Special theory of relativity applies to the elementary particles and their interactions.Elementary particles are those particles present in atoms whose substructure is not known. Examples of elementary particles are bosons, fermions.
Special theory of relativity is based on two main postulates given by Einstein in his paper in 1905.
The two postulates are as follows:
1) The principle of relativity: The laws of physics are same in all inertial frame of reference.
In other words, the laws of physics are same for all observers moving uniformly relative to one another.
2) The principle of constancy of the speed of light: The speed of light in free space( vacuum) has same value c in all inertial frames of references.
In other words, the value of speed of light is same for all observers irrespective of their relative motion with motion of light source.