In the Galilean transformation, there are two separate invariants that two inertial frame of reference always agree on between two separate events:
- length, given by the Pythagorean theorem
However, in special relativity, neither of those are invariant separately, since space and time are mixed up together.
Instead, there is a new unified invariant: the spacetime-interval, given by:
Note that this distance can be zero for two events separated.