Interval

The one thing in
spacetime
on which one may depend...

Interval – the separation in spacetime
between two events. The spacetime interval between two events is invariant among
reference frames. The interval is calculated by the
square root
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.