Galileo's and Einstein's Principle Of Relativity

Before the revolutionary theory of relativity given by einstein which deals with the predictions of some really advanced phenomenon like astronauts aging slower than people on Earth, there was this theory by Galileo which states that laws of motion are the same in all inertial frames. Galileo measured that all bodies accelerate at the same rate regardless of their size or mass. Isaac Newton founded classical mechanics on the view that space is distinct from the body and that time passes uniformly without regard to whether anything happens in the world. For this reason, he spoke of absolute space and absolute time, so as to distinguish these entities from the various ways by which we measure them (which he called relative spaces and relative times)

Special Theory Of Relativity

The special theory of relativity is limited to objects that are moving with inertial frames of reference- i.e. in the state of uniform motion with respect to one another, such that one cannot, by purely mechanical experiments, distinguish one from the other.

Galileo's theory of relativity

No mechanical experiment can distinguish between two uniformly moving frames of reference.

Einstein's theory of relativity

No experiment can distinguish between two uniformly moving frames of reference. In particular, the speed of light in a vacuum has the same value in two uniformly moving frames of reference.