Ten years before understanding that time is slowed down by mass,21 Einstein had realized that it was slowed down by speed.22 The consequence of this discovery for our basic intuitive perception of time is the most devastating of all. The fact itself is quite simple. Instead of sending the two friends from the first chapter to the mountains and the plains, respectively, let’s ask one of them to stay still and the other one to walk around. Time passes more slowly for the one who keeps moving.
