# Mixing of gravitational wave echoes

## TL;DR

This paper explores how gravitational wave echoes, potentially revealing black hole quantum structures, can exhibit complex mixing effects due to multiple barriers near the horizon, affecting their detectability.

## Contribution

It introduces a model where near-horizon structures cause mixing of echoes, challenging previous assumptions of simple, sequential echo decay.

## Key findings

- Echo signals can exhibit mixing and superpositions due to multiple barriers.
- Successive echo amplitudes may not decrease monotonically.
- Waveform complexity impacts gravitational wave data analysis.

## Abstract

Gravitational wave (GW) echoes, if they exist, would be a probe to the near-horizon quantum structure of black hole (BH), which has motivated the searching for the echo signals in GW data. We point out that the echo phenomenology related with the potential structure might be not so simple as expected. In particular, if the near-horizon regime of BH is modelled as a multiple-barriers filter, the late-time GW ringdown waveform will exhibit the mixing of echoes, even the superpositions. As a result, the amplitudes of successive echoes might not drop sequentially.