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 centre 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, travelling 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 centre 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.

Source: Institute for Advanced Study

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