Photographing the Unseeable: How We Imaged a Black Hole

On April 10, 2019, the Event Horizon Telescope collaboration released the first image of a black hole — or more precisely, of the shadow of a black hole against its glowing accretion disk. The target: M87*, the supermassive black hole at the center of the Messier 87 galaxy, 55 million light-years away and 6.5 billion times the mass of our Sun.

The Instrument That Made It Possible

No single telescope on Earth is large enough to resolve the event horizon of any known black hole. The angular resolution required — about 20 microarcseconds for M87* — would demand a telescope the size of Earth itself. So the EHT collaboration built one, conceptually: they synchronized radio dishes at eight observatories across six continents (including Antarctica), then used a technique called Very Long Baseline Interferometry to combine their signals as if they were a single Earth-sized dish.

The data collected in April 2017 across five nights of observation filled half a ton of hard drives, which were physically flown to processing centers in the United States and Germany. Atomic clocks at each site kept time to nanosecond precision. The correlation algorithms that combined the signals took two years to process.

What the Image Actually Shows

The iconic orange ring is synchrotron radiation from plasma in the accretion disk — superheated gas spiraling into the black hole at relativistic speeds, emitting radio waves as it goes. The dark central region is the black hole's shadow: not the event horizon itself (which is too small to resolve even with EHT) but the region from which no light can escape, surrounded by the bright photon ring where light orbits the black hole before falling in or escaping.

The image matched the predictions of general relativity almost exactly. The shadow's size, shape, and the brightness asymmetry of the ring (brighter on one side due to relativistic beaming as the plasma moves toward us) were all consistent with Einstein's equations applied to a black hole of the measured mass.

Sagittarius A* — Our Own Black Hole

In May 2022, the EHT released a second image: Sagittarius A*, the supermassive black hole at the center of our own Milky Way, 27,000 light-years away and 4 million solar masses. Imaging Sgr A* was technically harder than M87* — the gas around it varies on timescales of minutes rather than years, requiring the team to essentially average thousands of snapshots to produce a stable image. The result confirmed that our galaxy's central black hole looks exactly as general relativity predicts.