The first image of a black hole, unveiled by the Event Horizon Telescope (EHT) collaboration, provided visual evidence confirming that black holes, long predicted by Albert Einstein’s theory of general relativity, are real astronomical objects. The target was the supermassive black hole at the center of the galaxy Messier 87 (M87), a colossal object 6.5 billion times the mass of the Sun. The resulting image—a bright, ring-like structure surrounding a dark center—was the culmination of a decade-long international effort in engineering and computational physics.
The Extreme Challenge of Taking a Picture
Capturing an image of M87 presented an extreme technical challenge due to the immense distance and the black hole’s nature. The galaxy M87 is located approximately 55 million light-years away from Earth. Even with its enormous mass, the black hole’s apparent size in the sky is incredibly small, equivalent to trying to see a donut on the surface of the Moon.
To resolve such a tiny feature, an optical telescope would need a mirror dish roughly the size of the Earth. A black hole is defined by its event horizon, the boundary from which nothing, not even light, can escape, meaning the object itself is perfectly dark. Therefore, the image captured is not the black hole directly, but rather its shadow cast against the intensely glowing matter surrounding it. This shadow is a dark circular region where the black hole’s gravity has captured the light from the surrounding accretion disk.
The Event Horizon Telescope Network
To achieve the necessary resolving power, scientists created the Event Horizon Telescope, a global network of synchronized radio observatories. This array effectively transformed the entire planet into a single, Earth-sized virtual telescope. The collaboration linked eight existing radio observatories located in diverse and geographically distant locations.
These sites included telescopes in Spain, Chile, Antarctica, and Hawaii. The physical separation between these observatories provided the high angular resolution required to see the black hole’s shadow. By coordinating these telescopes, the EHT achieved a resolution comparable to locating an orange on the Moon’s surface. The infrastructure was designed to collect radio waves at a specific wavelength of 1.3 millimeters.
The Technique of Very Long Baseline Interferometry
The global network of dishes was united into a single instrument using a technique called Very Long Baseline Interferometry, or VLBI. VLBI works by simultaneously recording the radio waves from the black hole at each telescope site. The technique relies on the precise timing of the signal’s arrival at each dish to measure the difference in the wave fronts.
To achieve this extreme accuracy, each observatory was outfitted with highly stable atomic clocks, which time-stamp the recorded data. These clocks are stable enough to keep time to a precision of a few attoseconds, ensuring the recordings from thousands of kilometers apart are synchronized. The time-stamped signals from pairs of telescopes are later correlated to measure interference patterns. This process provides data points in a mathematical domain that represents the object’s structure, rather than an immediate visual picture.
Data Processing and Image Reconstruction
The simultaneous observations resulted in an enormous amount of raw data, totaling petabytes. This volume was too large to transmit electronically, necessitating the physical transportation of hard drives from the global sites to two central correlation facilities. Specialized supercomputers at these facilities performed the “data correlation,” meticulously aligning the time-stamped signals from all the telescopes.
The raw, correlated data provided a sparse set of measurements describing the black hole’s appearance. Sophisticated algorithms were then required to computationally fill in the missing information and reconstruct a coherent image. Teams used novel imaging methods to convert the abstract interference measurements into the final recognizable ring-and-shadow structure.