Consider a clock face. If that clock face started quickly moving away from you, what happens to how you read that clock? Just like the concept of the light from starts reaching you only after they have traveled a certain distance, the light from that clock face would have to travel longer and longer distances to reach you. The result—the clock would appear to click slower and time appears to move slower. If the clock were to move faster and faster, time would appear to slow down further and further. As the clock speed reaches the speed of light, time would appear to stand still as the clock is moving away from you as fast as the light is traveling away from the clock. Time stands still!

This concept is called time dilation and though you've just walked through a thought experiment showing how this works, it actually works in reality as well. In a real experiment, scientists synchronized two extremely accurate atomic clocks. One stayed on Earth motionless and the second was put on an airplane and flown around the Earth. To those who were on the airplane, time seemed to move at "normal" speed; however, when they returned, the clocks were out of sync. The clock that was moving lagged behind the non-moving clock. It is also of note that the clocks on the International Space Station usually lag behind stationary earth-bound clocks by about 0.007 seconds for every six months.

The equation that relates the contracted time to the time of the stationary observer is complex and is shown here just to show the relationship between the times is related to the velocity as compared to the speed of light.

\(\large\mathsf{ t = \frac{t_0}{\sqrt{1 - \frac{v^2}{c^2}}} }\)