In special relativity, the invariant interval is a fundamental quantity that measures the “distance” between two events in spacetime. Unlike measurements of time or space alone—which vary depending on the observer’s frame of reference—the invariant interval remains the same in all inertial frames.
This interval combines differences in both space and time between two events into a single quantity. Depending on the nature of the events, the interval can be:
- Timelike (events can be causally connected),
- Spacelike (events cannot influence each other),
- Lightlike or null (events are connected by a light signal).
The invariance of this interval is what ensures the consistency of physical laws across different observers. It’s a cornerstone of the geometry of spacetime, showing that while observers may disagree on how far apart or how long apart two events are, they all agree on the invariant interval that links them.