The first image of a black hole: A three minute guide


This is the first direct image ever taken of a black hole, courtesy of the Event Horizon Telescope. Creating this image required more data than any other scientific experiment in history. So how did they do it? Up until now most measurements of black holes were indirect, like this time lapse of stars at the centre of our galaxy. The stars seem to be orbiting around something… something with a humongous gravitational pull. Scientists predicted that thing was a black hole. Astronomers have also seen massive jets of matter emanating from the centres of galaxies, sometimes much larger than the galaxies themselves. They predict that the only thing powerful enough to create them would be black holes. The gravitational waves detected by LIGO in 2015 were the most direct and compelling evidence for black holes to date. But detecting gravitational waves is sort of like hearing a black hole. Scientists still wanted to see it. But how do you take a picture of a black object against a black background? The answer: back light it. Matter should emit radiation as it heats up while circling the edge of a black hole, the event horizon. With a big enough telescope scientists should be able to use that radiation as a backdrop to see the silhouette of the black hole itself. But it’s tricky. Even though the black hole is gigantic and has galactic scale impacts, it is so far away, to get an image of high enough resolution, scientists calculated they’d need a telescope the size of the Earth, which is obviously impossible. Enter the Event Horizon Telescope, which is not in fact one telescope, but a network of eight observatories spread around the world from Hawaii to Antarctica. Each observatory provides a tiny piece of the puzzle. Take repeated readings as the Earth spins, and you can capture more angles from each telescope. Once you have enough pieces of the puzzle, algorithms can fill in the gaps and build up the whole picture. This process is called interferometry. The observations used to create this image were made over a few nights in 2017. Each night each observatory gathered the same amount of data as the Large Hadron Collider does in a year and it has taken two years to piece it all together. With this image, we can see a black hole for the first time. But it’s much more than images. It provides precise tests of the predictions of general relativity. And what’s more, beyond the event horizon is a place where physics as we know it no longer works. This is the first direct image ever taken of a black hole but coded in its pixels could be the answers to questions about the whole universe.

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