The Event Horizon Telescope captured the first image of the Milky Way galaxy's supermassive black hole Sagittarius A* — our galaxy's "black hole heart."
Credit: ESO
Credit: ESO
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TechTranscript
00:00 Imaging a black hole seems like an impossible dream.
00:12 After all, they are black and do not emit light.
00:16 So how can we see them?
00:18 Well, with a telescope big enough, we could at least see the immediate surroundings of
00:23 the largest black holes.
00:26 The supermassive ones that are millions or even billions of times heavier than our sun.
00:32 Then we would be able to unveil some of the mysterious secrets these monsters hide.
00:39 Except that when you do the maths, you find that to observe even the closest supermassive
00:43 black holes, you'd need a telescope the size of the Earth.
00:48 Something beyond our wildest dreams.
00:52 Or maybe not.
00:55 A few years ago, 300 astronomers from nearly 80 institutes across the globe joined forces
01:01 and found a way to create a telescope as large as our planet.
01:05 And they did it without using new mirrors, screws or steel.
01:09 The Event Horizon Telescope, or EHT, is not a real telescope, but a virtual one.
01:25 The stroke of genius of the EHT collaboration was in using powerful radio telescopes that
01:30 already exist, including ALMA and APEX, co-owned by ESO.
01:37 They combined their observations in a way no one had ever attempted before, with a technique
01:42 called Very Long Baseline Interferometry.
01:48 This may sound like sci-fi, but it actually works, as the EHT team showed back in 2019.
01:56 That's when they revealed the supermassive object at the center of the M87 galaxy to
02:01 the world.
02:06 The very first image of a black hole.
02:14 To understand exactly how hard that was, let us drop in some facts.
02:20 First of all, you should know that the EHT telescopes could not see the black hole itself,
02:26 as it is invisible.
02:27 Rather, they picked up the radio signals from the hot glowing gas around it and imaged the
02:33 shadow the black hole casts on it.
02:38 To do this, the telescope antennas in the EHT array had to be pointed to exactly the
02:43 same position in the sky at exactly the same time.
02:47 The EHT can tell if one of these antennas is off by just a millimetre, and if the timing
02:52 is shifted by a trillionth of a second, even though the telescopes are located thousands
02:56 of kilometres apart.
03:01 Imaging the black hole in M87 then required combining the observations of all telescopes
03:06 in the network, using interferometry.
03:11 This technique works best if you have many telescopes, which wasn't the case.
03:18 The team had 8 observatories, though now the network has grown to 11.
03:27 So the EHT researchers had to develop special algorithms to be able to fill in the gaps
03:32 and reconstruct their image.
03:35 It was like staring at a puzzle with most pieces missing, trying to figure out what
03:40 the whole image would look like.
03:43 To determine if the result was scientifically bulletproof, they used a variety of methods.
03:49 Computer simulations to identify errors introduced by their telescope network, different teams
03:54 working in isolation on reconstructing the image in different ways, new techniques and
03:58 software.
03:59 It took years of work until they were sure they had done it right.
04:06 Only then did they show their image to the world.
04:18 The result was like peering at the black hole in M87 with a telescope almost the size of
04:24 the Earth, an instrument so powerful that it could see details as small as a doughnut
04:30 on the Moon.
04:36 So what's next for the EHT?
04:40 The team have already pointed their telescopes to a new target.
04:45 Sagittarius A*, the supermassive black hole at the heart of the Milky Way.
04:51 Our black hole.
04:55 Sagittarius A* is much closer to Earth than the supermassive black hole in M87.
05:02 So you may think that imaging it is a piece of cake by comparison.
05:06 Sorry to disappoint you.
05:07 It's even more difficult.
05:10 First, the centre of the Milky Way is obscured to us by clouds of dust and hot gas that scatter
05:17 the radio signals coming from around the black hole.
05:21 Furthermore, because Sagittarius A* is about 1500 times less massive than its cousin in
05:27 M87, its radio signals change far more rapidly in time.
05:32 Blobs of plasma orbit it in just a few minutes, whereas those in M87 orbit the black hole
05:39 every few days.
05:42 This forces astronomers to adapt their algorithms and to develop new techniques to get stable
05:46 images.
05:47 A bit like trying to read the brand on a basketball while spinning it on your finger.
05:56 In the end, the EHT team did manage to overcome all these obstacles.
06:01 So here it is.
06:02 The first image of Sagittarius A*, the black hole at the centre of the Milky Way.
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