Paths in spacetime that move at the speed of light are called null trajectories (because the proper time is zero) or lightlike trajectories (for obvious reasons). When we choose units where c = 1, null trajectories are drawn as diagonal lines that tilt at 45 degrees on a spacetime diagram, traversing equal amounts of space and time. Anything that is moving slower than the speed of light will traverse more time than space, so such paths are known as timelike trajectories, moving upward in the spacetime diagram. We can also consider spacelike trajectories, which move sideways and traverse more
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