In April 2019, the Event Horizon Telescope (EHT) collaboration released the first image of a black hole: the shadow of the supermassive black hole at the center of galaxy M87. In May 2022, EHT followed with an image of the Milky Way’s own central black hole, Sagittarius A* (Sgr A*). These images are not just visual achievements — they are the most precise tests of general relativity in strong-field conditions ever conducted.
## How EHT Works
Imaging a black hole requires extraordinary angular resolution. M87*’s event horizon subtends only about 42 microarcseconds on the sky — roughly equivalent to photographing a golf ball on the Moon.
The EHT achieved this resolution using Very Long Baseline Interferometry (VLBI): synchronizing observations from eight radio telescope sites around the globe (including ALMA in Chile, the South Pole Telescope, and observatories in Hawaii, Spain, and Mexico), effectively creating a virtual telescope with the diameter of Earth. Atomic clocks synchronized the observations; computer correlation analysis combined the data into images.
## M87*: The First Black Hole Image
M87*’s mass is approximately 6.5 billion solar masses. The 2019 EHT image shows a bright ring of emission (hot plasma bent by gravitational lensing) surrounding a central dark region (the black hole shadow). The ring’s size, shape, and asymmetry — the southern side is brighter due to Doppler boosting as plasma orbits at near-light speed — match general relativity predictions with high precision.
In 2023, EHT released a polarized light image of M87* revealing the spiral magnetic field structure near the black hole — the likely energy source powering M87’s relativistic jet, which extends more than 5,000 light-years from the nucleus. See [EHT collaboration](https://eventhorizontelescope.org/).
## Sgr A*: Our Own Black Hole
Sagittarius A*, the Milky Way’s central black hole, has a mass of approximately 4 million solar masses and lies 27,000 light-years away. It is harder to image than M87* despite being closer: the orbital timescale for plasma near Sgr A* is minutes to hours (versus days for M87*), causing rapid image variability during observations. The 2022 EHT image required advanced algorithms to handle this variability while recovering a consistent ring-and-shadow structure matching GR predictions.
Andrea Ghez (UCLA) and Reinhard Genzel (Max Planck) shared the 2020 Nobel Prize in Physics for tracking stellar orbits near Sgr A* over decades, proving it must be a supermassive black hole.
## What Comes Next
The next-generation EHT (ngEHT) will add observing stations and extend to shorter wavelengths, enabling higher resolution and time-resolved movies of black hole dynamics. ESA’s LISA gravitational wave detector, planned for 2034, will detect merging supermassive black holes across the observable universe. On the theoretical side, resolving the black hole information paradox — whether information is truly destroyed when matter falls into a black hole — remains one of the deepest open problems in fundamental physics.
For related reading, see [Gravitational Wave Astronomy](https://sunqi.org/gravitational-wave-astronomy-en/) and current research at [arxiv:astro-ph.HE](https://arxiv.org/list/astro-ph.HE/recent).
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