# On the ratio of lapses in bimetric relativity

## TL;DR

This paper derives the ratio of lapse functions in spherically symmetric bimetric relativity, a crucial step for enabling numerical simulations in the Hassan-Rosen theory.

## Contribution

It provides the first explicit calculation of the lapse ratio in the spherically symmetric case, addressing a key obstacle in numerical bimetric relativity.

## Key findings

- Derived the lapse ratio for spherically symmetric solutions
- Facilitates numerical evolution in bimetric theories
- Clarifies the relationship between lapse functions in Hassan-Rosen theory

## Abstract

The two lapse functions in the Hassan-Rosen bimetric theory are not independent. Without knowing the relation between them, one cannot evolve the equations in the 3+1 formalism. This work computes the ratio of lapses for the spherically symmetric case, which is a prerequisite for numerical bimetric relativity.