Orbital angular momentum of scalar field generated by gravitational scatterings

TL;DR

This paper investigates how gravitational scattering by black holes can generate orbital angular momentum in scalar waves, revealing that black hole rotation influences certain angular momentum components.

Contribution

It clarifies the mechanism of orbital angular momentum generation in scalar waves due to gravitational lensing, emphasizing the role of black hole rotation.

Findings

Frame dragging is not necessary for generating orbital angular momentum.

Rotating black holes produce specific angular momentum components.

Scalar wave deformation analyzed via geometrical optics approximation.

Abstract

It has been expected that astronomical observations to detect the orbital angular momenta of electromagnetic waves may give us a new insight into astrophysics. Previous works pointed out the possibility that a rotating black hole can produce orbital angular momenta of electromagnetic waves through gravitational scattering, and the spin parameter of the black hole can be measured by observing them. However, the mechanism how the orbital angular momentum of the electromagnetic wave is generated by the gravitational scattering has not been clarified sufficiently. In this paper, in order to understand it from a point of view of gravitational lensing effects, we consider an emitter which radiates a spherical wave of the real massless scalar field and study the deformation of the scalar wave by the gravitational scattering due to a black hole by invoking the geometrical optics approximation. We show that the frame dragging caused by the rotating black hole is not a necessary condition for generating the orbital angular momentum of the scalar wave. However, its components parallel to the direction cosines of images appear only if the black hole is rotating.