General relativity principle and uniqueness in Einstein equations

TL;DR

This paper discusses the principle of general relativity in Einstein equations, proposing a stronger condition for metric uniqueness and introducing the concept of an 'equivalence group' to clarify the physical content of covariance.

Contribution

It introduces the 'equivalence group' concept and provides a sufficient condition to ensure the equivalence of all coordinate systems, addressing the uniqueness of the metric.

Findings

Proposes the 'equivalence group' as a physically meaningful symmetry group.

Provides a sufficient condition for the equivalence of coordinate systems.

Clarifies the distinction between covariance and physical equivalence in Einstein equations.

Abstract

The issue of implementing the principle of general relativity in Einstein equations has been widely discussed, since Kretschmann's well-known criticism stated that general covariance of the Einstein equations is not suffice to express the principle of general relativity (the equivalence of all the coordinate systems). This failure is usually rooted in the fact that metric in Einstein equations is not univocally determined by the matter distribution. We show that the condition of univocal determination of the metric by the matter distribution is stronger than the requirement of equivalence of all coordinate systems. In order to separate the uniqueness problem in Einstein equations from the issue of the principle of general relativity, we define the "equivalence group" instead of the notion of covariance group which is empty of physical content. Moreover, we have complemented in a positive way Kretschmann's objection by supplementing Einstein equations with a sufficient condition for the equivalence of all the coordinate systems.