Einstein’s theory of relativity suggests that there is no fact of the matter as to when “now” is. Any measurement of time is relative to the perspective of an observer. In other words, if you are traveling very fast, the clocks of others are speeding up from your point of view. You will spend a few years in a spaceship but when you return to earth thousands or millions of years will have passed. Yet it seems odd, to say the least, to discount the well-being of people as their velocity increases. Should we pay less attention to the safety of our spacecraft, and thus the welfare of our astronauts, the faster those vehicles go? If, for instance, we sent off a spacecraft at near the velocity of light, the astronauts would return to earth, hardly aged, millions of years hence. Should we—because of positive discounting—not give them enough fuel to make a safe landing? And if you decline to condemn them to death, how are they different from other “residents” in the distant future?

Tyler Cowen, ‘Caring about the Distant Future: Why it Matters and What it Means’, University of Chicago Law Review, vol. 74, no. 1 (Winter, 2007), p. 10