The iconic first image of a black hole, released in 2019, might never have happened if not for a stroke of luck, according to one of the astronomers who helped create the image.
Speaking at in London, at Goethe University Frankfurt, Germany, who is on the executive committee of the Event Horizon Telescope (EHT) that created the image, revealed that the researchers didn鈥檛 know if their efforts would pay off until after they had spent years on the project. 鈥淚t would have been an interesting exercise in wasting our time,鈥� he says.
The EHT collaboration used eight radio telescopes on Earth to capture an image of the supermassive black hole at the centre of M87, a galaxy 55 million light years away. It showed the black hole encircled by a ring of hot, glowing matter being pulled towards it.
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鈥淭here were two big question marks which were behind the scenes that kind of kept us awake,鈥� says Rezzolla.
The first concern was that astronomers didn鈥檛 know for certain how large M87鈥檚 supermassive black hole was. Rezzolla and his team knew that their telescopes were powerful enough to see the black hole if it were massive enough, but predictions from theorists varied by a factor of two 鈥� one said it was around 4 billion times the mass of the sun, while another said it was 2 billion times as massive. Fortunately, the larger prediction turned out to be more accurate.
鈥淚n astronomy, a factor of two doesn鈥檛 really worry anyone, but in our case, it would have been a killer because we simply wouldn鈥檛 have enough resolution to see it,鈥� says Rezzolla.
The other potential obstacle was the weather on Earth. Water vapour in clouds is the perfect absorber of the radio waves that the telescopes detected to make the image. 鈥淵ou鈥檙e asking that you have good weather across these eight telescopes at the same time across the planet,鈥� says Rezzolla. 鈥淭hat was another source of concern.鈥�
During the team鈥檚 first imaging campaign in April 2017, there was nearly perfect weather, with no clouds. Such conditions were almost never repeated for the other rounds of EHT observations in 2018, 2020 and 2022. Without those initial observations under clear skies, the data may not have been good enough to make an image, says Rezzolla.
A final, personal concern for Rezzolla was whether he and his colleagues would be able to make sense of the image that was produced, if it did finally work. 鈥淲e knew what to expect, but if our observations revealed something different, then we [would have] had to explain that and that meant really going back to the blackboard and understanding how to explain what we saw,鈥� he says. 鈥淟uckily, that didn鈥檛 turn out to be the case.鈥�
In 2022, the EHT collaboration released a second image, showing the supermassive black hole at the centre of our own galaxy.
We can look forward to sharper, less blurry images of black holes in future, says Rezzolla. Three of the EHT telescopes have already been upgraded to take more frequent observations, which will allow for higher resolution, and a network of space-based satellites could eventually produce black hole images with very fine details, he says.
