 Null vector

A nonzero spacetime vector $k$ is null when its Minkowski norm vanishes, $k^a{k}_a=0$. Null tangent vectors describe the local directions of light rays.

Special relativity describes inertial frames using Lorentz transformations and Minkowski spacetime.

A null vector, often referred to as the zero vector, is a vector that has all its components equal to zero.