To photograph a black hole, astronomers turned Earth into one giant telescope.
A black hole's shadow is tiny as seen from Earth, far too small for any single telescope. So astronomers linked radio dishes across the planet, synchronised with atomic clocks, into one virtual instrument almost as wide as Earth. In 2019 they revealed the first image of a black hole.
The Event Horizon Telescope is a collaboration of more than 300 researchers at around 60 institutions. It uses a technique called very-long-baseline interferometry. Radio telescopes thousands of kilometres apart observe the same target at the same time, each recording its signal alongside timestamps from an extremely precise atomic clock. Combining the recordings later lets the network act like a single dish with an aperture the size of the planet, giving enough resolution to see structure on the scale of a black hole's event horizon.
The data volumes are so large that they are not sent over the internet. Hard drives are flown by commercial freight to central sites at MIT's Haystack Observatory and the Max Planck Institute for Radio Astronomy for processing. Data from the South Pole Telescope, recorded in April 2017, could not arrive until December, because no flights leave the South Pole during the southern winter.
On 10 April 2019, in six simultaneous press conferences, the team unveiled the first direct image of a black hole: M87*, at the heart of the giant galaxy Messier 87. It showed a bright ring around a dark central shadow, matching what general relativity predicts for light bent and captured around a spinning black hole. To check the result was reliable, four independent teams reconstructed the image using different methods. The measurements gave the black hole a mass of about 6.5 billion Suns and an event horizon around 40 billion kilometres across.
In May 2022 the collaboration released an image of Sagittarius A*, the black hole at the centre of our own galaxy, 27,000 light-years away and thousands of times smaller than M87*. The team has also published images of both black holes in polarised light. The array keeps adding telescopes and observing at shorter wavelengths to sharpen the view.
Source: Wikipedia — Event Horizon Telescope · Text summarised from Wikipedia (CC BY-SA 4.0)
