Event Horizon Telescope

The Event Horizon Telescope (EHT) is a planet-scale array of radio telescopes linked by very long baseline interferometry (VLBI) to function as a single Earth-sized dish. It produced the first direct images of black holes: M87* in 2019 and Sagittarius A* (the black hole at the centre of the Milky Way) in 2022. Both images show the predicted shadow — a dark region surrounded by a bright ring of photons bent around the black hole's extreme gravity.

Very Long Baseline Interferometry

A single radio telescope has limited angular resolution — its ability to distinguish fine detail is determined by its diameter divided by the wavelength of the light it collects. To image a black hole shadow (which subtends roughly 40 microarcseconds at cosmological distances) would require a radio dish thousands of kilometres across.

VLBI achieves this synthetically. Multiple radio telescopes separated by continental distances observe the same source simultaneously. The signals are recorded with atomic clock timestamps and later combined computationally — the timing differences between stations encode the interference pattern of the received waves. The effective resolution is determined by the separation between the most distant telescopes, not by any individual dish.

The EHT operates at 1.3 mm wavelength (230 GHz), chosen because Earth's atmosphere is transparent at this frequency and the black hole shadows are largest relative to surrounding emission. At this wavelength, a baseline spanning Earth's diameter (12,700 km) gives a resolution of ~25 microarcseconds — sufficient to resolve the shadow.

M87* — First Image (April 2019)

The first target was the black hole at the centre of Messier 87, a giant elliptical galaxy 55 million light-years away in the Virgo Cluster. M87* was chosen because:

Its mass (~6.5 billion solar masses) makes its shadow large enough to resolve
The galaxy is at a high galactic latitude — less intervening gas to scatter the radio waves
Its relativistic jet (a 5,000-light-year-long plasma jet) had been studied for decades

The image, released 10 April 2019, shows an asymmetric bright ring with a darker region (the shadow) at the centre. The asymmetry — one side brighter — is caused by Doppler beaming: the side of the accretion disk rotating toward us appears brighter. The shadow diameter matched predictions from general relativity to within measurement uncertainty.

The result was the first direct visual confirmation of the existence of the event horizon — the boundary beyond which nothing, including light, escapes.

Sagittarius A* — Second Image (May 2022)

The second target was Sgr A*, the black hole at the centre of our own Milky Way, 27,000 light-years away. Despite being much closer than M87*, it was harder to image:

Sgr A* is 1,500 times less massive (4 million solar masses), so its shadow is smaller
The galactic plane is full of interstellar gas that scatters radio waves
The accretion material around Sgr A* varies on timescales of minutes — fast enough to blur during the exposure time needed to build up the image

The EHT team developed new imaging techniques to average over the variability and correct for scattering. The resulting image, released 12 May 2022, shows the same ring structure seen in M87* — consistent with general relativity, and confirming the long-suspected compact object at the galactic centre.

For context: Sgr A* is 17 times the diameter of the Sun but contains 4 million solar masses, compressed to a region smaller than the orbit of Mercury.

The Array

The 2017 observing campaign that produced both images used eight telescope sites:

ALMA and APEX (Chile, Atacama plateau)
IRAM 30m (Spain, Sierra Nevada)
James Clerk Maxwell Telescope and Submillimeter Array (Hawaii)
Large Millimeter Telescope (Mexico)
South Pole Telescope (Antarctica)
Submillimeter Telescope (Arizona)

Antarctica was essential: it provides the long baseline needed for north-south resolution and is one of the world's driest sites (dry air is transparent at 1.3 mm; water vapour is not).

The data volume was too large to transmit over the internet — eight petabytes were physically shipped on hard drives from each site to correlation centres in Cambridge, Massachusetts and Bonn, Germany.

Significance

The EHT images confirm general relativity in the strong-field regime — the region immediately outside a black hole, where spacetime curvature is extreme. Every feature of the images (shadow size, ring brightness asymmetry, photon ring structure) matches GR predictions. If GR were wrong in this regime, the images would look different.