A nonzero vector in -dimensional Lorentzian space is said to be negative lightlike
if it has zero (Lorentzian) norm and if its first component
is negative. Symbolically, is negative lightlike if both
and
hold.
The collection of all negative lightlike vectors form the bottom 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.