Skip to main content

Research Repository

Advanced Search

Ambitwistor strings at null infinity and (subleading) soft limits

Geyer, Yvonne; Lipstein, Arthur E.; Mason, Lionel

Ambitwistor strings at null infinity and (subleading) soft limits Thumbnail


Yvonne Geyer

Lionel Mason


The relationship between BMS symmetries at null infinity and Weinberg's soft theorems for gravitons and photons together with their subleading extensions are developed using ambitwistor string theory. Ambitwistor space is the phase space of complex null geodesics in complexified space-time. We show how it can be canonically identified with the cotangent bundle of complexified null infinity. BMS symmetries of null infinity lift to give a Hamiltonian action on ambitwistor space, both in general dimension and in its twistorial four-dimensional representation. General vertex operators arise from Hamiltonians generating diffeomorphisms of ambitwistor space that determine the scattering from past to future null infinity. When a momentum eigenstate goes soft, the diffeomorphism defined by its leading and its subleading part are extended BMS generators realized in the world sheet conformal field theory of the ambitwistor string. More generally, this gives an explicit perturbative correspondence between the scattering of null geodesics and that of the gravitational field via ambitwistor string theory.


Geyer, Y., Lipstein, A. E., & Mason, L. (2015). Ambitwistor strings at null infinity and (subleading) soft limits. Classical and Quantum Gravity, 32(5), Article 055003.

Journal Article Type Article
Acceptance Date Dec 5, 2014
Online Publication Date Jan 30, 2015
Publication Date Mar 5, 2015
Deposit Date Feb 22, 2016
Publicly Available Date Jul 25, 2018
Journal Classical and Quantum Gravity
Print ISSN 0264-9381
Electronic ISSN 1361-6382
Publisher IOP Publishing
Peer Reviewed Peer Reviewed
Volume 32
Issue 5
Article Number 055003
Related Public URLs


Accepted Journal Article (482 Kb)

Copyright Statement
This is an author-created, un-copyedited version of an article published in Classical and Quantum Gravity. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at

You might also like

Downloadable Citations