Black Holes: Why Observations Cannot Confirm Their Existence ✛ ✦
Context
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.
Methods
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.
Results
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.
Implications
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.
Future development
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.
Impact
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.
Next steps
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.
Key open problems
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.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper analyzes LIGO-Virgo-KAGRA data, observations of the M87* shadow, and stellar orbits at the Galactic center, comparing predictions for Kerr geometry and ultracompact objects with the proximity parameter ε. It shows that quasinormal modes, the shadow, and other observable characteristics do not allow an unambiguous distinction between an event horizon and the surface of an ultracompact object.
Where its strength is
A systematic comparison of three independent types of observations with models of ultracompact objects reveals a fundamental indistinguishability of the event horizon in finite data, which sets clear bounds for interpreting future observations.
Where it could go next
- It might be interesting to see how refining the black hole mass from the shadow and ring affects the proximity parameter ε for ultracompact objects—this could quantitatively constrain how small the parameter must be for the object to remain indistinguishable from a black hole.based on: Shadow and Ring: How to Weigh a Black Hole More Precisely arXiv:2606.30753
- A related avenue is the white hole route: if it looks similar through a telescope, one could compare white hole predictions with the same LIGO and EHT data, adding another class of horizonless alternatives.based on: A White Hole in the Lens: What the Telescope Will See arXiv:2607.20945
- It would be interesting to test how singularity-free black hole models, which are nearly indistinguishable from classical ones, agree with observed quasinormal modes—this could show how broad the class of black hole mimics is.based on: Black Holes Without Singularities: Almost Indistinguishable arXiv:2607.10713
Worth trying
- Try applying the proximity ε formalism to M87* shadow data, accounting for the refined mass from paper 2606.30753.
- Compare predictions for white holes with LIGO gravitational-wave events, especially during the ringdown stage.
- Assess which parameter values in singularity-free models yield quasinormal modes compatible with observations.
Why this may matter
If the boundary between black holes and their mimics remains indistinguishable in principle, this would force a reconsideration of the ontological status of the event horizon and possibly a reformulation of what we consider an observational discovery in astrophysics.
Nearby works in our archive
- Shadow and Ring: How to Weigh a Black Hole More Preciselysimilarity 0.67
- A White Hole in the Lens: What the Telescope Will Seesimilarity 0.66
- Black Holes Without Singularities: Almost Indistinguishablesimilarity 0.65
- The Gentle Boiling of the Cosmossimilarity 0.65
- A Black Hole's Shadow with a Secret from Other Dimensionssimilarity 0.65
- A Black Hole in a Fur Hatsimilarity 0.65
The closest work in the archive sits at 0.67 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2605.13901v1
General relativity has achieved remarkable experimental and observational success. Critically, recent data from the LIGO-Virgo-KAGRA, Event Horizon Telescope, and GRAVITY collaborations are often credited with \textit{demonstrating} the existence of black holes, but in fact they only provide evidence for objects that should be regarded as black hole candidates. While current data are in striking agreement with the predictions for Kerr black holes, they can only rule out specific alternative models of compact objects rather than provide conclusive proof of black holes. More fundamentally, and independent of whether or not black holes exist, general relativity itself imposes limits on what can be observationally established. Essentially, no observational data is sufficient to confirm the existence of black holes.
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.
Key numbers
- Black hole masses in GW150914: over 25 M☉
- Signal delay between LIGO detectors: ~3 ms
- Closeness parameter ε: can be arbitrarily small
- Hubble time: ~13.8 billion years
- Typical horizon size of a stellar black hole: a few kilometers