Over four nights in April 2017, eight different radio-wave telescopes across the globe fixed their gazes on a single infinitesimally small point—the black hole at the center of the Messier 87 galaxy (M87*). Around 55 million light-years from Earth, M87* and the hot plasma surrounding it create only a minuscule smudge on the night sky. To resolve an image from that distance would require a 13,000-kilometer telescope—roughly the diameter of Earth.
Alternatively, the researchers of the Event Horizon Telescope (EHT) Project stitched together an Earth-sized telescope using a set of radio-wave telescopes distributed around the globe. These telescopes were synchronized with incredible precision via hydrogen maser atomic clocks installed at each location. As the Earth spun, the telescopes each collected fragments of the radio-wave transmissions coming from that infinitesimally small point.
The circumstances were auspicious. Cloud cover or technical difficulties at any of the locations would have significantly curtailed their results. As it was, they obtained 5 petabytes of data, which is roughly equivalent to the storage space found on 20,000 laptops.
Two years, four data teams, and a bank of supercomputers transformed this immense cache of raw data into a single remarkable image. The debut black hole image resembled a blurry coffee cup stain (if your coffee happened to be a radioactive vermilion color). As underwhelming as it may seem to some, this blurry image contains unprecedented evidence for Einstein’s theory of relativity and a glimpse at how supermassive black holes, like M87*, hold galaxies together.
From this picture, scientists have been able to determine that M87* has a mass of roughly 6.5 billion suns. That entire mass is crushed into a single infinitely dense point called the singularity. Within the singularity, all of our established laws of physics collapse. The abnormality of the singularity warps space-time around it, creating some of the most surreal physical phenomena in our universe. The singularity itself cannot be imaged since light that ventures too close to this point is invariably dragged into the center by the immense gravity and absorbed. This property creates a circle of absolute darkness, called the event horizon, that separates the singularity and its inconceivable physics from the ordered universe. Nothing can travel faster than light, so nothing can escape the event horizon.
But if the singularity is completely cloaked in darkness, then what are we seeing in the M87* image? As a black hole spins through space, it can devour stars and other cosmic materials that wander too close. But because the black hole is spinning, the cosmic material doesn’t just fall straight into it. Instead, it spirals around the black hole, heating up from friction and slowly descending towards the singularity. This creates a glowing hot plasma halo around the event horizon, called an accretion disk.
The accretion disk is the smudgy vermillion ring we see in the image of M87*. The shadow in the center of the ring is the black hole itself. Because the accretion disk is superheated, it emits light and radio waves, some of which escape the black hole and are flung out into space. After a 55-million-year journey, some of the radio waves make it to Earth where they can be picked up by our radio telescopes.
Supermassive black holes, like M87*, are some of the most enigmatic entities in our universe. Scientists know that stellar black holes form when a star five to fifty times more massive than our sun runs out of fuel and implodes—most of its mass collapses under gravity to form the singularity. Alternatively, supermassive black holes are millions or billions of times more massive than our sun, and scientists can only guess at how they may have formed. There are many conflicting theories concerning how these cosmic behemoths came into being. Some scientists claim that they formed from massive mega-stars that may have existed in the nascent years of the universe. Others have suggested that dense regions of an elusive substance called dark matter may have created gravity sinks, which pulled in cosmic gas and conglomerated into massive black holes. The answer, whatever it may be, will be fundamental to our understanding of how the universe was formed.
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