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.

Back on Earth, Mission Control judges that your shipboard clocks run slow. What do you judge about clocks at Mission Control?
a. They run fast.
b. They keep time at the same rate as your clocks.
c. They run slow.
d. You can’t tell anything about their clocks.