If, for instance, we sent off a spacecraft at nearly the velocity of light, the astronauts would return to Earth, hardly aged, many millions of years hence. Should we pay less attention to the safety of our spacecraft, and thus to the welfare of our astronauts, the faster those vehicles go? Should we—as a result of positive discounting—not give them enough fuel to make a safe landing? And if you decline to condemn these brave astronauts to death, how are they different from other residents of the distant future? Instead of letting our speedy
