Principle of Relativity

In the year 1905, Albert Einstein published a paper called "Zur Elektrodynamik bewegter Körper" (Engl. On the Electrodynamics of Moving Bodies) in which he formulated the fundamental theorems of the special theory of relativity. The two postulates of Einsteins, on which the theory of relativity is based, can be stated as follows. The first postulate matches with the daily experience. If a car moves at a constant speed along a straight line, it is impossible to "feel" the motion of the car. There is indeed no physics experiment that can be performed inside the car that shows that the car is in motion. Only during the time of acceleration or deceleration a force acting on the driver can be observed. The second postulate is based on an experiment performed by Michelson and Morley in the year 1887. With the help of an interferometer, it could be proved that the speed of light does not change, no matter how fast an object is moving. All effects of the special theory of relativity can be derived from these two postulates.