  We study geometric structures associated with shear-free null geodesic
congruences in Minkowski space-time and asymptotically shear-free null geodesic
congruences in asymptotically flat space-times. We show how in both the flat
and asymptotically flat settings, complexified future null infinity acts as a
"holographic screen," interpolating between two dual descriptions of the null
geodesic congruence. One description constructs a complex null geodesic
congruence in a complex space-time whose source is a complex world-line; a
virtual source as viewed from the holographic screen. This complex null
geodesic congruence intersects the real asymptotic boundary when its source
lies on a particular open-string type structure in the complex space-time. The
other description constructs a real, twisting, shear-free or asymptotically
shear-free null geodesic congruence in the real space-time, whose source (at
least in Minkowski space) is in general a closed-string structure: the caustic
set of the congruence. Finally we show that virtually all of the interior
space-time physical quantities that are identified at null infinity (center of
mass, spin, angular momentum, linear momentum, force) are given kinematic
meaning and dynamical descriptions in terms of the complex world-line.
