Washington, D.C. · 10 April 2019 — The Event Horizon Telescope (EHT) collaboration unveiled the first-ever direct image of a black hole, a landmark achievement in astrophysics that transformed a theoretical construct into an observable physical object. The image, revealed in simultaneous press conferences on four continents, shows the shadow of the supermassive black hole at the centre of the galaxy Messier 87 (M87), located approximately 55 million light-years from Earth.
The black hole, which possesses a mass roughly 6.5 billion times that of the Sun, appears as a bright ring of light surrounding a dark central region — the "shadow" cast by the event horizon, the boundary beyond which nothing, not even light, can escape.
A Telescope the Size of Earth
The image was captured using a technique called Very Long Baseline Interferometry (VLBI), which links multiple radio telescopes across the globe to create a virtual telescope with an effective aperture equal to the diameter of Earth. The EHT network comprised eight observatories at six geographic locations during the April 2017 observing campaign:
| Observatory | Location |
|---|---|
| ALMA and APEX | Atacama Desert, Chile |
| IRAM 30m | Pico Veleta, Spain |
| JCMT and SMA | Maunakea, Hawaii |
| LMT | Sierra Negra, Mexico |
| SMT | Mount Graham, Arizona |
| SPT | South Pole, Antarctica |
By observing at a wavelength of 1.3 millimetres, the EHT achieved an angular resolution of approximately 20 microarcseconds — equivalent to resolving the head of a matchstick on the Moon as seen from Earth. This extreme resolution was necessary because the black hole's shadow, despite the object's enormous mass, appears extraordinarily small on the sky due to its vast distance.
What the Image Shows
The bright ring in the image is not the black hole itself but light emitted by superheated gas — plasma — swirling around the black hole at relativistic speeds in the accretion disk. The black hole's intense gravity bends this light around the event horizon, creating the ring-like structure. The dark central region is the shadow of the event horizon, which is approximately 2.5 times larger than the event horizon itself due to gravitational lensing.
The asymmetry of the ring — brighter at the bottom — is consistent with the Doppler boosting effect: the portion of the accretion disk rotating toward the observer appears brighter due to relativistic beaming. The diameter of the ring, approximately 42 microarcseconds, matches the predictions of Einstein's General Theory of Relativity for a black hole of this mass.
Confirming Einstein on a Cosmic Scale
The image represents a landmark confirmation of Albert Einstein's General Theory of Relativity, first published in 1915. The theory predicts that the shadow of a black hole should be approximately 2.5 times the diameter of the event horizon, with a specific size and shape determined by the black hole's mass and spin. The observed shadow of the M87 black hole matches these predictions.
The discovery was published in a series of six papers in a special issue of The Astrophysical Journal Letters on 10 April 2019, detailing the observations, calibration methods, imaging algorithms, and theoretical interpretation.
The Science of Imaging the Unseeable
Producing the image required not only a planet-scale telescope but also years of algorithm development. Because the EHT has a limited number of participating observatories, the data does not provide a complete image — it provides sparse measurements that must be reconstructed using computational algorithms.
The collaboration used multiple independent imaging algorithms to ensure the result was not an artefact of any single method. Four teams, working in isolation from one another, independently reconstructed images from the same data. All four produced the same ring-like structure, providing confidence that the image reflects a real physical feature rather than a computational artefact.
The M87 Black Hole vs. Sagittarius A*
The EHT's primary targets were two supermassive black holes: the one at the centre of M87 and Sagittarius A* (Sgr A*), the black hole at the centre of our own Milky Way galaxy. While Sgr A* is closer to Earth (approximately 26,000 light-years away), it is also far less massive — about 4 million solar masses compared to M87's 6.5 billion — making its shadow appear similar in angular size but its variability on much shorter timescales, complicating the imaging process.
The collaboration chose to release the M87 image first because the longer timescale of variability around the larger black hole made the data more amenable to static imaging. An image of Sgr A* was subsequently released by the EHT in May 2022.
A Global Collaboration
The EHT project involved over 200 researchers from approximately 60 institutions across 20 countries and regions. The collaboration spanned continents, languages, and scientific disciplines, requiring coordinated observations during narrow weather windows at each participating observatory.
Key funding and institutional support came from the National Science Foundation (USA), the European Research Council, the Max Planck Society (Germany), the East Asian Observatory, and numerous national research agencies.
Implications and Next Steps
The EHT image opens a new window on the study of black holes and relativistic physics. Future observations and analysis are expected to address several open questions:
- Black hole spin: The current image does not precisely constrain the spin of the M87 black hole. Additional observations and analysis of the ring's asymmetry could provide spin measurements.
- Magnetic fields: Polarimetric analysis of EHT data, published in 2021, revealed the magnetic field structure near the black hole, providing insights into how jets of plasma are launched from the accretion disk.
- Time variability: Multi-epoch observations will track changes in the accretion flow, testing models of black hole feeding and jet formation.
- Tests of general relativity: More precise measurements of the shadow's size and shape could test for deviations from Einstein's predictions in the strong-gravity regime.
The EHT collaboration continues to expand, with additional observatories joining the network and plans to observe at shorter wavelengths (0.87 mm) to achieve even higher resolution in future campaigns.
Sources
- Event Horizon Telescope Collaboration, "First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole", The Astrophysical Journal Letters, Vol. 875, No. 1, 10 April 2019
- National Science Foundation, "Astronomers Capture First Image of a Black Hole", Press Release, 10 April 2019
- European Southern Observatory, "First Ever Black Hole Image Released", Press Release, 10 April 2019
- NASA, "Black Hole Image Makes History", 10 April 2019
- Event Horizon Telescope,
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