The existence of black holes and spacetime singularities is a fundamental issue in science. Despite this, observations supporting their existence are scarce, and their interpretation is unclear. In this Perspective we outline the case for black holes that has been made over the past few decades, and provide an overview of how well observations adjust to this paradigm. Unsurprisingly, we conclude that observational proof for black holes is, by definition, impossible to obtain. 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 coming years and decades, we will enter an era of precision gravitational-wave physics with more sensitive detectors. Just as accelerators have required larger and larger energies to probe smaller and smaller scales, more sensitive gravitational-wave detectors will probe regions closer and closer to the horizon, potentially reaching Planck scales and beyond. What may be there, lurking?
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V.C. acknowledges financial support provided under the European Union’s H2020 ERC Consolidator Grant ‘Matter and strong-field gravity: New frontiers in Einstein’s theory’, grant agreement No. MaGRaTh–646597. Research at Perimeter Institute is supported by the Government of Canada through Industry Canada and the Province of Ontario through the Ministry of Economic Development Innovation. This article is based on work from COST Action CA16104 ‘GWverse’ and MP1304 ‘NewCompstar’, supported by COST (European Cooperation in Science and Technology). This work was partially supported by FCT-Portugal through project IF/00293/2013, and by the H2020-MSCA-RISE-2015, grant No. StronGrHEP-690904.
The authors declare no competing financial interests.
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Cardoso, V., Pani, P. Tests for the existence of black holes through gravitational wave echoes. Nat Astron 1, 586–591 (2017). https://doi.org/10.1038/s41550-017-0225-y
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