A nonzero vector v = (v_0, v_1, ..., v_(n - 1)) in n-dimensional Lorentzian space R^(1, n - 1) is said to be positive lightlike if it has zero (Lorentzian) norm and if its first component v_0 is positive. Symbolically, v is positive lightlike if both -v_0^2 + v_1^2 + ... + v_(n - 1)^2 = 0 and v_0>0 hold. The collection of all positive lightlike vectors form the top half of the light cone.

light cone | lightlike | Lorentzian inner product | Lorentzian space | metric signature | negative lightlike | negative timelike | positive timelike | spacelike | timelike