What invariant quantity separates events in Minkowski spacetime?

The invariant quantity separating events in Minkowski spacetime is the spacetime interval.

For two events, the interval combines their spatial separation with the time between them. Using one common sign convention, its squared form is Δs² = c²Δt² − Δx² − Δy² − Δz². Although observers can disagree about the distance or time measured separately, all inertial observers calculate the same interval.

The interval classifies the relationship between events. A timelike interval allows a slower-than-light object or signal to connect them; a lightlike or null interval corresponds to light; and a spacelike interval means that no causal signal traveling at or below light speed can connect them. The light cone is the boundary between these categories.

The interval is not the same as ordinary Euclidean distance. Depending on the metric-signature convention, its squared value can be positive, negative, or zero, and physicists may reverse every sign without changing the underlying physics. This is why “spacetime interval” is the canonical answer, while rest wavelength, charge, and angular momentum describe different physical concepts.