Photographing the invisble
Depicting a black hole? Many astrophysicists considered it impossible, until 2019, when a team of the Event Horizon Telescope showed the first picture of a black hole. This is the story about how a creative idea resulted in an earth-sized radio telescope.
Einstein’s General Theory of Relativity was only one month old when Karl Schwarzschild used it to calculate the simplest star possible. It had a surprising point of no return: compress a star as far as it can go and its gravity becomes all-devouring, according to Schwarzschild’s formula. Even light cannot escape from the event horizon.
A mathematical curiosity, according to physicists. A 1.4 million kilometres wide sun would have to be compressed to a ball with a diameter of 6 kilometres. Which is possible, if a very heavy star burns out and collapses under its own weight. Astrophysicists have discovered all sorts of possible black holes, like a dancing double star with an invisible small partner, or the incredibly bright radio sources in the centre of some galaxies. That much energy can only be generated when matter is devoured by a black hole.
In the nineties, Heino Falcke, Professor in Astrophysics, was already fascinated by black holes during his PhD: “It was during my PhD that I understood that we do not fully comprehend gravity yet. In reality, its workings and how this ties in with the Standard Model of particle physics is a big mystery.” Black holes are the most extreme case of gravity imaginable. “Theories can only be truly tested with black holes.”
What do black holes look like? In the previous century, theorists like David Hilbert, Max von Laue and James Bardeen calculated the way in which the enormous gravity of a black hole transforms the light beams of a near star or of the small accretion disk of matter that orbits it. Because of this lens effect, it is possible to look directly behind the black hole.
The calculations are difficult, but seeing and photographing a black hole is even more difficult. Black holes are too small and too far away for a regular telescope. The closest collapsed star is at a thousand lightyears – thousand billion kilometres - distance from Earth and only a couple of kilometres wide.
Shadow of a black hole
Even for the Hubble telescope, blackholes are impossibly small. However, they can give themselves away. As a young PhD student in 1993, Heino Falcke calculated the amount of radio emission that a black hole generates when it devours interstellar dust and gas, during which a small amount of the particles manages to escape in a so-called jet. According to his formulas, you are looking at piping hot gas that skims the black hole in the right wavelength. Because of that, you shine light on the black hole and you can show the darkness.
The radio image becomes extra interesting because of the enormous gravity: it bundles the radio waves like a lens. Instead of a scattered cloud or a jet stream, you should be able to see a clear ring around a dark area. Falcke realised that this is the direct image of the edge of a black hole: in two publications from 2000 he and two colleagues named this effect the shadow of a black hole. It is five times as large as the black hole itself, and should be observable with a radio telescope. As long as the telescope is large enough.
Network of radio telescopes
Astrophysicist Reinhard Genzel and his team, and later astrophysicist Andrea Ghez (winners of the Nobel Prize in Physics 2020), had discovered stars, which were orbiting the radio source Sagittarius A* in the centre of the Milky Way with a great velocity. This showed that the radio source had to be very heavy: probably a super heavy black hole. 4 million times as heavy as the Sun and 24.5 kilometres wide; replace the Sun with Sagittarius A* and its shadow would cover half the sky.
Thomas Kirchbaum, a colleague of Falcke, just carried out radio measurements of Sagittarius A*. He was able to measure the radio signal of the super heavy black hole because of the VLBI technique (Very Long Baseline Interferometry). It allowed him to connect radio telescopes that are thousands of kilometres apart. The result were measurements that are just as precise as ones made with a radio telescope with the size of the in-between distance. However, the black hole proved to be too small to see details.
Nevertheless, Falcke concluded that although Sagittarius A* is difficult to photograph, it is not impossible. He launched a campaign to convince his colleagues. Based on newer observations, the black hole in the centre of our Milky Way appeared to be heaver, and thus larger, than was previously thought. Falcke pointed out that the black hole enlarges its own image due to the shadow effect, and it must be measurable on a radio wavelength of millimetres. But only if Falcke’s astrophysical models are correct.
Falcke admits that he “was not sure that we could really make a photograph of a black hole when I suggested it in 1995. I was also convincing myself. The story had to become stronger before I could convince other people.”
The best visible black hole in the sky is the enormous one in the centre of the Milky Way, Sagittarius A*, named after its constellation. Together with his colleagues, Falcke calculated the ideal wavelength to look at this monster of 4 million sun masses. At 1 millimetre, radio waves that are close to the event horizon become visible. VLBI required large updates to operate at that wavelength – and thus required funding. “We had to show that our method was worth the risk to receive that funding.”
Central to Falcke’s prediction was that lower radio wavelengths allow you to look closer to a black hole, up to the event horizon. That is exactly what observers later see when they look at Sagittarius A* with a small VLBI network. In 2014, Falcke showed that this prediction is correct, together with his colleague Geoff Bower. In 2008, Shep Doeleman of the American MIT confirmed earlier measurements of Thomas Kirchbaum, which show that there were indeed 1 millimetres radio waves coming from the direct surroundings of the black hole.
With this proof in their pocket, Falcke and his colleagues Rezzolla and Kramer received an ERC Synergy Grant, a prestigious European research fund of 14 million euros.
In 2013, Falcke started a collaboration with Doeleman and together with 13 parties all over the world, they form EHT: Event Horizon Telescope. A collaboration of 150 astrophysicists and 8 telescopes in Europe, North and South America, and on the South Pole. Falcke is the chairman of the science council of EHT, Doeleman became the director. Years of work on the theoretical models and measuring instruments followed. Examples are modern data storage for the enormous load of measuring data, radio detectors for the desired wave length of 1 millimetres, atom clocks in order to tune the telescopes perfectly to each other, and digitisers that convert analogue radio measurements to computer data. EHT uses, a.o., equipment made for SETI, the search for extra-terrestrial life.
EHT research group in Nijmegen, november 2018
The EHT network was ready to start measuring in 2017. In the mean time, the focus had been on not just the black hole in the centre of the Milky Way. The team also researched the black hole in galaxy M87. Although it is a thousand times further away, it is also a thousand times heavier and larger than Sagittarius A*. The exact mass has not been defined, but it proved to be a golden find. In our own Milky Way, large interstellar dust clouds obstruct the view, but M87 provides a crystal clear image. After two years of analysing in secret, EHT went public. On 10 April 2019, Heino Falcke and Shep Doeleman, in Brussels and Washington respectively, presented the first image of a black hole.
“All astrophysicists fantasise about what it would be like to see a real black hole”, Falcke says, “but many accept that it will not happen. They are so small and so far away. Twenty years ago, not every astrophysicist was convinced that black holes really existed. And now we can really see them. It has been a whirlwind!”