Scattering equations and Kawai-Lewellen-Tye orthogonality

Several recent developments point to the fact that rational maps from n-punctured spheres to the null cone of D-dimensional momentum space provide a natural language for describing the scattering of massless particles in D dimensions. In this paper we identify and study equations relating the kinematic invariants sab and the puncture locations σc, which we call the scattering equations. We provide an inductive algorithm in the number of particles for their solutions and prove a remarkable property which we call Kawai--Lewellen--Tye (KLT) orthogonality. In a nutshell, KLT orthogonality means that ``Parke--Taylor'' vectors constructed from the solutions to the scattering equations are mutually orthogonal with respect to the KLT bilinear form. We end with comments on possible connections to gauge theory and gravity amplitudes in any dimension and to the high-energy limit of string theory amplitudes.