What’s the world’s most famous photograph of 2019 to date? If you’re reading this, you almost certainly chose the image of the black hole at the center of the galaxy M87, revealed to us on April 10.
On May 10, Columbia’s Pupin Hall audience was treated to a talk by journalist/author Seth Fletcher about this recently celebrated first direct image of a black hole. Fletcher is the chief features editor at Scientific American and author of two books. His latest, Einstein’s Shadow: A Black Hole, a Band of Astronomers and the Quest to See the Unseeable, which recounts this project, is available at Amazon. As he explained, Fletcher was embedded with the research team during the multi-year effort to develop the Event Horizon Telescope (EHT), capable of producing such images of black holes by using the entire Earth as a very, very long baseline interferometer array of radio telescopes.
Fletcher began by recounting the history of black hole theory, beginning with Karl Schwarzchild’s solution to Einstein’s general relativity equations soon after these were published. Schwarzchild found that a body of sufficient mass constrained to a small enough volume would have an escape velocity in excess of the speed of light and therefore not be visible. These were first called Schwarzchild Singularities, but the name did not stick.
In 1939, J. Robert Oppenheimer, of Manhattan Project fame, theorized that a sufficiently massive star would eventually collapse to form what we today call a black hole. But it was the post-war development of radio astronomy which led to the general acceptance of the possibility of black holes actually existing, due to the discovery of quasars. In 1967, the physicist John Wheeler gave black holes their current name. It was British astrophysicist Donald Lynden-Bell who in 1969 proposed that quasars were in fact super-massive black holes.
Our own galaxy’s central black hole, Sagittarius A*, was discovered in 1974 by astronomers Bruce Balick and Robert Brown using the baseline array of the National Radio Interferometer Observatory.
In the case of M87, at a distance of over 50 million light years, Fletcher presented the best image we could get of its incredibly massive black hole (6.5 billion solar masses) by the Hubble telescope. It’s not impressive.
Enter the concept of the VLBI array. A Very Long Baseline Interferometer is an array of a series of radio antennae separated by geography but used in conjunction. The greater the baseline (separation), the greater the resolution. By the early 1970s their usefulness had been demonstrated, particularly in observing quasars and notably, to confirm the existence of plate tectonics on the Earth.
In 1979 the French physicist Jean-Pierre Luminet theorized what a black hole would look like if we could observe it. As Fletcher pointed out, the black and white drawing based on his calculations is in retrospect remarkably accurate.
EHT Director Shep Doeleman had begun work at MIT’s Haystack Observatory in Massachusetts in 1992, working for Haystack co-founder Alan Rogers’s project of getting the wavelength at which radio telescopes can observe down to the submillimeter level. It would prove not to be easy.
In 2000, astrophysicists Heino Falcke, Fulvio Melia, and Eric Agol published a paper theorizing that with an Earth-sized high frequency VLBI array, it should be possible to see Sagittarius A*. In 2004, Doeleman, Falcke, and fellow astronomer Geoff Bower made a presentation outlining a road map to seeing Sagittarius A* using VLBI.
Fletcher offered several anecdotes, recounting how much of the next decade was spent adding telescopes in multiple locations to the array. In addition to refining the software, the team had to deal with getting trucks loaded with sensitive equipment past a plethora of low-tech physical challenges in inhospitable environments, including sheep on a mountain road, and even armed gangs of thieves.
To take an EHT image, they needed to take into account at each location the weather, geometry, and timing of each observatory on the Earth and their collective target in the sky, technical issues at each location, and even general fatigue before making a decision to do an imaging run. By April 2017 the EHT was ready to do its first run. A year later, Doelemann’s team still had not finished correlating the 2017 data, which was no less a task than obtaining it. They did another imaging run in 2018. Finally, on April 10, 2019, the team was able to make the statement, “We report the first image of a black hole.”
Following Fletcher’s presentation, a lively Q&A session ensued.
 If you wish to know more about how this was done, read Fletcher’s book!