The observational evidence for horizons: from echoes to precision gravitational-wave physics

TL;DR

The paper discusses the challenges of observationally confirming black hole horizons, emphasizing the potential of future precision gravitational-wave detectors to probe regions near horizons and test fundamental physics.

Contribution

It highlights the limitations of current observations for black holes and outlines how upcoming gravitational-wave detectors could explore near-horizon physics at unprecedented scales.

Findings

Current evidence for black holes is inconclusive.

Future detectors will improve sensitivity to near-horizon regions.

Potential to probe Planck-scale physics near horizons.

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 evidence 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?