“…The Minkowski space R 4 1 is the vector space R 4 endowsed with the pseudo-scalar product x, y = −x 0 y 0 + x 1 y 1 + x 2 y 2 + x 3 y 3 , for any x = (x 0 , x 1 , x 2 , x 3 ) and y = (y 0 , y 1 , y 2 , y 3 ) in R 4 1 (see, e.g., [9]). We say that a non-zero vector x ∈ R 4 1 is spacelike if x, x > 0, lightlike if x, x = 0 and timelike if x, x < 0, respectively.…”