When an object moves through space relative to us, its clock runs slow compared to ours. That is, the speed of its motion through time slows down. Here’s the leap: Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed—that of light. […] We are presently talking about an object’s combined speed through all four dimensions—three space and one time—and it is the object’s speed in this generalized sense that is equal to that of light. [… All] objects that are at rest relative to us and to each other move through time—they age—at exactly the same rate or speed. If an object does move through space, however, this means that some of the previous motion through time must be diverted. Like the car traveling at an angle, this sharing of motion implies that the object will travel more slowly through time than its stationary counterparts, since some of its motion is now being used to move through space.