How the EHT Built a Telescope the Size of Earth Across 6 Continents to Photograph a Black Hole

The Event Horizon Telescope has an angular resolution of about 20 microarcseconds. To put that in terms a human eye can feel: if your vision worked at that resolution, you could read the date stamped on a coin sitting on the surface of the Moon. No single dish on Earth comes close to this. The largest steerable radio telescope on the planet, the 100-metre Green Bank Telescope in West Virginia, has a resolution thousands of times coarser. The EHT achieved what it did without building anything larger than what already existed. It used the Earth itself as the aperture.

The technique is called interferometry, and the principle is both elegant and demanding. Two radio telescopes separated by a large distance, observing the same source at the same frequency, will each receive the same incoming wavefront, but at slightly different times, because the wavefront reaches one dish before the other. By recording both signals with atomic-clock precision and then combining them mathematically, astronomers can extract interference patterns that carry information equivalent to what a single dish the size of the separation between them would have collected. The longer the baseline, the finer the resolution. The EHT's baseline is the diameter of the Earth: roughly 12,700 kilometres. That is the aperture. The collecting area is still the sum of the individual dishes, but the resolving power belongs to the planet.

This is Very Long Baseline Interferometry, or VLBI, and radio astronomers have used it for decades on shorter baselines. What the EHT did was push it to its physical limit, the largest baseline available on a solid surface, and apply it to the hardest target in observational astronomy.

Eight observatories participated in the April 2017 observations that produced the first EHT image. They were spread across North America, South America, Europe, Africa, Antarctica, and Hawaii, effectively every continent except Australia. The sites were chosen partly for their existing equipment and partly for their elevation and atmospheric dryness, because water vapour absorbs millimetre-wavelength radio waves and ruins the signal. The Atacama Large Millimeter Array in Chile sits at 5,058 metres above sea level. The South Pole Telescope operates at the geographic South Pole, where the atmosphere above is among the driest on Earth. The Submillimeter Telescope on Mount Graham in Arizona, the IRAM 30-metre telescope in Spain's Sierra Nevada, the James Clerk Maxwell Telescope and Submillimeter Array in Hawaii, the Large Millimeter Telescope in Mexico, and the Submillimeter Observatory in Hawaii rounded out the array.

Each station observed at a wavelength of 1.3 millimetres, the millimetre-wave radio band where the galactic centre and the core of M87 are relatively transparent. All stations had to observe simultaneously, which meant coordinating across time zones, weather windows, and equipment schedules on three continents in a single week. The window opened in April 2017. Five of the eight sites had usable weather. That was enough.

Interferometry requires that the signals from every station be compared. To compare them, you must first store them. Each EHT station recorded data onto high-capacity hard drives at a rate of 64 gigabits per second, the equivalent of filling a laptop hard drive roughly every two seconds. Over the observation run, the array accumulated approximately five petabytes of raw data. Five petabytes is five million gigabytes. No internet connection available at the South Pole or the Atacama plateau could transmit that volume in any reasonable time. The drives were physically flown to correlation centres at MIT Haystack Observatory in Massachusetts and the Max Planck Institute for Radio Astronomy in Bonn. The South Pole data had to wait for the Antarctic winter to end before a flight could carry it out. The correlation process, aligning the signals from every station to nanosecond precision using the timestamps from hydrogen maser atomic clocks, took months.

On 10 April 2019, the EHT collaboration released the first direct image of a black hole's shadow. The target was the supermassive black hole at the centre of Messier 87, a giant elliptical galaxy in the Virgo cluster, 55 million light-years from Earth. The black hole has a mass of approximately 6.5 billion times that of the Sun. Its shadow, the dark region where light cannot escape, spans about 40 microarcseconds on the sky. The image showed a bright ring of superheated plasma surrounding a dark centre, exactly consistent with the predictions of general relativity. The asymmetry in the ring's brightness, brighter in the south, matched models of a rotating black hole with a jet pointed away from Earth.

In 2022, the EHT released a second image: Sagittarius A*, the black hole at the centre of our own Milky Way, 26,000 light-years away and about 4 million solar masses. Imaging it was technically harder than M87 because material orbits it much faster, the plasma completes an orbit in minutes rather than weeks, so the source was flickering on timescales shorter than a single observation. The team had to develop new imaging methods to handle a source that was changing while they watched it.

The next generation Event Horizon Telescope, known as the ngEHT, is designed to add more stations to the array and observe at shorter wavelengths, which will improve both resolution and imaging speed. Candidate sites include locations in Africa, Asia, and the Arctic. More stations mean more baselines, and more baselines mean the imaging algorithm has more constraints to work with, producing sharper and more reliable reconstructions. There is also serious scientific discussion about extending baselines into space, a radio telescope in Earth orbit would give a baseline longer than the planet's diameter and push resolution below 10 microarcseconds. A space-ground VLBI mission called the Event Horizon Imager has been studied by the European Space Agency.

The imaging produced by the EHT is not a photograph in the conventional sense. No single pixel in the image corresponds to a single photon arriving at a single detector. The image is a mathematical reconstruction, a best-fit solution to an underdetermined problem, the data constrain what the source could look like, and the algorithm finds the simplest image consistent with all the measurements. The EHT team used three independent imaging pipelines and compared their outputs. All three converged on the same ring. That convergence, across independent methods and independent teams, is what makes the result trustworthy.

The geometry of general relativity predicted the shadow's size and shape decades before any telescope could test it. The EHT's measurement of M87*'s shadow diameter matched the prediction to within 10 percent. A camera the size of Earth, assembled from dishes that had never been designed to work together, confirmed the most extreme prediction of the most tested theory in physics, using hard drives flown on cargo planes and atomic clocks ticking in the Atacama desert at five kilometres above the sea.