Surveys in Differential Geometry

Volume 20 (2015)

Geometry and physics of null infinity

Pages: 99 – 122



Abhay Ashtekar (Institute for Gravitation and the Cosmos, and Department of Physics, Penn State University, University Park, Pennsylvania, U.S.A.)


In asymptotically Minkowski space-times, one finds a surprisingly rich interplay between geometry and physics in both the classical and quantum regimes. On the mathematical side it involves null geometry, infinite dimensional groups, symplectic geometry on the space of gravitational connections and geometric quantization via Kähler structures. On the physical side, null infinity provides a natural home to study gravitational radiation and its structure leads to several interesting effects such as an infinite dimensional enlargement of the Poincaré group, geometrical expressions of energy and momentum carried by gravitational waves, emergence of non-trivial ‘vacuum configurations’ and an unforeseen interplay between infrared properties of the quantum gravitational field and the enlargement of the asymptotic symmetry group. The goal of this article is to present a succinct summary of this subtle and beautiful interplay.


Einstein equations, Cauchy problem, asymptotically flat spacetimes, stability, asymptotics, null infinity, ADM, gravitational radiation, memory effect, Christodoulou memory effect, classical theory, quantum theory, isolated gravitating system

2010 Mathematics Subject Classification

35A01, 35L51, 35Q76, 83C05, 83C30, 83C35

Published 7 July 2015