Published in 'The Astrophysical Journal Letters', the study outlines a method leveraging gravitational lensing, a process where light bends around black holes due to their immense gravity. This bending causes light from a single source to follow multiple paths to Earth - some rays taking direct routes, while others loop around the black hole, creating observable "echoes" that arrive at different times.

"Light circling around black holes and producing echoes has been theorized for years, but these echoes have yet to be directly observed," said lead author George N. Wong, Frank and Peggy Taplin Member at the Institute's School of Natural Sciences and Associate Research Scholar at the Princeton Gravity Initiative. "Our method provides a roadmap for capturing these measurements, potentially transforming our understanding of black hole physics."

The technique isolates these faint echo signatures from the stronger direct light using advanced interferometric telescopes, such as the Event Horizon Telescope. Both Wong and co-author Lia Medeiros, a NASA Einstein Fellow at Princeton University, have previously collaborated with the Event Horizon Telescope.

To validate their technique, Wong, Medeiros, and colleagues, including James Stone and Alejandro Cardenas-Avendano, conducted high-resolution simulations. These simulations modeled light around a supermassive black hole similar to M87', the black hole at the center of the M87 galaxy, located 55 million light-years from Earth. Their findings demonstrated that this method could accurately determine the echo delay, suggesting broad applicability to other black holes.

"This method offers a new way to verify light orbiting a black hole while also allowing us to measure the black hole's core properties," Medeiros explained.

Understanding these characteristics is crucial. "Black holes significantly influence the universe's evolution," Wong stated. "They don't just pull matter in - they emit large amounts of energy, impacting galaxy formation, star development, and the overall structure of galaxies. Knowing black hole mass and spin distributions, and how they change over time, deepens our cosmic insight."

Determining a black hole's mass and spin poses challenges, particularly due to the nature of the accretion disk, a swirling mass of hot gas spiraling toward the black hole. According to Wong, light echoes offer an independent metric, enabling scientists to derive reliable mass and spin estimates. Medeiros added, "These independent measurements help us form trustworthy assessments of black hole properties."

The potential to detect light echoes could also enhance tests of Einstein's theories of gravity. "With this method, we may find unusual results that challenge our understanding," said Medeiros. "Such findings would allow us to verify if black holes align with general relativity."

The study suggests that these echoes might be observed through a "very long baseline interferometry" approach, involving telescopes on Earth and in space. Wong noted that the technique could enable a manageable, efficient mission to collect essential black hole data.

Research Report:Measuring Black Hole Light Echoes with Very Long Baseline Interferometry

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