Match each concept to its definition. I I the mass of an object measured at rest with respect to the observer; also called the proper mass the length of an object or distance between two points as measured by an observer who is stationary relative to the object or distance the slowing down of time as observed from a different reference frame the occurrence of two or more events at the same time the shortening of distances in an inertial frame of reference moving relative to an observer in another inertial frame of reference time dilation simulta neity proper length rest mass length contraction