{"id":12212,"date":"2022-07-04T18:00:14","date_gmt":"2022-07-04T17:00:14","guid":{"rendered":"https:\/\/www.einstein-online.info\/?post_type=spotlight&p=12212"},"modified":"2026-02-19T16:25:04","modified_gmt":"2026-02-19T15:25:04","slug":"eht","status":"publish","type":"spotlight","link":"https:\/\/www.einstein-online.info\/en\/spotlight\/eht\/","title":{"rendered":"How the Event Horizon Telescope observes black holes"},"content":{"rendered":"\n

Nobel laureates Reinhard Genzel<\/a> (Max Planck Institute for Extraterrestrial Physics in Garching, Germany) and Andrea Ghez (University of California in Los Angeles, USA) had already demonstrated through stellar motions<\/a> that a black hole<\/a> resides at the center of the Milky Way<\/a>. Since May 2022, direct, visual proof of its existence has been available as well, as researchers revealed the first image of the supermassive black hole at the center of our galaxy.<\/p>\n\n\n

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\"Image
This image of Sagittarius A*, the black hole at the center of the Milky Way, made headlines around the world in May 2022. \u00a9 EHT Collaboration<\/figcaption><\/figure>\n<\/div>\n\n\n

Image of an invisible object<\/h2>\n\n\n\n

The black hole cannot be imaged directly, because it does not emit light. However, the image shows glowing gas orbiting the black hole at high speed \u2013 the so-called accretion disk<\/a>. This glowing gas emits radio waves<\/a> that can be observed with ground-based telescopes. The dark region in the center is not, as one might think, the event horizon of the black hole, but its \u201cshadow\u201d: Light is deflected by the gravitational force<\/a> of the black hole. The smallest possible distance at which it can travel around the black hole without falling in determines the extent of the dark central region in this image.<\/p>\n\n\n

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\"Visualisation
Visualization of a black hole.\u00a9 NASA’s Goddard Space Flight Center\/Jeremy Schnittman<\/figcaption><\/figure>\n<\/div>\n\n\n

The Event Horizon Telescope<\/h2>\n\n\n\n

This image was enabled by Very Long Baseline Interferometry (VLBI)<\/a>. With this technique, radio telescopes around the world are combined into a single virtual telescope \u2013 the Event Horizon Telescope (EHT)<\/a>. This was necessary, because there are two ways to achieve the highest possible resolution: short wavelength<\/a> or large telescope size. With a wavelength of 1.3 millimeters, the EHT is already very close to the limit of what is technically possible. The physical size of the construction is also limited, since above a certain diameter it could no longer support its own weight. Thus, the only option to improve the resolution even further is to create a virtual telescope. This is achieved when several telescopes observe the same astronomical source at the same time. Radiation<\/a> from the source reaches each telescope at slightly different times, which are precisely measured. A supercomputer combines the data streams from the individual telescopes, creating a virtual telescope \u2013 in the case of the EHT, one of the size of the Earth. The principle resembles that of optical interferometry, which is used at the VLTI, for example. The EHT thus allows, in principle, to detect an object the size of a ping-pong ball on the Moon.<\/p>\n\n\n\n

In 2017, when the image of Sagittarius A* was taken, the following observatories were part of the EHT:<\/p>\n\n\n\n