Black holes are often described as cosmic gluttons, consuming everything that drifts too close — including light itself. This is what makes the images of the supermassive black holes at the centers of the galaxy M87 and our own Milky Way so remarkable. Captured by the Event Horizon Telescope (EHT) collaboration a few years ago, these observations marked a major milestone in astronomy.
“What you see on these images is not the black hole itself, but rather the hot matter in its immediate vicinity,” explains Prof. Luciano Rezzolla of Goethe University Frankfurt, whose team played a crucial role in the discovery. “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.”
Einstein’s Theory and the Mystery of Black Holes
These striking images reveal what scientists call the “shadow” of a black hole, offering a new way to probe the physics behind these mysterious cosmic giants. For over a century, Einstein’s general theory of relativity has been the foundation of our understanding of space and time. It predicts the existence of black holes and 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,” Rezzolla notes. “Some of these approaches require the presence of matter with very specific properties or even the violation of the physical laws we currently know.”
Testing Einstein’s Ideas With Black Hole Shadows
In collaboration with colleagues from the Tsung-Dao Lee Institute in Shanghai (China), Rezzolla and his team proposed a new way to test these alternative theories. Their work, published in Nature Astronomy, outlines how future black hole observations could help confirm or challenge Einstein’s model of gravity. Until now, there has not been enough data to verify or reject competing ideas, but that may soon change through detailed analysis of black hole shadow images.
“This requires two things,” Rezzolla explains. “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.”
Simulations Reveal How Theories Diverge
To tackle this, the team produced a thorough framework describing how different theoretical types of black holes would vary from Einstein’s predictions and how those differences would appear in images. They used advanced three-dimensional computer simulations to reproduce the motion of matter and magnetic fields in the warped spacetime surrounding black holes. From these simulations, they created synthetic images of the glowing plasma that circles these immense objects.
“The central question was: How significantly do images of black holes differ across various theories?” says lead author Akhil Uniyal of the Tsung-Dao Lee Institute. The researchers identified clear patterns that, with sharper images in the future, could help scientists determine which theory best matches reality. Although today’s EHT resolution cannot yet detect these fine distinctions, improvements in technology will gradually make such comparisons possible. To prepare for this, the physicists developed a universal description of black holes that can encompass many different theoretical frameworks.
Einstein’s Theory Still Holding Strong — for Now
“One of the EHT collaboration’s most important contributions to astrophysics is turning black holes into testable objects,” Rezzolla emphasizes. “Our expectation is that relativity theory will continue to prove itself, just as it has time and again up to now.” So far, the findings remain consistent with Einstein’s theory, although uncertainties in measurement mean that only a few exotic ideas have been ruled out. For example, the black holes in M87 and the Milky Way are almost certainly not “naked singularities” (without an event horizon) or wormholes. Still, Rezzolla notes, “Even the established theory must be continuously tested, especially with extreme objects like black holes.” If Einstein’s model were ever shown to fail, it would mark a revolutionary moment in physics.
A New Era of Cosmic Observation
The EHT provides an unprecedented opportunity for these investigations. By combining data from multiple large radio telescopes across the world, it effectively creates a telescope as large as Earth, capable of capturing fine details around black holes. Plans are already underway to add more observatories to the network and, eventually, to include a radio telescope in space, which would greatly boost its resolution.
Such advancements could make it possible to perform truly definitive tests of competing black hole theories. According to the new study, this would require achieving an angular resolution of less than one millionth of an arcsecond — roughly equivalent to spotting a coin on the surface of the Moon from Earth. While that level of precision is not yet possible, scientists expect it to be within reach in the coming years, potentially unlocking a new chapter in our understanding of gravity and the universe itself.
