WHAT IS RELATIVITY?In physics, the part of the universe under our study is called a system and the place from where we observe the system is called a reference frame.
Now, the same system may be observed from more than one reference frame. A natural question here is - Are the properties ( speed, acceleration, mass, length, time intervals etc. ) of the system as observed by one reference frame are the same as observed by the others. In general, NO.
Relativity is basically about relating the properties observed by one of the reference frame to the other.
Galileo gave a set of equations relating the space and time co-ordinates of two reference frames. These were known as Galilean Transformations. The equations were quite accurate ( classical mehanics ) for frames with lower values of relative velocity, but strictly speaking, incorrect.
In 1905, Albert Einstein gave his revolutionary Special Theory of relativity in which the subject of relativity was studied with a deep examination of the concepts of time and space. The correct set of equations, known as Lorentz Transformations was used in the theory. He also gave the two basic postulates of relativity in this theory.