General relativity has passed many successful tests. Recent results from the LIGO-Virgo-KAGRA, Event Horizon Telescope, and GRAVITY collaborations are often taken as proof that black holes exist, but they actually only show the presence of candidate objects. The data agree impressively with predictions for Kerr black holes, but this only excludes certain alternative models of compact objects—it doesn't provide a final proof. More fundamentally, general relativity imposes principled limits on what we can observe: no observational data can ever confirm the existence of black holes. Thus, observational cosmology hits an epistemological limit set by the theory itself.
Rapid advances in observational astrophysics—including detection of gravitational waves with LIGO, built with contributions from Rainer Weiss, and imaging with radio interferometers like the Event Horizon Telescope—have fostered a widespread belief that black holes have been definitively discovered. Yet, as Karl Popper emphasized, scientific theories are never verified, only falsified. With black holes, we face a fundamental limitation: no finite set of observations can distinguish an event horizon from the surface of an ultracompact object if the difference is described by an arbitrarily small parameter.
The analysis draws on three types of data: 1) gravitational-wave merger signals from LIGO-Virgo-KAGRA, especially the ringdown stage, described by quasi-normal modes—damped oscillations of curved spacetime, first derived for the spherically symmetric case by Karl Schwarzschild; 2) shadow observations of the supermassive object in M87 at 1.3 mm wavelength, revealing a region of strong gravitational lensing tied to the accretion disk and unstable photon circular orbits; 3) measurements of stellar orbits near the Galactic center, showing gravitational redshift. Predictions for pure Kerr geometry are compared with models of ultracompact objects characterized by a closeness parameter ε.
The ringdown quasi-normal modes after mergers of black hole candidates align perfectly with Kerr metric solutions. However, the same modes can be produced by any object possessing a light ring—a region of unstable photon orbits. For instance, some neutron star models with extremely stiff equations of state can host a light ring and exhibit a similar ringdown. Moreover, echo signals that could betray a surface are exponentially suppressed and, over finite observation times, are buried in noise. The M87* shadow from EHT is also not unique: any sufficiently compact object with a photon sphere will cast a similar image, and details of the accretion flow add further uncertainty. Even historical candidates in quasars only hint at high luminosity and compactness, not a horizon. As for radiation predicted by Stephen Hawking, its potential detection would merely confirm quantum effects in strong fields, not prove the object is a black hole.
Thus, we must acknowledge that the term 'black hole' in an observational context is premature. A rigorous scientific stance calls for speaking of black hole candidates. This does not diminish GR's triumphs; rather, it underscores its power as a theory that successfully describes extreme gravitational fields. But it also means that proving the existence of an event horizon remains a fundamentally unattainable goal within classical observational methods.
In the future, with next-generation gravitational-wave detectors like the Einstein Telescope and space-based LISA, along with advances in submillimeter radio interferometry, we will probe spacetime ever closer to the horizon. Theoretical efforts will focus on non-universal features of emission—such as polarization anomalies in echoes or spectral signatures from quantum effects near the horizon. Yet the fundamental limitation will persist: we can never fully rule out the existence of an ultracompact object.
This issue directly impacts black hole astrophysics, cosmology, and quantum gravity. It also forces a reinterpretation of data in the context of dark matter, where primordial black holes are considered as candidates, and in accretion disk physics.
Research is needed to identify specific signatures incompatible with a horizon, such as modifications to the Hawking spectrum from quantum geometry fluctuations, and to develop statistical methods for model discrimination.
Connected to unresolved issues in black hole thermodynamics and the information paradox: if the horizon is not an absolute boundary, evaporation and information preservation can be described without dramatic scenarios like firewalls. It also resonates with the search for quantum gravity.
🎯 If we were to watch an object falling into a black hole, gravitational redshift and time dilation would make its image redden and freeze at the horizon, never disappearing for an external observer.
🎬 In science fiction, the event horizon is often crossed: in Dan Simmons' novel 'Hyperion,' black holes serve as gateways, and in the movie 'Interstellar,' characters traverse a wormhole supposedly formed from a black hole with a 'soft' singularity. Both exploit the hope that the horizon is not the end of the road.