A nonzero vector in -dimensional Lorentzian space is said to be positive lightlike
if it has zero (Lorentzian) norm and if its first component
is positive. Symbolically, is positive lightlike if both
and
hold.
The collection of all positive lightlike vectors form the top half of the light
cone.
Misner, C. W.; Thorne, K. S.; and Wheeler, J. A. Gravitation.
San Francisco, CA: W. H. Freeman, p. 53, 1973.Ratcliffe,
J. G. Foundations
of Hyperbolic Manifolds. New York: Springer-Verlag, 2006.