One of the real mysteries of life is to find an object which is absolutely at rest. Every object in this universe is in motion and even this motion is not absolute because when we talk about rest or motion we specify a frame of reference or a point of observation from where the object is observed. Now, let us have an example. A train passing by a tree is considered to be in motion with respect to the tree considering it as an inertial frame but still, the tree is in motion as it rotates along with the earth. This confirms that whenever we talk about rest or motion, we always need a frame of reference. There are two types of the frame of references:

(i) Inertial frame of reference
(ii) Non-inertial frame of reference                                           The frame of Einstein One of the real mysteries of life 

An inertial frame of reference: If a frame is moving with uniform velocity in a straight line then it is said to be an inertial frame of reference. It is a non-accelerated.

A non-inertial frame of reference: If a frame is moving with non-uniform velocity then it is said to be the non-inertial frame of reference. It is accelerated.

                                                                                              The frame of Einstein One of the real mysteries of life 

Now the question arises, “Is there any possibility of finding an inertial frame?”. The answer is ‘NO’. There is no possibility of having an inertial frame of reference. We treat the earth as an inertial frame of reference but as it rotates around the sun, it is accelerated. Scientists consider distant stars as an inertial frame of reference as they have very-very small angular velocity. Another example to show that earth is non-inertial is Coriolis force. As we discussed earlier, the inertial frame is non-accelerated.

In other words, we can say that it is free from all forces but as we all know each particle experiences a gravitational force. This again shows that it is impossible to have an inertial frame of reference. We can treat a man-made frame moving with uniform velocity in a straight line for a short interval of time as an inertial frame but still, it is problematic as it rotates along with the earth. Einstein solved out this problem. According to him absolute rest or absolute motion is a meaningless concept. Every motion is relative.

His simple idea solved this. He said “If a small frame is falling freely under the uniform gravitational force, all forces disappear because the frame and any object inside the frame falling with the same acceleration. It is accelerated but as there is no force, it can be considered as an inertial frame”. This idea of Einstein gave rise to a new theory named ‘General theory of relativity’ which is experimentally verified and it is the well-established theory of gravity. This signifies that a simple, small and basic idea can lead to a great scientific achievement.

The frame of Einstein One of the real mysteries of life