# General theory of relativity And why we need it

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 position or moving with constant velocity. But what if body is accelerating so in this case we can not use special relativity for accelerating 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 “space time“. To understand this point we must understand what is space time. So let’s see.

**Concept of space time**

To understand space time, 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 dimension space with time and that four dimension structure is known as “space time”.

Now another key point in General Relativity is “Minkowski Space“. This topic contains lots of complex mathematics, so I am not described it here but i will give you mathematical 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 space time. So by principle of equivalence gravity has a curve effect on space time. Then we need to ask ourselves a question what is gravity ? The answer is mass wrapping space time and it’s effect caused on anybody to follow a straight line which is curved by space time. 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 space time.

### Summary

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

2. Gravity bends Light.

3. Gravity is a bending of space time due to mass.

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

Thank you for Reading…

**Important PDF – **

Nice information in easy language..

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