In general s 2 can assume any real number value. If s 2 is positive, the spacetime interval is referred to as timelike. Since spatial distance traversed by any massive object is always less than distance traveled by the light for the same time interval, positive intervals are always timelike. If s 2 is negative, the spacetime interval is said to be spacelike. Spacetime intervals are equal to zero when x = ± ± c t . In other words, the spacetime interval between two events on the world line of something moving at the speed of light is zero. Such an interval is termed lightlike or null. A photon arriving in our eye from a distant star will not have aged, despite having (from our perspective) spent years in its passage.