Enrique Garcia works on a supercomputer with an oxygen tank on his back. The machine stands inside a 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 supercomputer in the world, and the air around it is so thin that its cooling fans must push twice the normal amount of air so that the processors do not overheat. The people who look after it need help to breathe too.
Nobody builds at that height for comfort. The Chajnantor plateau was chosen because it is large, very flat and very dry. Some areas there receive only about 3 millimetres of rain in an average year, and there is often almost no water vapour in the air above. The price of this dryness is an altitude of about 5,000 metres, where people often have to wear oxygen masks. The desert around the site, the Atacama, is the driest desert on Earth outside the polar regions, with around 15 millimetres of rain a year. Some weather stations there have never recorded any rain at all.
Dry air matters because water vapour is the main gas that blocks the radio waves that these telescopes study. Sitting above a large part of the atmosphere, ALMA looks through air that usually contains very little water.
What was built there is probably the most ambitious ground telescope project ever: 20 countries working together, a site 16 kilometres wide, ten years of construction and more than a billion dollars. One thing was a surprise, though. Although the Atacama is extremely dry, a place 5,000 metres high also brings strong winds and heavy snow.
The altitude changes everything about daily work. Iván López, ALMA’s safety manager, has described a team of about 250 people, of whom around 50 do not always get enough oxygen. Workers travel from the base at 2,900 metres up to the antennas at 5,100 metres in less than an hour, every day, which is very different from the slow way climbers get used to height. So the technical building now receives oxygen from its own plant, and special backpacks let workers carry an oxygen tank anywhere. Because the gas is very dry, staff use nasal sprays. People always work in teams of at least two, and everyone works eight days on site and then six days off at sea level. The machines suffer too: parts often last half as long as expected.
In 2017 this thin, dry air became part of something bigger. The Event Horizon Telescope connects radio dishes all over the planet so that together they work like one telescope the size of the Earth. The eight telescopes stood in Hawaii, Mexico, Arizona, Spain, Chile and Antarctica. Each one produced about 350 terabytes of data a day, stored on hard drives that were then flown to special supercomputers to be combined.
ALMA’s part was decisive. Adding it made the whole network ten times more sensitive, which turned the project into one that could produce a real 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 the result was shown at press conferences around the world: a ring of glowing gas with a dark centre, the shadow of the huge black hole in the galaxy Messier 87. The object is 55 million light-years away and has a mass 6.5 billion times that of the Sun.
The picture does not show the black hole itself, because no light escapes from it. What we see is the hot gas turning around it.
Luciano Rezzolla, of Goethe Universität in Germany, said that comparing theory with observations is always a dramatic moment for a theorist, and that it was a relief and a source of pride to see that the observations matched their predictions so well.