Enrique Garcia works on a supercomputer with an oxygen tank on his back. The machine he looks after sits 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 air around it is so thin that its cooling fans must push twice the normal volume of air to stop the processors overheating. The people who maintain it need help breathing too.
Nobody builds at that altitude for comfort. The Chajnantor plateau was chosen because it offered something rare: a large, very flat, very dry plain high in the Chilean Andes, where some areas record as little as 3 millimetres of rain in an average year and there is often almost no water vapour overhead. The price of that dryness is an altitude of about 5,000 metres, where oxygen masks are often needed and where there was little experience of managing a large, technically complex building project. The surrounding Atacama is the driest non-polar desert on Earth, with around 15 millimetres of rain a year and some weather stations that have never recorded rain at all.
For astronomers working at millimetre wavelengths, water vapour is the enemy, because it is the main gas that makes the atmosphere opaque at those wavelengths. Sitting above a large part of the atmosphere, on the edge of the driest desert on the planet, ALMA looks through air that very often contains almost no water.
What was built there is probably the most ambitious ground-based astronomical project ever attempted: a collaboration of 20 countries, spread over 16 kilometres, a decade in construction and more than a billion dollars in cost. It has met or exceeded almost every scientific expectation. One thing that did not match the predictions was the climate, since a site 5,000 metres up also brings the high winds and heavy snowfall that come with altitude.
The altitude shapes everything about how the observatory is staffed. Iván López, ALMA’s safety manager, has described an operation of around 250 people, about 50 of whom are exposed to intermittent hypoxia, the 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 under an hour, and they do it daily, which is nothing like the slow acclimatisation climbers use. The solutions are practical: the technical building is permanently oxygenated by an on-site liquid oxygen plant, backpacks were designed so that a worker can carry an oxygen tank anywhere, and staff are given nasal sprays because the gas is so dry. Drivers are advised to take a co-pilot, staff work in teams of at least two following exact procedures, and everyone works eight days on site followed by six days off at sea level. The machinery suffers too: parts often last half as long as expected. Mining companies face similar problems but publish little, so the observatory has learned as it went.
In 2017 that thin, dry column of air became part of something larger. The Event Horizon Telescope links radio dishes across the planet, synchronising them with atomic clocks and using the Earth’s rotation to form, in effect, a single telescope the size of the planet. The eight telescopes stood on volcanoes in Hawaii and Mexico, on mountains in Arizona and the Spanish Sierra Nevada, in the Chilean Atacama and in Antarctica. Each produced around 350 terabytes of data a day, stored on hard drives that were then flown to specialised supercomputers to be combined. The resulting resolution is enough to read a newspaper in New York from a café in Paris.
ALMA’s contribution was decisive. Adding it as an anchor station raised the sensitivity of the whole network by a factor of ten, which turned the network from an instrument that could detect structure into one that could make 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 around the world, the collaboration released the result: 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 55 million light-years from Earth with a mass 6.5 billion times that of the Sun.
The image does not show the black hole itself. Black holes are black because no light escapes them, so what is visible is the hot gas swirling around the hole. For decades these objects had lived in equations and in computer models of the kind used in films. The plan now is to add new stations on other continents, and eventually satellites in space, for sharper images and perhaps even moving pictures of the plasma.
For those who had spent years predicting what such a ring should look like, the comparison was an uncomfortable moment. “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.