The one thing in spacetime on which one may depend...
An interval is the separation in spacetime
between two events. The spacetime interval between two events is invariant among
reference frames. The interval is calculated by the
of the difference in the squares of the spacetime time separation and the spacetime space separation between a pair of events. If the interval is positive, meaning the time separation is greater than the space separation, the interval is called time-like. If the interval is negative it is called space-like. If the interval is zero it is called light-like, or a null interval. Only events that are separated by time-like or light-like intervals may have a causal relationship, one being the cause of the other.