According to Einstein's theory of special relativity, the observed time for events is dependent upon the speed of the observer relative to the speed of the reference frame in which the events are occurring. For objects moving at relativistic speeds, there is an observable stretching out of time relative to an observer in a stationary reference frame.

Suppose that a spaceship was equipped with a pair of mirrors separated by one meter, an accurate clock which measures time to the nanosecond, and a source of light. Suppose that when the light source is turned on, a flash of light is emitted which bounces off the mirrors and that the nanosecond clock could accurately measure the time for the flash of light to make four complete back-and-forth bounces off the mirrors. Furthermore, suppose that the event of the flash of light making four back-and-forth bounces could be observed from the reference frame of the spaceship and from the reference frame of an observer who is at rest relative to the moving space ship. What would the two observers see? Would the speed of the spaceship have any influence upon the observations? Use the animation below to investigate these questions.


  1. Use the user input box to modify the speed of the spaceship; use values between 0 and 97.4 % the speed of light.
  2. Click on the Run button to observe the animation.
  3. Record values for the distance and time of the light beam to make four bounces off the mirror as viewed from the two reference frames. Use the distance/time ratio to determine the speed of light as measured by observers in each reference frame.
  4. Click on the Reset button and repeat for several values of spaceship speed. Use the Reset button if you encounter any problems.

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