Researchers have created a cutting-edge technique for identifying light echoes from black holes. This new approach simplifies the process of measuring black holes’ mass and spin and marks a significant advancement, as it functions independently from many previous methods employed by scientists to explore these characteristics.
A group of astrophysicists, spearheaded by researchers from the Institute for Advanced Study, has devised a groundbreaking method for detecting light echoes from black holes. This innovative technique will make the measurement of black hole mass and spin more straightforward and signifies a considerable progress since it does not rely on many traditional approaches used by scientists in the past.
The findings, published today in The Astrophysical Journal Letters, unveil a technique that could potentially offer direct confirmation of photons orbiting black holes due to a phenomenon known as “gravitational lensing.”
Gravitational lensing occurs when light travels close to a black hole and its trajectory is altered by the black hole’s intense gravitational field. This effect allows the light to take various paths from its origin to an observer on Earth. Some light rays might take a direct path, while others may loop around the black hole once or several times before reaching us, causing light from the same source to arrive at different times, thus creating an “echo.”
“The idea that light orbits around black holes, creating echoes, has been proposed for years, but we have not yet observed these echoes,” says the lead author of the study, George N. Wong, who is a member of the Institute’s School of Natural Sciences and an Associate Research Scholar at the Princeton Gravity Initiative at Princeton University. “Our method provides a framework for measuring these echoes, which could significantly alter our understanding of black hole physics.”
This technique enables the identification of faint echo signals against the stronger direct light captured by prominent interferometric telescopes, such as the Event Horizon Telescope. Both Wong and one of his collaborators, Lia Medeiros, who is a Visitor in the Institute’s School of Natural Sciences and a NASA Einstein Fellow at Princeton University, have been heavily involved in the Event Horizon Telescope Collaboration.
To validate their technique, Wong and Medeiros, together with James Stone, a Professor in the School of Natural Sciences, and Alejandro Cárdenas-Avendaño, a Feynman Fellow at Los Alamos National Laboratory and former Associate Research Scholar at Princeton University, conducted high-resolution simulations that captured tens of thousands of “snapshots” of light moving around a supermassive black hole similar to that located in the center of the M87 galaxy (M87*), which is approximately 55 million light-years from Earth. Their simulations demonstrated that their method could effectively infer the delay period of light echoes in the simulated data, and they believe this technique may be applicable to other black holes as well.
“Our method will not only confirm when light circling a black hole has been detected, but it will also serve as a new instrument for assessing the fundamental properties of these black holes,” explains Medeiros.
Grasping these properties is crucial. “Black holes significantly influence the universe’s evolution,” says Wong. “While we often concentrate on how black holes draw in matter, they also expel substantial energy into their environment. They play a pivotal role in galaxy development by influencing the formation of stars, the timing of these events, and the overall evolution of galaxy structure. Understanding the distribution of black hole masses and spins, as well as how this distribution evolves over time, greatly enriches our comprehension of the universe.”
Measuring the mass or spin of a black hole can be challenging. The characteristics of the accretion disk, which is the rotating structure of heated gas and other material spiraling toward a black hole, can complicate these measurements, Wong points out. However, light echoes provide an independent metric for assessing mass and spin, and having several measurements can yield a reliable estimate for these parameters, as stated by Medeiros.
Furthermore, detecting light echoes may enable researchers to more accurately test Albert Einstein’s theories of gravity. “With this approach, we might uncover results that make us think, ‘this is unusual!'” adds Medeiros. “Analyzing such data could help us determine if black holes conform to the predictions of general relativity.”
The team’s findings indicate that it might be feasible to detect echoes using a pair of telescopes—one on the ground and one in space—working together in a method known as “very long baseline interferometry.” This interferometric mission could be relatively “modest,” according to Wong. Their technique offers a practical and effective means to collect critical and trustworthy information about black holes.
