Tests for the existence of horizons through gravitational wave echoes

TL;DR

This paper discusses the potential of gravitational wave echoes as a method to test the existence of horizons and distinguish black holes from alternative compact objects, emphasizing future observational prospects.

Contribution

It introduces the concept of using gravitational wave echoes to test horizon existence, highlighting the importance of future high-sensitivity detectors for probing near-horizon physics.

Findings

Gravitational wave echoes could indicate horizonless compact objects.

Current observations are consistent with black hole models but cannot confirm horizons.

Future detectors may detect echoes, testing horizon existence.

Abstract

The existence of black holes and of spacetime singularities is a fundamental issue in science. Despite this, observations supporting their existence are scarce, and their interpretation unclear. We overview how strong a case for black holes has been made in the last few decades, and how well observations adjust to this paradigm. Unsurprisingly, we conclude that observational proof for black holes is impossible to come by. However, just like Popper's black swan, alternatives can be ruled out or confirmed to exist with a single observation. These observations are within reach. In the next few years and decades, we will enter the era of precision gravitational-wave physics with more sensitive detectors. Just as accelerators require larger and larger energies to probe smaller and smaller scales, more sensitive gravitational-wave detectors will be probing regions closer and closer to the horizon, potentially reaching Planck scales and beyond. What may be there, lurking?