Relativity Lite

4 | Relativity Lite Note that we use the Greek letter tau ( τ ) for this time, since we know there will be two different times to consider. H = c τ Figure 1. A spaceship with light bouncing between lower and upper mirrors, called a light clock . To someone standing on the Earth (see figure 2) the light will be seen to follow a diago- nal path because the spaceship moves relative to the Earth between the time that the light is emitted near the bottom and when it is reflected from the mirror near the top of the spaceship. r = v t H = c τ d = c t Figure 2. As seen from the Earth, light bouncing between lower and upper mirrors of a moving spaceship follows a diagonal path. Since the speed of light is the same in the Earth’s reference frame as in the reference frame of the ship, this diagonal distance is d = c t , where t is the time interval measured from the Earth. Finally, the ship travels a distance r = v t with respect to the Earth.