Zero rest-mass fields and the Newman-Penrose constants on flat space

TL;DR

This paper investigates the relationship between initial data for zero rest-mass fields on flat space and their Newman-Penrose constants at null infinity, revealing conditions under which these constants are correlated or independent.

Contribution

It clarifies the correspondence between initial data and NP constants using Friedrich's framework, showing the role of spherical harmonic expansion and time symmetry.

Findings

NP constants relate to second highest spherical harmonic data

No natural correspondence between future and past NP constants for generic data

Time-symmetric data yields identical NP constants at future and past null infinity

Abstract

Zero rest-mass fields of spin 1 (the electromagnetic field) and spin 2 propagating on flat space and their corresponding Newman-Penrose (NP) constants are studied near spatial infinity. The aim of this analysis is to clarify the correspondence between data for these fields on a spacelike hypersurface and the value of their corresponding NP constants at future and past null infinity. To do so, Friedrich's framework of the cylinder at spatial infinity is employed to show that, expanding the initial data in terms spherical harmonics and powers of the geodesic spatial distance $\rho$ to spatial infinity, the NP constants correspond to the data for the second highest possible spherical harmonic at fixed order in $\rho$. In addition, it is shown that for generic initial data within the class considered in this article, there is no natural correspondence between the NP constants at future and past null infinity ---for both the Maxwell and spin-2 field. However, if the initial data is time-symmetric then the NP constants at future and past null infinity have the same information.