All observers agree upon relative speed. Distance depends on an observer’s motion. Proper length is the distance between two points measured by an observer who is at rest relative to both of the point...All observers agree upon relative speed. Distance depends on an observer’s motion. Proper length is the distance between two points measured by an observer who is at rest relative to both of the points. Earth-bound observers measure proper length when measuring the distance between two points that are stationary relative to the Earth. Length contraction is the shortening of the measured length of an object moving relative to the observer’s frame.
