An international team, spearheaded by Prof. Luciano Rezzolla at the Goethe-Universität, has invented a novel approach to investigate whether black holes conform to Einstein's theory of relativity or to other, more unconventional theories. The study was published in the journal Nature Astronomy.
At the current resolution of telescopes, black holes predicted by different theories of gravity still look very similar. Future telescopes will make the differences more visible, making it possible to distinguish Einstein’s black holes from others. Image Credit: ©L. Rezzolla/Goethe-Universität.
The images of black holes are not merely captivating visuals; they may act as a "testing ground" for alternative gravitational theories in the future.
The researchers performed highly intricate simulations and established measurable criteria that can be evaluated with future, even more advanced telescopes. In the coming years, this method could determine whether Einstein's theories remain valid even in the most extreme areas of the universe.
Black holes are often described as cosmic gluttons, so powerful that not even light can escape their pull. That’s precisely why the images captured by the Event Horizon Telescope (EHT) of the black holes at the centers of the Milky Way and the M87 galaxy marked such a major breakthrough when they were released a few years ago.
What you see on these images is not the black hole itself, but rather the hot matter in its immediate vicinity. As long as the matter is still rotating outside the event horizon, before being inevitably pulled in, it can emit final signals of light that we can, in principle, detect.
Luciano Rezzolla, Professor, Goethe University Frankfurt
The images fundamentally depict the shadow cast by the black hole. This discovery now provides the chance to thoroughly investigate the theories surrounding these extraordinary cosmic entities. Einstein’s general theory of relativity remains the cornerstone of modern physics when it comes to describing space and time. It predicts the existence of black holes as specific solutions to its equations, along with their defining characteristics, most notably the event horizon, a boundary beyond which nothing, not even light, can escape.
There are, however, also other, still hypothetical theories that likewise predict the existence of black holes. Some of these approaches require the presence of matter with very specific properties or even the violation of the physical laws we currently know.
Luciano Rezzolla, Professor, Goethe University Frankfurt
In collaboration with colleagues from the Tsung-Dao Lee Institute in Shanghai, China, the physicist based in Frankfurt has presented a novel approach to evaluate these alternative theories. There has been a lack of substantial data to either disprove or validate these theories, a situation that the researchers aim to rectify in the future by utilizing shadow images of supermassive black holes.
“This requires two things. On the one hand, high-resolution shadow images of black holes to determine their radius as accurately as possible, and on the other hand, a theoretical description of how strongly the various approaches deviate from Einstein’s theory of relativity,” explained Rezzolla.
The researchers have now provided a detailed account of how various kinds of theoretical black holes differ from the principles of relativity and how this divergence is manifested in the shadow images. To explore this phenomenon, the team performed intricate three-dimensional computer simulations that emulate the interactions of matter and magnetic fields within the warped spacetime around black holes. Utilizing these simulations, the scientists subsequently created synthetic images of the luminous plasma.
The central question was: How significantly do images of black holes differ across various theories?
Akhil Uniyal, Study Lead Author, Tsung-Dao Lee Institute
The physicists were able to establish distinct criteria that, with forthcoming high-resolution measurements, could frequently enable a decision to be made in support of a particular theory. Although the discrepancies in images remain too minor with the present resolution of the EHT, they consistently grow with enhanced resolution. The physicists formulated a universal characterization of black holes that encompasses a variety of theoretical perspectives.
“One of the EHT collaboration’s most important contributions to astrophysics is turning black holes into testable objects. Our expectation is that relativity theory will continue to prove itself, just as it has time and again up to now,” emphasized Rezzolla.
The findings are consistent with Einstein's theory. The level of measurement uncertainty remains significantly high, allowing only a limited number of highly unusual scenarios to be excluded. For example, the two black holes located at the center of M87 and the Milky Way are improbable candidates for what are termed naked singularities (which lack an event horizon) or wormholes, merely two among the numerous other theoretical options that require investigation.
“Even the established theory must be continuously tested, especially with extreme objects like black holes,” added the physicist.
Proving Einstein’s theory is invalid would represent a profound shift in physics.
The Event Horizon Telescope (EHT) provides exceptional opportunities for such measurements. This collaboration of multiple large radio telescopes around the world achieves a resolution comparable to a telescope the size of Earth, for the first time allowing a clear view into the immediate vicinity of black holes. In the future, there are plans to incorporate additional telescopes on Earth into the EHT.
Scientists are also optimistic about the possibility of a radio telescope in space, which would greatly enhance the overall resolution. With such high-resolution observations, researchers could begin to rigorously test competing theories about black holes. This level of detail requires angular resolutions finer than one millionth of an arcsecond, roughly equivalent to spotting a coin on the Moon from Earth. Although this surpasses current capabilities, it is anticipated to be attainable within a few years.
Journal Reference:
Uniyal, A., et al. (2025) The future ability to test theories of gravity with black-hole shadows. Nature Astronomy. DOI: 10.1038/s41550-025-02695-4. https://www.nature.com/articles/s41550-025-02695-4.