Event Horizon Telescope

The Event Horizon Telescope is a global network of radio telescopes that uses very-long-baseline interferometry (VLBI) to achieve the highest angular resolution possible from Earth. Its primary goal is to observe the immediate environment of supermassive black holes, particularly the event horizon and the shadow cast by the black hole against surrounding hot gas.¹ The collaboration involves over 300 researchers from 80 institutions across the globe, operating as a single virtual telescope with a resolution equivalent to that of a dish the size of Earth.²

In April 2017, the EHT observed two supermassive black holes: M87* in the center of the Virgo A galaxy and Sagittarius A* (Sgr A*) in the Milky Way. On April 10, 2019, the collaboration released the first-ever image of a black hole, showing the shadow of M87* surrounded by a bright ring of emission. On May 12, 2022, the EHT unveiled the image of Sgr A*, confirming that it is a black hole and providing strong evidence for Einstein's general relativity. These images have enabled tests of gravitational theories and studies of black hole accretion and jet formation.

The EHT employs very-long-baseline interferometry (VLBI), a technique that synchronizes radio telescopes across the globe to act as a single instrument. Each telescope records data at a rate of several gigabits per second, and the data are physically shipped to central processing facilities (at MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy) for correlation.³ The telescopes observe at a wavelength of 1.3 mm (230 GHz), which penetrates the Earth's atmosphere and provides the necessary resolution. The array includes facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA), the South Pole Telescope, and the IRAM 30-meter telescope.

Beyond the headline images, the EHT has produced a wealth of scientific results. For instance, the polarization measurements of M87* revealed the magnetic field structure near the black hole, which is crucial for understanding relativistic jets.⁴ The EHT also observed the quasar 3C 279 and the blazar OJ 287, providing insights into their jet physics. A notable challenge was imaging Sgr A*, which varies on timescales of minutes, requiring sophisticated imaging algorithms. The collaboration has also developed open-source software, such as the eht-imaging library, which is now widely used in the field. Additionally, the EHT has inspired the next-generation project (ngEHT), which aims to add more telescopes and observe at multiple frequencies to produce movies of black hole activity.

The EHT is currently expanding its capabilities. The next-generation EHT (ngEHT) plans to include additional telescopes and observe at 230 GHz and 345 GHz, improving sensitivity and image fidelity. This will enable time-lapse movies of black hole accretion flows and jets, and allow the study of a larger sample of black holes. The collaboration also aims to test general relativity more precisely and explore the photon ring, a thin bright ring predicted by theory.⁵

Very-long-baseline interferometry (VLBI)

A technique that combines signals from multiple radio telescopes to simulate a single telescope with a diameter equal to the maximum separation between them.

Event horizon

The boundary around a black hole beyond which nothing, not even light, can escape.

Black hole shadow

The dark region seen against the bright emission from hot gas near a black hole, caused by the bending of light by gravity.

Supermassive black hole

A black hole with a mass millions to billions of times that of the Sun, typically found at the centers of galaxies.