Before starting the general theory of relativity, we have to understand that what is space time?, it is very important.

According to Albert Einstein , space and time can't be isolated, means without space time has no meaning, and without time , space has no meaning. You can understand this statement by following example:

Imagine you are going to date your girlfriend. And you thought for a place for meeting both ie. Taj Mahal palace at 4 "O" clock evening.

But you message your girlfriend only that lets meet at TAJ MAHAL PALACE. What do you think , will she come.... No, she will not come , because she has no idea about time that when she has to come there?

But again you message that lets meet at 4 "O" clock at evening, again she will not come , because she has no idea about space , she will not understand that where has to come.

So by this example we can conclude that space and time are interrelated to each other. If we remove anyone then there was no meaning of other. Space and time is not different terms, this is a single term that is spacetime. And for occuring any event, space and time both are required.

By his general theory of relativity , Einstein says that space time is interrelated or interwoven such that it's behave like a 2D flat curvature, and it is directly related to energy and momentum , whatever matter and radiation present. This relationship is described by Einstein field equation, which is system of differential equations.

According to the Einstein's theory of general relativity, space time are interwoven such that it makes a net and this net has the elastic property due to this property it behaves like a infinitly stretched membrane like a trampoline. Which is shown as below:

*(Here you have to remembered that general theory of relativity is applicable only when space time is taken as 2D flat curvature).*
All the celestial bodies such as galaxies , stars , planets , asteroids , black holes or everything which are present in the universe are lie in this infinitly stretched membrane of spacetime. And in the trampoline like membrane, we know that higher masses objects create more slope and get fixed at the centre. And if we put small objects in the slope, then they all roll towards the higher mass. And we have all must seen this phenomenon somewhere in our daily life events. Which can be seen below.

Exactly same phenomenon has also occurs in the universe. The objects which have higher mass then they distort the space time more and create more slope, and objects which have small masses then they distort the space time less and create small slope.

And if the small objects get stuck in the slope of higher masses objects, then they start free falling with rolling motion towards the higher mass objects , and this free falling of objects in the slope or by the slope is coined as gravity. And every free falling body has some acceleration and this acceleration is called acceleration due to gravity. But why gravity create then you can say simply that space time slope is the cause of creation of gravity, but why this slope created , they again you can say that slope is created because depression of space time due to large mass objects.

In our solar system sun has very large mass , and because it has large mass then it depressed the space time more and create more slope. And all the small objects such as mercury , Venus , earth, mars, Jupiter and all planets with asteroids , and other small objects get stuck into the slope of sun , and due to this all the planets and small objects are in free falling towards the sun with rolling motion. This rolling motion gives the rotation to the planets and object such as asteroids. And same as, Our natural satellite the moon is also get stuck in the slope of the earth , and continuously free falling towards the earth . But anyone can ask question that why do they not collide?

In the answer of this question , we can say that , everything in the universe has been moving. And moon is free falling in the slope of earth because earth has greater mass than moon, and same , earth is free falling in the slope of sun, and sun is free falling in the space time slope of our galaxy the milky way, and our milky way is free falling in the slope of the black hole, which is in the centre of our galaxy. And may be this black hole is free falling in the slope of other black hole. Everything in this universe is moving due to distortion in the space time curvature because of mass.

Then we can say that higher mass gives higher depression and higher depression gives higher space time slope and higher space time slope gives higher gravity.

If we talk about the Newton's laws of Gravitation, then we can find that Newton only measure the magnitude of force, and which type of force it is. Newton has no idea about the space time distortion due to mass and creation of slope,he has no idea that planets are free falling in the slope of space time.

He think that somewhere a force exist which attracts the planets and any other objects , and give a acceleration , and he says this acceleration as acceleration due to gravity. Then he derived a formula for this attraction or force named force of gravity or gravitational force in terms of mass and distance between them. Which is as follows:

F= GMm/r^2

But he did not know ,why mass shows the property of gravity? In reality mass have no property of gravity , we have to understand the term gravity, gravity is a effect which is created due distortion in the space time curvature due to mass. This distortion this slope of space time create the effect of gravity.

And Newton only measure the magnitude of gravity as force, but Einstein shows that why gravity exist and how is it create?

This all explaination is only given by the general theory of relativity. Also read

Special theory of relativity
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