Poynting vector, super-Poynting vector, and principal observers in electromagnetism and general relativity

TL;DR

This paper explores the concept of principal observers in electromagnetism and general relativity, analyzing their properties, relationships, and algorithms for identification, with a focus on the Poynting and super-Poynting vectors across different field types.

Contribution

It provides a comprehensive survey of principal observers, establishes their relation to arbitrary observers, and introduces algorithms to identify them based on electromagnetic and gravitational field data.

Findings

Principal observers exist for non-null electromagnetic and certain gravitational fields.

Super-Poynting vector analogy holds for Petrov type D but only partially for type I.

In Petrov type D Einstein-Maxwell fields, Poynting and super-Poynting vectors are aligned, and principal observers coincide.

Abstract

In electromagnetism, the concept of Poynting vector as measured by an observer is well known. A mathematical analogue in general relativity is the super-Poynting vector of the Weyl tensor. Observers for which the (super-)Poynting vector vanishes are called principal. When, at a given point, the electromagnetic field is non-null, or the gravitational field is of Weyl-Petrov type I or D, principal observers instantaneously passing through that point always exist. We survey characterizations of such observers and study their relation to arbitrary observers. In the non-null electromagnetic case it is known that, given any observer, there is a principal observer which moves relative to the first in the direction of his Poynting vector. Replacing Poynting by super-Poynting yields a possible gravitational analogue; we show that this analogy indeed holds for any observer when the Petrov type is D, but only for a one-dimensional variety of observers when the Petrov type is I. We provide algorithms to obtain the principal observers directly from the electric and magnetic fields (in the electromagnetic case) or electric and magnetic parts of the Weyl tensor (in the gravitational case) relative to an arbitrary observer. It is found that in Petrov type D doubly aligned non-null Einstein-Maxwell fields (which include all classical charged black hole solutions) the Poynting and super-Poynting vectors are aligned, at each point and for each observer, and the principal observers coincide. Our results are illustrated in simple examples.