Special Theory of Relativity is Albert Einstein’s theory of how light behaves in relation to different perspectives. Suppose we want to find the length of an airplane when we are on board. All we have to do is put the end of a tape measure at the airplane’s nose and look at the number on the tape at the airplane’s tail. But what if we are standing on the ground and the airplane is in flight? Now things are more difficult. According to Einstein, our measurements from the ground of length, time, and mass in the airplane would differ from those made by somebody moving with the airplane.