Direct images of black hole taken for the first time
Results show no deviation from theory of general relativity
On Wednesday, the Astrophysical Journal Letters published a special issue featuring four images that have a lot in common: they show a dark splotch surrounded asymmetrically by a bright orange ring, all against a black background. These are the first images of a black hole ever taken. Specifically, the pictures show a supermassive black hole in the center of Messier 87 (M87), a galaxy in the Virgo cluster. M87 is about 55 million light years away from Earth.
The images were the result of a years-long effort led by astronomer Shep Doeleman at the Harvard-Smithsonian Center for Astrophysics. The project involved an international team of over 200 astronomers, including members of the MIT community working at the MIT Haystack Observatory in Westford, Massachusetts.
Black holes are astronomical objects that form when the density of mass in a given place is high enough to exhibit a gravitational field so strong that nothing, not even light, can escape beyond a certain distance. This distance is known as the “event horizon.”
Because nothing escapes from a black hole, it was doubted that astronomers would ever be able to observe black holes except by indirect methods. The pictures published on Wednesday directly contradict this expectation.
To make this direct measurement, astronomers took advantage of the extreme gravity that the black hole exhibits. This gravity causes a dense gas to form outside the event horizon as matter falls in, similar to water going down a drain. The density of the gas and resulting pressure make the matter extremely hot, causing it to spew out radiation.
Using a network of eight telescopes scattered across the globe, collectively known as the Event Horizon Telescope, the researchers were able to detect this radiation and use it to form the images. This was no small feat, as viewing the black hole with the resolution in these photos required about the equivalent magnification as would be needed to read a newspaper in Tokyo from San Francisco. To magnify something by this amount, one would need a radio receiver the size of the Earth itself. The collaboration got around this requirement by synchronizing the eight telescopes so that they effectively act as one big telescope.
These images show no deviation from theoretical predictions, providing another piece of evidence to support the theory of general relativity, first developed by Albert Einstein in 1916. In the theory of general relativity, gravity is a result of the curvature and interdependence of space and time, collectively known as “spacetime.” The curvature is caused by interactions involving mass and energy. Objects can freely travel through this bent spacetime on locally straight lines, which may appear curved to an observer if one does not otherwise account for the curvature of spacetime.
The images demonstrate a proof-of-principle that one can directly image black holes by observing light. What’s next? According to their publications, the researchers are interested in improving the resolution of the images and studying them to develop a deeper understanding of black holes.