Astronomers Reveal Historic First Silhouette of Black Hole
On April 10, 2019, the team behind the Event Horizon Telescope (EHT) unveiled the above image of the supermassive black hole M87* at the center of the Messier 87 galaxy. This marks the first time the silhouette of a black hole has ever been recorded, an unprecedented feat which is being said to “open a new era of astrophysics.”
As part of the press release, EHT project director Shep Doeleman of the Harvard-Smithsonian Institute for Astrophysics said he was “delighted to be able to report [...] that we have seen what we thought was unseeable.”
These images provide conclusive proof of the existence of black holes, which in turn serves as evidence for various other theories. Chief among them is Albert Einstein’s century-old theory of general relativity which, at its publication in 1915, described gravity as a phenomenon created by the geometry of spacetime. Einstein believed that sufficient amounts of highly compressed mass would distort the fabric of spacetime to the point where not even light could escape the gravitational pull. Such cases were termed “singularities” or “black holes.”
Einstein himself predicted that, if black holes were to exist, they would be spherical in form and appear as a shadow surrounded by a ring of light. Beyond proving the existence of black holes in general, these newly captured images confirm the existence of supermassive black holes (M87* is 6.5 billion times more massive than our Sun), which have been theorized to be at the center of all galaxies.
The EHT used to take the images is, in actuality, an array of eight radio telescopes positioned across the Western Hemisphere, capturing a total of several petabytes of data. These telescopes include facilities in the western United States, Mexico, Chile, France, Spain, Greenland, and Antarctica. The collected data were sent to the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the MIT Haystack Observatory in Westford, Massachusetts.
The telescope uses a technique known as Very-Long-Baseline Interferometry (VLBI), in which the precise calculation of distances and time intervals between multiple radio telescopes allows for their data to be combined accurately. This results in a level of resolution in the data which would otherwise take a single radio telescope the size of the maximum distance between those linked through VLBI. In this case, the combination of installations used can be compared to an Earth-sized telescope capturing a single image.
Although the images were only recently shown, the EHT actually recorded the raw data in 2017 during a period of clear weather. While difficulties with weather and the technical aspects of some telescopes have precluded further observation since then, the organization hopes to “mount observing campaigns of increasing power and sensitivity, aiming to bring black holes into focus” over the coming years.
The images of M87* are actually the second set to have been published by the EHT Collaboration this year, although they are certainly the more dramatic of the two. On January 27th 2019, images of the supermassive black hole Sagittarius A* (Sgr A*; at the center of our own Milky Way galaxy) were released, compiled from data also recorded in 2017. While the actual images do not show the silhouette of the black hole in the same way as those of M87* does (likely due to Sgr A*’s relatively low mass of only 4 million times our Sun’s), they nonetheless provided a base of what to expect for the ultimate unveiling of the images of M87*.