The constant c , the speed of light, converts time t units (like seconds) into space units (like meters). The squared interval Δ Δ s 2 is a measure of separation between events A and B that are time separated and in addition space separated either because there are two separate objects undergoing events, or because a single object in space is moving inertially between its events. The separation interval is the difference between the square of the spatial distance separating event B from event A and the square of the spatial distance traveled by a light signal in that same time interval Δ Δ t . If the event separation is due to a light signal, then this difference vanishes and Δ Δ s = 0 .