The spreadsheet will calculate the effects of Einstein's postulate of a fixed speed of light upon the time taken to travel to distant stars and return to earth, and on the apparent shortening or length contraction that the space traveler would exhibit.
For example, at 0.99 of the speed of light, a journey to the star Sirius, located 8.9 light years from earth, would take 17.4 years as seen by an earthbound observer. For the space traveler, the journey would only take 2.5 years as recorded by a clock aboard the spacecraft. In other words, the traveler will return to earth biologically aged only 2.5 years compared to when they left. These effects are real, and have been proven using experiments with atomic clocks.
Traveling at 0.99 of the speed of light, a 5-foot, 5-inch traveler will appear to shrink to a length of 9.2 inches as seen by an earthbound observer. This contraction lies in the direction of forward motion. The traveler would not notice any difference.
This spreadsheet can be given to students as an activity, or it can be used as a demonstration by the teacher. It includes tabs with directions, the main sheet, and a key.