Physical meaning of null spacetime interval

[vanhees71]

That's what preach all the time: Hammer into your students the fact that the indefinite fundamental form of Minkowski space does NOT induce a metric on $\mathbb{R}^4$ as a Euclidean scalar product does. Light-like separated events thus are, by definition, never the same event. As you say, it describes the emission and absorption event of light signals, as described by the retarded propagator of the electromagnetic field in classical electrodynamics.