Angular momentum, BMS charges, twistors and scattering

The BMS charges form a well-defined set of asymptotic kinematic quantities of significant interest; there has been considerable work interpreting the charges as conserved quantities conjugate to BMS motions.  However, the charges were originally proposed as part of a treatment of angular momentum, and this interpretation is more problematic.  While there is a formal parallel between the BMS and Poincare structures, attempts to interpret the BMS charges as spin and center of mass run into difficulties.  I will explain the infinite-dimensional ambiguity in the charge-based center of mass, and the related failure of supertranslation invariance for the charge-based spin [1].

Twistors give an approach to angular momentum which codes the same information, but in a different way [2].  Gauge problems are factored out; there are no ambiguities; one has appealing treatments of center of mass, spin and changes of origin.  I will show how the sorts of scattering usually considered can be naturally interpreted as maps on the asymptotic twistor spaces.