# Revisiting the Darmois and Lichnerowicz junction conditions

## TL;DR

This paper clarifies the true relationship between Darmois and Lichnerowicz junction conditions, emphasizing their equivalence only in strict admissible coordinates and correcting misconceptions caused by earlier misinterpretations.

## Contribution

It provides a clear, historically informed correction to the misconception that Darmois and Lichnerowicz conditions are always equivalent, emphasizing the importance of coordinate definitions.

## Key findings

- Equivalence holds only in Gaussian-normal coordinates.
- Loose definitions of Lichnerowicz conditions impose additional restrictions.
- Previous proofs of equivalence were based on strict admissible coordinates.

## Abstract

What have become known as the "Darmois" and "Lichnerowicz" junction conditions are often stated to be equivalent, "essentially" equivalent, in a "sense" equivalent, and so on. One even sees not infrequent reference to the "Darmois-Lichnerowicz" conditions. Whereas the equivalence of these conditions is manifest in Gaussian-normal coordinates, a fact that has been known for close to a century, this equivalence does not extend to a loose definition of "admissible" coordinates (coordinates in which the metric and its first order derivatives are continuous). We show this here by way of a simple, but physically relevant, example. In general, a loose definition of the "Lichnerowicz" conditions gives additional restrictions, some of which simply amount to a convenient choice of gauge, and some of which amount to real physical restrictions, away from strict "admissible" coordinates. The situation was totally confused by a very influential, and now frequently misquoted, paper by Bonnor and Vickers, that erroneously claimed a proof of the equivalence of the "Darmois" and "Lichnerowicz" conditions within this loose definition of "admissible" coordinates. A correct proof, based on a strict definition of "admissible" coordinates, was given years previous by Israel. It is that proof, generally unrecognized, that we must refer to. Attention here is given to a clarification of the subject, and to the history of the subject, which, it turns out, is rather fascinating in itself.