Enrique Garcia tends a supercomputer with an oxygen tank strapped to his back. The machine is housed in the technical building at ALMA’s Array Operations Site, on the Chajnantor plateau in northern Chile, 5,100 metres above sea level. It is the highest-altitude supercomputer in the world, and the building that holds it is said to be the second highest building in the world. The air up there is so thin that the cooling fans must move twice the usual volume of it to stop the processors overheating. The people who look after the machine need help breathing too.
Nobody chooses such a height for the comfort of it. The Chajnantor plateau was selected because it offered something rare: a wide, very flat, very dry plain high in the Chilean Andes, where parts of the area record as little as 3 millimetres of rain in an average year and where there is often almost no water vapour in the sky above. The price of that dryness is an altitude of roughly 5,000 metres, where oxygen masks are often needed and where almost nobody had ever run a large, technically complex construction project before. The Atacama that surrounds the site is the driest non-polar desert on Earth. It receives around 15 millimetres of rain a year on average, and some of its weather stations have never recorded any rain at all.
For astronomers working at millimetre and submillimetre wavelengths, water vapour is the enemy. It is the main gas that makes the atmosphere opaque at those wavelengths, which is why so much work goes into measuring the precipitable water vapour above a possible site before any concrete is poured. Researchers ran systematic comparisons using identical instruments at three places: Maunakea in Hawaii, the South Pole and the area around Cerro Chajnantor. The Chajnantor conditions came out clearly better than those on Maunakea. Perched above a large part of the atmosphere, on the edge of the driest desert on the planet, ALMA looks out through air that very often contains extremely little water.
What was eventually built there is probably the most ambitious ground-based astronomical project ever attempted. Twenty countries took part, the array spreads over 16 kilometres at 5,000 metres of elevation, it took a decade to build and it cost more than a billion dollars. It has met or surpassed almost every scientific goal set for it. The one thing that did not behave as predicted was the weather. The Atacama is normally one of the driest places in the world, but a site 5,000 metres up also attracts the strong winds and heavy snow that come with the altitude.
The height determines how the observatory is staffed. Iván López, ALMA’s safety manager, has described an operation of about 250 people, of whom roughly 50 are exposed to intermittent hypoxia, the medical condition in which the body does not get enough oxygen. Workers travel from the Operations Support Facility at 2,900 metres up to the antennas at 5,100 metres in less than an hour, and they do it every day. That is nothing like the slow acclimatisation that climbers rely on. The remedies are practical and distinctly unglamorous. The technical building is now permanently supplied with oxygen by a liquid oxygen plant on site. Backpacks were designed so that a worker can carry an oxygen tank anywhere. Because the gas is very dry, staff are given nasal sprays to keep their airways moist. Drivers on the road up and down are advised to take a co-pilot. People work in teams of at least two, following precise procedures broken down into small listed tasks. Everyone follows a roster of eight days on site and six days off at sea level. The equipment suffers as well: components that were expected to last a certain number of years often last half that, and the time allowed for any given task has had to be revised. Mining companies face comparable problems, but they have published very little of what they know, so the observatory has had to learn as it went, leaning on staff who had worked in Hawaii and at the nearby APEX telescope.
In 2017 that thin, dry column of air became part of something much larger. The Event Horizon Telescope links radio dishes across the planet using a technique called very long baseline interferometry. Hydrogen maser atomic clocks keep the separate facilities in step, and the rotation of the Earth is used to fill in the gaps, so that the dishes act, in effect, as a single telescope the size of the planet, observing at a wavelength of 1.3 millimetres. The eight telescopes that contributed stood on volcanoes in Hawaii and Mexico, on mountains in Arizona and in the Spanish Sierra Nevada, in the Chilean Atacama and in Antarctica. Each produced about 350 terabytes of data a day. The data were stored on helium-filled hard drives, which were then flown to specialised supercomputers called correlators at the Max Planck Institute for Radio Astronomy and at MIT Haystack Observatory, where the recordings were combined. The resulting angular resolution, 20 micro-arcseconds, is fine enough to read a newspaper in New York from a café in Paris.
ALMA’s part in this was decisive. Adding it as an anchor station made the whole network ten times more sensitive, and that is what turned the Event Horizon Telescope from an instrument that could detect structure into one that could produce an image. “ALMA is the most sensitive facility in the EHT, and its 66 high-precision antennas were critical in making the EHT a success,” said Xavier Barcons, Director General of the European Southern Observatory.
On 10 April 2019, at press conferences held simultaneously around the world, the collaboration published the result in six papers that appeared together in The Astrophysical Journal Letters. The image showed a ring of glowing plasma with a dark patch at its centre: the shadow of the supermassive black hole at the heart of Messier 87, a massive galaxy in the Virgo cluster. The object lies 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun. Its event horizon, the boundary that gives the telescope its name, is about 2.5 times smaller than the shadow it casts and measures just under 40 billion kilometres across.
The picture does not show the black hole itself. Black holes are black because no light escapes from them, so the hole cannot be seen at all. What can be seen is the hot gas circling it, pulled around by gravity close to the horizon. For decades such objects had existed only in theory, in equations and in the kind of computer models used to make films. What the array did was to move the event horizon out of mathematics and into the class of things that can be observed directly. The intention is to carry on: new stations on other continents, and eventually satellites in space, to produce sharper images and perhaps even moving pictures of the hot plasma.
For the scientists who had spent years predicting what such a ring ought to look like, the moment of comparison was an anxious one. “The confrontation of theory with observations is always a dramatic moment for a theorist. It was a relief and a source of pride to realise that the observations matched our predictions so well,” said Luciano Rezzolla of Goethe Universität in Germany.