The Event Horizon Telescope (EHT) is a project to create a large telescope array consisting of a global network of radio telescopes and combining data from several very-long-baseline interferometry (VLBI) stations around the Earth. The aim is to observe the immediate environment of the supermassive black hole Sagittarius A* at the center of the Milky Way, as well as the even larger black hole in the center of the supergiant elliptical galaxy Messier 87, with angular resolution comparable to the black hole's event horizon.^[1]

The first image of the black hole inside galaxy Messier 87 was published on April 10, 2019.^[2] The black hole was given the name Pōwehi, meaning "embellished dark source of unending creation" in Hawaiian.^[3]

Overview

The EHT is composed of many radio observatories or radio telescope facilities around the world to produce a high-sensitivity, high-angular-resolution telescope. Through the technique of very-long-baseline interferometry (VLBI), many independent radio antennas separated by hundreds or thousands of miles can be used in concert to create a virtual telescope with an effective diameter of the entire planet.^[4] The effort includes development and deployment of submillimeter dual polarization receivers, highly stable frequency standards to enable very-long-baseline interferometry at 230–450 GHz, higher-bandwidth VLBI backends and recorders, as well as commissioning of new submillimeter VLBI sites.^[5]

Each year since its first data capture in 2006, the EHT array has moved to add more observatories to its global network of radio telescopes. The first image of the Milky Way's supermassive black hole, Sagittarius A*, was expected to be produced in April 2017,^[6][7] but because the South Pole Telescope is closed during winter (April to October), the data shipment delayed the processing to December 2017 when the shipment arrived.^[8]

Data collected on hard drives are transported by airplane (a so-called sneakernet) from the various telescopes to the MIT Haystack Observatory in Massachusetts, USA, and the Max Planck Institute for Radio Astronomy, Bonn, Germany, where the data are cross-correlated and analyzed on a grid computer made from about 800 CPUs all connected through a 40 Gbit/s network.^[9]

Scientific results

The Event Horizon Telescope Collaboration announced its first results in simultaneous press conferences worldwide on April 10, 2019.^[13] The announcement featured the first-ever direct image of a black hole, which showed the supermassive black hole at the center of Messier 87, provisionally designated M87*.^[11] The scientific results were presented in a series of six papers published in The Astrophysical Journal Letters.^[14] Katie Bouman, an American computer scientist who was a graduate when she began work on the project, delivered a TED Talk on using algorithms to put together pictures from data.^[15]

The image provided a test for Albert Einstein's general theory of relativity under extreme conditions.^[4][7] Studies have previously tested general relativity by looking at the motions of stars and gas clouds near the edge of a black hole. However, an image of a black hole brings observations even closer to the event horizon.^[16] Relativity predicts a dark shadow-like region, caused by gravitational bending and capture of light, which matches the observed image. The published paper states: "Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity."^[17] Paul T.P. Ho, EHT Board member, said: "Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter, and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well."^[14]

The image also provided new measurements for the mass and diameter of M87*. EHT measured the black hole's mass to be approximately 6.5 billion solar masses and measured the diameter of its event horizon to be approximately 40 billion km, roughly 2.5 times smaller than the shadow that it casts, seen at the center of the image.^[14][16] From the asymmetry in the ring, EHT inferred that the matter on the brighter south side of the disk is moving towards Earth, the observer. This is based on theory that approaching matter appears brighter because of a relativistic light beaming effect. Previous observations of the black hole's jet showed that the black hole's spin axis is inclined at an angle of 17° relative to the observer's line of sight. From these two observations, EHT concluded the black hole spins clockwise, as seen from Earth.^[18]

Contributing institutions

Some contributing institutions are:^[19][20]

Further reading

• Non-technical: The Black Hole at the Center of Our Galaxy (2001), Fulvio Melia (Princeton University Press), ISBN 0691095051
• Technical: The Galactic Supermassive Black Hole (2008), Fulvio Melia (Princeton University Press), ASIN B008VQXCN2

• Official website