This is what a black hole looks like

In a huge breakthrough for the field of astronomy, researchers representing the Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — have unveiled the first direct visual evidence of a supermassive black hole and its shadow.

The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution, so that although the telescopes are not physically connected, they are able to synchronise their recorded data with atomic clocks which precisely time their observations. It is the result of years of international collaboration, and offers scientists a new way to study the most extreme objects in the universe — including black holes.

Revealed in a series of six papers published in a special issue of The Astrophysical Journal Letters, the researchers have imaged a black hole 55 million light-years from Earth, at the centre of Messier 87 — a massive galaxy in the nearby Virgo galaxy cluster. It is an extraordinary cosmic object with an enormous mass — 6.5 billion times that of the Sun — but an extremely compact size, warping spacetime and super-heating any material that surrounds it.

Multiple calibration and imaging methods had revealed a ring-like structure with a dark central region — the black hole’s ‘shadow’ — that persisted over multiple independent EHT observations. Heino Falcke, Chair of the EHT Science Council, explained, “If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein’s general relativity that we’ve never seen before.

“This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects and allowed us to measure the enormous mass of M87’s black hole.”

EHT board member Paul TP Ho added, “Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well. This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.”

Thirteen partner institutions worked together to create the EHT, with key funding provided by the US National Science Foundation (NSF), the European Research Council (ERC) and funding agencies in East Asia. Its creation required the upgrading and connecting of a worldwide network of eight pre-existing telescopes deployed at a variety of high-altitude sites, including volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert and Antarctica.

A technique called very-long-baseline interferometry (VLBI) enabled the synchronisation of these telescope facilities around the world and exploited the rotation of our planet to form one huge, Earth-size telescope observing at a wavelength of 1.3 mm, allowing the EHT to achieve an angular resolution of 20 micro-arcseconds — enough to read a newspaper in New York from a cafe in Paris. Petabytes of raw data from the telescopes were meanwhile combined by highly specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.

Australia’s own Dr Adam Deller, an Associate Professor of Astrophysics at Swinburne University of Technology, developed the DiFX software correlator, which the project used for data analysis. He described the breakthrough as “an amazing technical feat”, in which the EHT team “used radio telescopes that are thousands of kilometres apart and lined up the signals they receive to around a millionth of a millionth of a second. When they do, they’re able to make phenomenally sharp images.

“When you think about it, the photons that travelled almost 60 million years to hit the EHT telescopes in 2017 and make this image were extremely lucky — they were born right next door to a monstrous black hole and nearly, nearly fell in. The shadow we see in the image is the absence of photons right next to them that weren’t so lucky; they fell over the black hole’s event horizon and no-one will ever see them again.”

EHT Project Director Sheperd S Doeleman concluded, “We have achieved something presumed to be impossible just a generation ago. Breakthroughs in technology, connections between the world’s best radio observatories and innovative algorithms all came together to open an entirely new window on black holes and the event horizon.”

Image caption: The first image ever taken of a supermassive black hole — M87, at the centre of the Virgo A galaxy. Image credit: Event Horizon Telescope.

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