For over a hundred years, Albert Einstein’s general relativity theory has influenced our perception of gravity and the universe’s most extreme entities: black holes. But what if not all black holes are identical? A novel investigation by physicists from Goethe University Frankfurt and the Tsung-Dao Lee Institute in Shanghai indicates that we might soon possess the means to find out.
Their investigation, published in [Nature Astronomy: 10.1038/s41550-025-02695-4](https://doi.org/10.1038/s41550-025-02695-4), utilizes sophisticated computer simulations to forecast how black holes would manifest under varying gravity theories. The objective: to juxtapose these models with real images obtained by the Event Horizon Telescope (EHT), the global network of radio telescopes that captured the inaugural images of supermassive black holes in the galaxies M87 and our Milky Way.
## Observing the Inobservable
Physicist Luciano Rezzolla, a co-author of the paper, articulates the EHT’s accomplishment with striking clarity. The telescope array does not image the black hole itself, he observes, but rather the brilliant, swirling plasma just beyond its point of no return. Those glowing gas streams outline the “shadow” of the black hole, disclosing its gravitational impression against the surrounding luminosity.
> “What you see on these images is not the black hole per se, but rather the heated matter in its nearby surroundings,” remarked Rezzolla. “As long as the matter remains in rotation outside the event horizon, prior to being inevitably drawn in, it can emit final light signals that we can, in theory, detect.”
That shadow, it turns out, is more than a mere cosmic image. It represents a testable prediction of Einstein’s equations. However, according to Rezzolla and lead author Akhil Uniyal, alternative, more speculative theories also foresee black holes, some of which may vary slightly in shape or size, based on how gravity operates in their frameworks.
## When Shadows Convey Physics
The team modeled these discrepancies by employing three-dimensional magnetohydrodynamic frameworks that replicate how matter and light interact near black holes. Their results imply that while the existing EHT images lack sufficient resolution to differentiate between Einstein’s black holes and their exotic counterparts, the next generation of telescopes could significantly alter this scenario.
> “The main question was: How markedly do black hole images vary across different theories?” Uniyal explained. “With forthcoming high-resolution measurements, we could frequently favor a specific theory.”
The research anticipates that as telescope resolution enhances, particularly with the introduction of orbital instruments that extend Earth’s virtual lens, scientists will be capable of spotting image discrepancies as minor as 2 to 5 percent. This, the authors contend, is enough to verify or dismiss entire categories of gravitational theories. The ultimate aim of the EHT is to resolve features smaller than one millionth of an arcsecond, akin to detecting a coin on the Moon from Earth.
Currently, Einstein’s theory remains unchallenged. However, Rezzolla underscores that this is precisely why testing it is crucial: only by attempting to disprove a theory do we determine its validity. In just a few years, we may finally discern whether black holes, those cosmic consumers gobbling light itself, are uniformly fashioned from the same spacetime material.