String Theory Seminars

Causality Constraints in Quantum Field Theory (Tata-Infosys Lectures Series - Lecture 2)

by Dr. Sandipan Kundu (Cornell University)

Friday, April 21, 2017 from to (Asia/Kolkata)
at A304
Causality places nontrivial constraints on QFT in Lorentzian signature, for example fixing the signs of certain terms in the low energy Lagrangian.  In conformal field theory, I will show how causality is encoded in crossing symmetry and reflection positivity of Euclidean correlators, and derive constraints on the interactions of low-lying operators directly from the conformal bootstrap. I will explain the connection between the causality constraints in the lightcone limit and the averaged null energy condition. In particular, causality implies that the averaged null energy must be positive in interacting quantum field theory in flat spacetime. I will also show that in four-dimensions, conformal field theories with large-N and a sparse spectrum exhibit universal, gravity-like behavior and consequently, the anomaly coefficients satisfy a=c.