Last June, members of the Event Horizon Telescope (EHT) team convened in Cambridge, Massachusetts, to see if they could combine the data from eight telescopes into a single, clear image.
The researchers had their work cut out for them. Over the course of four days in April 2017, the EHT telescopes had stared at the supermassive black hole of Messier 87, an elliptical galaxy 55 million light-years away in the Virgo cluster. Three orders of magnitude as massive as the one at the center of our galaxy but also three orders as distant, M87’s central black hole (M87*) has an apparent diameter of 40 microarcseconds, roughly the size of the date on a quarter in Los Angeles as seen from Washington, DC. The telescopes—some single dishes, others multi-instrument arrays, and all susceptible to systematic noise—had viewed the tiny target from slightly different angles and had encountered varying degrees of atmospheric turbulence when collecting the 1.3-mm-wavelength photons. The technique of linking distant radio telescopes to form a virtual telescope the size of the distance between them, known as very long baseline interferometry, wasn’t new. But no one had ever tried to crunch the data from so many telescopes at such short wavelengths to view something just 40 μas across.
As we now know, the EHT team succeeded in “seeing the unseeable,” as project leader Sheperd Doeleman of MIT put it during a 10 April press conference in Washington. Just as he and his colleagues had hoped, the team was able to resolve superheated plasma streaking just outside the photon orbit radius, the distance from the center of a black hole at which any inward-moving photon no longer has a chance to escape (as opposed to the event horizon, where nothing, regardless of its motion, can escape). The details of how the image came to be, particularly the computational processing and the simulations that validated the derived image, appear in a series of papers published in Astrophysical Journal Letters.