A LASER CLOCK

Suppose that we have a space ship equipped with a laser/sensor on one wall and a mirror on the opposite wall (Fig. 1). Imagine that the laser/sensor and the mirror are positioned so that the light ray from the laser must travel perpendicular to the axis of the ship, perpendicular to its walls, and (once we get it moving) perpendicular to its direction of motion. The laser and mirror are adjusted so that they are separated by 3.00 m. Because the speed of light in air is approximately 3.00 × 10⁸ m/s, it takes 1.00 × 10^-8 s, or 10.0 nanoseconds (10.0 ns), for the light ray to get across the ship from the laser to the mirror and another 10.0 ns for the ray to return to the sensor. The ray therefore requires 20.0 ns to make one round trip from the laser/sensor to the mirror and back again.
Our laser emits pulses of extremely brief duration, far shorter than the time required for the beam to get across the ship. We might even suppose that the beam emits just a few photons in each burst! We measure the time increment using an extremely sophisticated oscilloscope so that we can observe the pulses going out and coming back and measure the time lag between them. This is a special clock; its timekeeping ability is based on the speed of light, which Einstein proposed is constant no matter from what point of view it is observed. There is no better way to keep time.

Fig. 1. A space ship equipped with a laser clock. This is what an observer in the ship always sees.