Video · + 2 min read
This video gives a gentle explanation and builds intuition for the two postulates of relativity. It is recommended to watch it. Read the explanation below. Then re-watch the video. Definitions for specialized terminology are numbered after the explanation. 1. Principle of Relativity
One cannot perform an experiment to distinguish between uniform (1) translational motion (2) and a rest state (3). In other words, one can only describe the relative motion between two objects.
2. Invariance of c
Between any two reference frames (4) in which motion is constant, c - the speed at which light moves in a vacuum - remains a unchanged. Another way to describe this is to say that "c is invariant between inertial (5) reference frames." This is exceptional because we would expect that if we were moving at a velocity v, on say a train, and we have a laser pointer (or some other light source) pointed in the direction of our motion that the light would move at c + v. But observations of the speed of light tell us that this doesn't happen. It always is c (in a vacuum) regardless of all other motion. This postulate is what can be considered special about Special Relativity.
Notes
(1) Uniform motion is motion without acceleration. Recall that acceleration is the quantity which describes any change in speed or direction.
(2) Translational motion is motion that is in a straight line. Recall that we are only talking about motion without acceleration, and if an object changes its direction then it is accelerating, even if its speed remains the same.
(3) A rest state describes a scenario in which an object is not moving at all.
(4) Because of the first postulate, we must describe motion with respect to something else which we assume to be in a rest state. We call this the frame of reference. The whole description of motion is called a reference frame.
(5) Inertial reference frames are reference frames in which one does not experience acceleration.