You’ve been named captain of NASA’s first interstellar mission since the Voyager robotic spacecraft. You board your spaceship, accelerate quickly to 0.8c, and cruise at constant speed toward Proxima Centauri, the closest star to our Sun. Proxima Centauri is 4 light-years distant as measured in the two stars’ common rest frame. On the way, you conduct various medical experiments to determine the effects of a long space voyage on the human body.

How much do you age during your interstellar journey?
a. 3 years
b. just under 4 years
c. just over 4 years d. 5 years