CLOCK IN MOTION

Imagine now that we are outside the ship and are back on Earth. We are equipped with a special telescope that allows us to see inside the ship as it whizzes by at a significant fraction of the speed of light. We can see the laser, the mirror, and even the laser beam itself because the occupants of the space vessel have temporarily filled it with smoke to make the viewing easy for us. (They have pressure suits on so that they can breathe.)
What we see is depicted in Fig. 1. The laser beam still travels in straight lines, and it still travels at 3.00 × 10⁸ m/s relative to us. This is true because of Einstein’s axiom concerning the speed of light and the fact that light rays always appear to travel in straight lines as long as we are not accelerating. However, the rays have to travel farther than 3.00 m to get across the ship. The ship is going so fast that by the time the ray of light has reached the mirror from the laser, the ship has moved a significant distance forward. The same thing happens as the ray returns to the sensor from the mirror. As a result of this, it will seem to us, as we watch the ship from Earth, to take more than 20.0 ns for the laser beam to go across the ship and back.
As the ship goes by, time appears to slow down inside it, as seen from a “stationary” point of view. Inside the ship, however, time moves at normal speed. The faster the ship goes, the greater is this discrepancy. As the speed of the ship approaches the speed of light, the time dilation factor can become large indeed; in theory, there is no limit to how great it can become. You can visualize this by imagining Fig. 1 stretched out horizontally so that the light rays have to travel almost parallel to the direction of motion, as seen from the “stationary” reference frame.

Fig. 1. This is what an external observer sees as the laser clock– equipped space ship whizzes by at a sizable fraction of the speed of light.