Before knowing relativity Let us imagine a boy is on a car with a ball in his hand and let another boy standing on the footpath watcing him. Now lets imagine the car is moving with a speed of 60km/hr. Now if the boy throws the ball in the direction of car with a speed of 20km/hr then for the boy on the footpath, the speed of the ball will be the speed by which the boy on the car throws the ball + the speed of the car i.e 80km/hr. This is due to relative motion of the ball and the car.

Playlist of Space-Time Relativity-----https://www.youtube.com/watch?v=7CeNjYzpmgM&list=PLkBcs7ozK06ennoeGvX2lRNJJfWppq9Nd

Frame of reference----------https://youtu.be/s7VbXhD73bQ

Now if we imagine that the boy have a flashlight in his hand and he flashes it in the direction of motion of the car, then for the boy on car the speed of the flash light should be the speed of light but for the boy on the footpath the speed of the flash should be the speed of light + the speed of car. But this is not true. The speed of flashlight is equal to the speed of light for both of them. Here relative motion doesnot work and speed of light is constant. This phenomenon is explained by Special theory of Relativity.

So Let’s get started.

Right from his childhood, Sir Einstein was excited about how the world would look if he sat on light. And with this question in mind he made many new discoveries such as Brownian motion, existence of atoms, etc. But despite of these theories and researches, Einstein couldnot gain the name and fame which he should have got as a Physicist.

Now before understanding Special theory of relativity, we must understand what frame of reference is and how with respect to it motion is relative and most importantly time is relative and not absolute.. We made a detailed video on Frame of reference, explaining all these phenomenon, you can watch it to understand distinctly.

Special theory of relativity is applicable only for Inertial frame of reference where a body moves with a constant velocity with respect to another frame of reference.

This theory is based around two main assumptions or postulates.
The first one is that the law of Physics are the same in all inertial frame of reference.

It doesn’t matter to which frame of reference you belong but the law of Physics will be same for all the frame of reference. Like in case of our example if we consider the boy on the car along with the car is stationary then with his respect it seems that the boy on the footpath is moving along the all the surrounding. And if we consider the boy on the footpath is stationary then the boy on the car along with the car is moving. In both aspect there is motion and hence physics will apply the same.

The second postulate is that the speed of light in a vacuum is the same for all observers irrespective of their frame of reference which is about three hundred million metres per second.