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# GENERAL THEORY OF RELATIVITY AND WHY WE NEED GENERAL RELATIVITY

(Last Updated On: May 18, 2018)

Hello all,

Today we are going to talk about General theory of relativity and why we need it? So let’s start…

Problem with Special Relativity :

In 1905 Albert Einstein wrote a paper “on the electrodynamics of moving bodies” this paper is also known as the special theory of relativity. In this paper Einstein explains that why time is not absolute and why speed of light “C” is constant for every frame of reference. But problem with this theory is the theory is only applicable for inertial frame of references is means it is only applicable if Body is in rest positin or moving with constant velocity. But what if body is accelerating so in this case we can not use special relativity for accelerating body that’s why we need General theory of relativity for accelerating objects or body.

General Relativity :

To understand General relativity we must understand what is equivalence  principal of Einstein :

If a boxes flowing towards earth with the gravity “g” then inside that box you would feel weightless means you will float. On the other hand if you are in outer space you will also feel weightless.

Similarly if you are in the box which standing on the earth so you are subjected to the gravitational attraction “g” is also equivalence to you in rocket which is accelerating with the acceleration “g”.

Einstein said there is no experiment you could do distinguish whether you are stationary inside the box on the surface of the earth being subjected to the force “g” or whether you will being accelerating in outer space in a rocket with “g”.

This is called Equivalence Principle of Einstein. The equivalence principal let us to the conclusion : light bands in the presence of the gravitational field or we can say that Gravity bends light. The speed of light is constant so it doesn’t change speed but it does change direction. It doesn’t change its speed but it does change it’s wavelength and frequency. That’s called “Doppler Effect”.

Which is one major point in General Relativity that Gravity bends Light. The other point is that is Gravity is a bending of “SPACETIME”. To understand this point we must understand what is spacetime. So let’s see.

Concept of SPACETIME :

To understand SPACETIME we first have to understand what is space ? “Space is the region in all direction and contain everything within” or we can say that “Space is the unlimited expand of physical dimensions in which all material objects are located”. There are 3 dimensions of space we feel, means we can go “up & down, left & right, forward & backward” directions. When we add “TIME” in three dimensional space then it is fourth dimension. And the combination of three dimensio space with time and that four dimension structure is known as “SPACETIME”.

Now another key point in General Relativity is “Minkowski Space”. This topic contains lots of complex mathamatics so I am not described it here but i give you mathamatical version of “Minkowski Space” so if you want to understand this you can check pdf link under the article. In general i can say that Minkowski Space is a Hyperbolic Function which describes the values of Distance & Time for any observers they agree on this function.

This is all about spacetime. So by principle of equivalence gravity has a curve effect on spacetime. Then we need to ask ourselves a question what is gravity ? The answer is mass wrapping spacetime and it’s effect caused on anybody to follow a straight line which is curved by spacetime. The effect is apply even on light and diverts its direction. That’s the effect of gravity.

So Einstein’s theory of General Relativity states the gravity doesn’t actually exist at all as we understand it, it is an illusion the illusion is caused when we follow a straight line in curved spacetime.

Summery :

1. Speed of light “C” is constant for every frame of reference.

2. Gravity bends Light.

3. Gravity is a bending of SPACETIME due to mass.

Note : This is only Theoretical part of General Relativity if you want Mathamatical part then you have to understand Einstein’s Field Equation. So if you want to know about that stay connected with us.

ImpoImpor pdf :

1. Einstein’s paper of speacial Relativity: https://drive.google.com/file/d/16ysr5GBK5J-nP8g_lxPEnaBVrTCDT2ci/view?usp=drivesdk