The second postulate of special relativity is that the speed of light c is the same in all inertial frames of reference, independent of the relative motion of the observer and the light source Relativity is the study of how observers in different reference frames measure the same event

An inertial frame of reference is a reference frame in which a body at rest remains at rest and a body in motion moves at a constant speed in a straight line unless acted upon by an outside force The first postulate of special relativity is that the laws of physics are the same in all inertial frames of reference.

first postulate of special relativity — laws of physics are the same in all inertial frames of reference. event — occurrence in space and time specified by its position and time coordinates ( x , y , z , t ) measured relative to a frame of reference. inertial frame of reference — reference frame in which a body at rest remains at rest and a body in motion moves at a constant speed in a straight line unless acted on by an outside force. second postulate of special relativity — light travels in a vacuum with the same speed c in any direction in all inertial frames.