Minkowski-space/Several vectors/Definition

Lightlike vector

Let $V$ be a Minkowski space, endowed with the Minkowski form $⟨-,-⟩$. A vector $v\in V$ fulfilling

$⟨v,v⟩=0$

is called lightlike, a vector $v\in V$ fulfilling

$⟨v,v⟩<0$

is called timelike, and a vector $v\in V$ fulfilling

$⟨v,v⟩>0$

is called spacelike.