  We introduce new tools for studying modular flow in AdS/CFT. These tools
allow us to efficiently extract bulk information related to causality and
locality. For example, we discuss the relation between analyticity in modular
time and entanglement wedge nesting which can then be used to extract the
location of the Ryu-Takayanagi (RT) surface directly from the boundary theory.
Probing the RT surface close to the boundary our results reduce to the recent
proof of the Quantum Null Energy Condition. We focus on heavy probe operators
whose correlation functions are determined by spacelike geodesics. These
geodesics interplay with the RT surface via a set of rules that we conjecture
and give evidence for using the replica trick.
