An observer moving with a light clock in a spaceship sees a light flash bouncing up and down between parallel mirrors in 1 nanosecond. an observer at rest outside the spaceship sees the same up-and-down flash in

More than 1 nanosecond.

Explanation:

According to the theory of special relativity, the observed time for an event by person that moves fast is slower (dilation of time) than the observed time by another person that is in a stationary reference. Therefore, the time is relative upon the speed of the observer.    

So, the answer to the question is the observer at rest outside the spaceship sees the same up-and-down flash in more than 1 nanosecond.        

I hope it helps you!

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