Scientists are proposing a method to test Einstein's general theory of relativity by analyzing high-resolution images of black hole shadows. Their new simulations suggest that future telescope advancements could reveal if alternative gravity theories hold true. The work, led by researchers from Goethe University Frankfurt and the Tsung-Dao Lee Institute, appears in Nature Astronomy.
Black holes, described as cosmic gluttons that consume everything including light, have been imaged by the Event Horizon Telescope (EHT) collaboration, capturing supermassive black holes in the galaxy M87 and the Milky Way. These images show not the black holes themselves, but the hot matter in their vicinity. "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 contributed to the discovery. As matter rotates outside the event horizon before being pulled in, it emits detectable light signals.
Einstein's general theory of relativity, foundational for over a century, predicts black holes and their event horizons, beyond which nothing escapes. However, other hypothetical theories also predict black holes but may require specific matter properties or violate known physical laws. "There are, however, also other, still hypothetical theories that likewise predict the existence of black holes," Rezzolla notes.
In a study published in Nature Astronomy, Rezzolla and colleagues from the Tsung-Dao Lee Institute in Shanghai outline a framework to test these alternatives using black hole shadow images. This requires high-resolution images to measure shadow radii accurately and theoretical models of deviations from Einstein's predictions. The team used three-dimensional simulations to model matter and magnetic fields around black holes, generating synthetic images of glowing plasma.
"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. They identified patterns that future sharper images could distinguish, though current EHT resolution is insufficient. A universal description of black holes was developed to encompass various frameworks.
So far, observations align with Einstein's theory, ruling out naked singularities and wormholes in M87 and the Milky Way black holes. "One of the EHT collaboration's most important contributions to astrophysics is turning black holes into testable objects," Rezzolla emphasizes. "Even the established theory must be continuously tested, especially with extreme objects like black holes."
The EHT, combining global radio telescopes to act as an Earth-sized instrument, plans expansions including a space telescope for better resolution—potentially under one millionth of an arcsecond—to enable definitive tests.