Focusing bounds for CFT correlators and the S-matrix

TL;DR

This paper establishes a new focusing bound for CFT correlators and the S-matrix derived from bulk focusing in AdS/CFT, which is stronger than causality constraints and applies to various theories including string theory.

Contribution

It introduces a novel focusing bound relating bulk and boundary operators in AdS/CFT, extending the implications of the focusing theorem to CFT correlators and the flat space S-matrix.

Findings

Focusing in the bulk implies a bound on CFT $n$-point functions.

The focusing bound is stronger than causality constraints.

String theory S-matrix and conformal Regge theory correlators also satisfy the focusing bound.

Abstract

The focusing theorem in General Relativity underlies causality, singularity theorems, entropy inequalities, and more. In AdS/CFT, we show that focusing in the bulk leads to a bound on CFT $n$-point functions that is generally stronger than causality. Causality is related to the averaged null energy condition (ANEC) on the boundary, while focusing is related to the ANEC in the bulk. The bound is derived by translating the Einstein equations into a relation between bulk and boundary light-ray operators. We also discuss the consequences of focusing for the flat space $S$-matrix, which satisfies a similar inequality, and give a new derivation of bounds on higher derivative operators in effective field theories. The string theory $S$-matrix and CFT correlators in conformal Regge theory also satisfy the focusing bound, even though in these cases it cannot be derived from the standard focusing theorem.