Not only does time change with speed, so do length and mass. As objects speed up, they get shorter and heavier. At nine-tenths of the speed of light, for instance, a yardstick would only be 0.44 yards long, and a one-pound bag of sugar would weigh nearly 2.3 pounds—from a stationary observer’s point of view. (Of course, this doesn’t mean that you would be able to bake more cookies with the same bag of sugar. From the bag’s point of view, its weight stays the same.)
