Astrophysicists use echoes of light to illuminate black holes

The research, published today in The Astrophysical Journal Letters, introduces a method that could provide direct evidence of photons circling black holes due to an effect known as "gravitational lensing."

Gravitational lensing occurs when light passes near a black hole and its path is bent by the black hole's strong gravitational field. The effect allows the light to take multiple paths from a source to an observer on Earth: some light rays might follow a direct route while others could loop around the black hole once -- or multiple times -- before reaching us. This means that light from the same source can arrive at different times, resulting in an "echo."

"That light circles around black holes, causing echoes, has been theorized for years, but such echoes have not yet been measured," says the study's lead author, George N. Wong, Frank and Peggy Taplin Member in the Institute's School of Natural Sciences and Associate Research Scholar at the Princeton Gravity Initiative at Princeton University. "Our method offers a blueprint for making these measurements, which could potentially revolutionize our understanding of black hole physics."

The technique allows the faint echo signatures to be isolated from the stronger direct light captured by well-known interferometric telescopes, such as the Event Horizon Telescope. Both Wong and one of his co-authors, Lia Medeiros, Visitor in the Institute's School of Natural Sciences and NASA Einstein Fellow at Princeton University, have worked extensively as part of the Event Horizon Telescope Collaboration.

To test their technique, Wong and Medeiros, working alongside James Stone, Professor in the School of Natural Sciences, and Alejandro Cárdenas-Avendaño, Feynman Fellow at Los Alamos National Laboratory and former Associate Research Scholar at Princeton University, ran high-resolution simulations which took tens of thousands of "snapshots" of light traveling around a supermassive black hole akin to that at the center of the M87 galaxy (M87*), which is located around 55 million light-years away from Earth. Using these simulations, the team demonstrated that their method could directly infer the echo delay period in the simulated data. They believe that their technique will be applicable to other black holes, in addition to M87*.

"This method will not only be able to confirm when light orbiting a black hole has been measured, but will also provide a new tool for measuring the black hole's fundamental properties," explains Medeiros.

Understanding these properties is important. "Black holes play a significant role in shaping the evolution of the universe," says Wong. "Even though we often focus on how black holes pull things in, they also eject large amounts of energy into their surroundings. They play a major role in the development of galaxies, affecting how, when, and where stars form, and helping to determine how the structure of the galaxy itself evolves. Knowing the distribution of black hole masses and spins, and how the distribution changes over time, greatly enhances our understanding of the universe."

Measuring the mass or spin of a black hole is tricky. The nature of the accretion disk, namely the rotating structure of hot gas and other matter spiraling inward towards a black hole, can "confuse" the measurement, Wong notes. Light echoes provide an independent measurement of the mass and spin, however, and having multiple measurements allows us to produce an estimate for those parameters "that we can really believe in," states Medeiros.

Detecting light echoes might also enable scientists to better test Albert Einstein's theories of gravity. "Using this technique, we might find things that make us think 'hey, this is weird!'" adds Medeiros. "The analysis of such data could help us to verify whether black holes are indeed consistent with general relativity."

The team's results suggest that it may be possible to detect echoes with a pair of telescopes -- one on Earth and one in space -- working together to perform what can be described as "very long baseline interferometry." Such an interferometric mission need only be "modest," states Wong. Their technique provides a tractable, practical method to gather important, reliable information about black holes.