Black Hole Shadow Puts General Relativity to the Test

If a picture is worth a thousand words, what might the first horizon-scale image of a black hole tell us? A new paper by researchers from the Event Horizon Telescope (EHT) collaboration, which famously imaged M87’s central black hole, has provided a number of enlightening answers. Based on an analysis of the black hole’s shadow, the team conducted a unique test of general relativity, deepening understanding about the unusual properties of black holes and ruling out many alternatives. This research, published in Physical Review Letters, was led by Dimitrios Psaltis (IAS Member, 2001–03) of the University of Arizona, Lia Medeiros of the Institute for Advanced Study (IAS), and Feryal Özel (IAS Member, 2002–05) and Pierre Christian, both of the University of Arizona, and was co-authored by the EHT collaboration.

The intense gravity of a black hole curves spacetime, acting as a magnifying glass and causing the black hole shadow to appear larger. By measuring this visual distortion, the research team found that the size of the black hole shadow corroborates the predictions of general relativity. A test of gravity at the edge of a supermassive black hole represents a first for physics and offers further proof that Einstein’s theory remains intact even under the most extreme conditions.

“This is really just the beginning. We have now shown that it is possible to use an image of a black hole to test the theory of gravity,” explained Medeiros. “This test will be even more powerful once we image the black hole in the center of our own galaxy and in future EHT observations with additional telescopes that are being added to the array.”

The black hole shadow is unlike the shadows encountered in everyday life. Whereas a physical object casts a shadow by preventing light from passing through it, a black hole can create the effect of a shadow by siphoning light towards itself. While light cannot escape from the interior of a black hole, it is possible—though unlikely—for light to escape from the region surrounding the event horizon, depending on its trajectory. The result is a murky no man’s land just beyond the point of no return, which appears to observers as a shadow.

Gravitational tests have been conducted in a variety of cosmic settings. During the 1919 solar eclipse, the first evidence of general relativity was seen based on the displacement of starlight, traveling along the curvature of spacetime caused by the sun’s gravity. More recently, tests have been conducted to probe gravity outside the solar system and on a cosmological scale. Examples include the detection of gravitational waves at the Laser Interferometer Gravitational-Wave Observatory (LIGO). Gravitational waves propagate through the fabric of spacetime like ripples on a pond given the dynamic nature of spacetime as predicted by general relativity.

The new EHT paper focuses on a previously unexplored parameter space for black hole research. In addition to providing a brand-new test for all alternative formulations of gravity, it also connects the constraints from black hole images to those from other gravitational experiments. The supermassive black hole at the center of M87 studied by the EHT collaboration is 6.5 billion times more massive than the sun. In contrast, gravitational wave detectors monitor stellar mass black holes that range from five to several dozen solar masses. Such diverse perspectives are essential to a more comprehensive understanding of the underlying nature of black holes.

The nearly circular shape of the black hole shadow, as observed, may also lead to a test of the general relativistic no-hair theorem, which states that a black hole is described entirely by its mass, spin, and electrical charge. In other words, two black holes that possess the same mass, spin, and electrical charge would be considered indistinguishable—similar to the identical nature of like subatomic particles. Should geometric irregularities be detected it would potentially indicate the existence of additional black hole properties beyond mass, spin, and electrical charge.

In a separate study, “A Parametric Model for the Shapes of Black Hole Shadows in Non-Kerr Spacetimes,” published in The Astrophysical Journal this year, Medeiros, Psaltis, and Özel explore the size and shape of the black hole shadow by modeling several different spacetimes and theories of gravity. The black hole shadow depends only on the geometry of the surrounding spacetime and not on the astrophysics of the accretion process.

The ongoing work of the EHT collaboration and its members will continue to bring light to both the hidden framework and visible features of black holes.

The published PRL article is available here.

About the Institute
The Institute for Advanced Study is one of the world's foremost centers for theoretical research and intellectual inquiry. Located in Princeton, N.J., the IAS is dedicated to independent study across the sciences and humanities. Founded in 1930 with the motto "Truth and Beauty," the Institute is devoted to advancing the frontiers of knowledge without concern for immediate application. From founding IAS Professor Albert Einstein to the foremost thinkers of today, the IAS enables bold, nonconformist, field-leading research that provides long-term utility and new technologies, leading to innovation and enrichment of society in unexpected ways.

Each year, the Institute welcomes more than 200 of the world's most promising researchers and scholars who are selected and mentored by a permanent Faculty, each of whom are preeminent leaders in their fields. Comprised of four Schools—Historical Studies, Mathematics, Natural Sciences, and Social Science—IAS has produced an astounding record of introducing new understanding and is responsible for undeniable progress across disciplines and generations, from the development of one of the first stored-program computers to the establishment of art history as a discipline in the United States. Among its present and past Faculty and Members are 34 Nobel Laureates, 42 of the 60 Fields Medalists, and 19 of the 22 Abel Prize Laureates, as well as many MacArthur Fellows and Wolf Prize winners.

About the EHT
The international EHT collaboration announced the first-ever image of a black hole at the heart of the radio galaxy Messier 87 (M87) on April 10, 2019, by creating a virtual Earth-sized telescope. The EHT links existing telescopes using novel systems—creating a new instrument with the highest angular resolving power that has yet been achieved. This technique has been used to measure the size of the black-hole shadow in M87 in the center of the Virgo A galaxy. Over the coming years, the international EHT team will mount observing campaigns of increasing resolving power and sensitivity, aiming to bring black holes into focus.