# Quasi-local mass at null infinity in Bondi-Sachs coordinates

## TL;DR

This paper computes the Wang-Yau quasi-local mass at null infinity in Bondi-Sachs coordinates, providing a local description of gravitational radiation and connecting it to the news function, thus offering new insights into gravitational energy.

## Contribution

It introduces a method to evaluate quasi-local mass at null infinity in Bondi-Sachs coordinates, linking it to gravitational radiation via the news function.

## Key findings

- Quasi-local mass converges to a limit at null infinity.
- The mass is expressed in terms of the news function.
- Provides a local perspective on gravitational radiation.

## Abstract

There are two important statements regarding the Trautman-Bondi mass at null infinity: one is the positivity, and the other is the Bondi mass loss formula, which are both global in nature. In this note, we compute the limit of the Wang-Yau quasi-local mass on unit spheres at null infinity of an asymptotically flat spacetime in the Bondi-Sachs coordinates. The quasi-local mass leads to a local description of the radiation that is purely gravitational at null infinity. In particular, the quasi-local mass is evaluated in terms of the news function of the Bondi-Sachs coordinates.