As one might imagine of a massive, collapsed star within which the laws of physics seemingly break down, black holes occupy a special place in human culture — spurring all kinds of creative science fiction, creative real physics theories, and creative metaphors. Yet even as we gape in awe at the astronomical bodies first known to theorists as "dark stars," we still have not been able to image the incredibly massive black hole at the center of our own galaxy, and around which the rest of the galaxy's stars spin.
Until now, that is.
Scientists from the Event Horizon Telescope Collaboration (EHT) announced on Tuesday that they had coordinated eight synchronized telescopes around the world to capture an image of Sagittarius A*. Four million times more massive than our own sun, Sagittarius A* has long been theorized to be a supermassive black hole, but experts could not know for sure because of the difficulties of imaging that part of the sky with great precision. The new image shows a ring of orange and golden light that becomes particularly bright in three locations. In the center, and colored black, is a slightly-bean shaped hole.
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"Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a telltale signature: a dark central region (called a 'shadow') surrounded by a bright ring-like structure," the astronomers explained in a statement. One of them, EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics in Taipei, added in the statement that "We were stunned by how well the size of the ring agreed with predictions from Einstein's Theory of General Relativity. These unprecedented observations have greatly improved our understanding of what happens at the very centre of our galaxy, and offer new insights on how these giant black holes interact with their surroundings."
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The first-ever direct image of a black hole was obtained in 2019, and was the culmination of years of astrophotography and research. That black hole was the supermassive black hole at the center of the galaxy M87, which has a mass of about 6.5 billion suns and is 50 million-light years away. Sagittarius A* is a mere 27,000 light-years away, far, far closer; yet because of its position in the night sky and the relative clear field of view, it was easier to image a distant galaxy's black hole before seeing our own.
It must be emphasized that, while we are in one sense seeing the black hole, in another sense the very concept of "seeing" one of these objects is absurd. The term "event horizon" is more than the title of a classic 1997 sci-fi horror film. It refers to the threshold which surrounds every black hole, and through which nothing can escape. The gravitational force is so strong that everything which falls in stays within the black hole forever.
This, naturally, includes light itself. That is why, to "see" a black hole, what you really observe are the objects that orbit around it, sometimes very close and very fast. That includes the accretion disk, the accumulation of space junk — gas and dust – that whips around most black holes near the event horizon, generating a faint aura of light from the heat of collision and motion.
"Some of it's falling in, some of it just forms this disc around it and that stuff glows," Seth Fletcher, the Scientific American chief features editor who wrote a book about the EHT, recently explained. "The black hole, because of the way it warps space, time around it, because of the incredible force of gravity, it casts a shadow against that glowing matter — and so that's actually what we see in this picture."
Saying that this is a long-distance photograph would be a drastic understatement. We live in a barred spiral galaxy, or one that swirls around with waving arms and has a bar-shaped structure in the middle composed largely of stars. Our planet is located in one of those spiral arms, and Sagittarius A* is 27,000 light-years away. This means that if one were traveling at the speed of light, it would take us 27 millennia to reach Sagittarius A* from our celestial home.
The research team from EHT which made this discovery published their findings in the scientific journal The Astrophysical Journal Letters. In closing the announcement of their discovery, they noted that more than 300 people from across the globe participated in this effort, and EHT has more ambitions.
"Since those observations, the EHT has continued to observe and to grow in its capabilities through the addition of new stations, the widening of bandwidth, and the introduction of a higher frequency capability," they wrote. "Existing and new observations with the EHT of Sgr A* and M87* coupled with innovations in analysis and theoretical modeling will drive discovery in these unique laboratories for black hole physics."
While black holes that weigh at least 4 times the mass of our sun are routinely created in supernova explosions of massive stars, supermassive black holes like Sagittarius A*, which weigh millions or billions of solar masses, emerge differently: either through a bizarre process of gas coalescing in the early universe; or when thousands of stellar black holes merge over billions of years; or a combination of both. Supermassive black holes have unique properties compared to their much smaller peers. Some scientists theorize that, because of the great distance between its event horizon and its central singularity, if one fell into a supermassive black hole, one would not immediately be stretched to oblivion by its immense tidal forces as one would in a quotidian black hole of 4 to 100 solar masses. Rather, an observer falling into a supermassive black hole might have a few hours or more of time before they were killed by tidal stretching; in that span, if they were to look back out of the black hole, and towards the event horizon, they would see future events in nearby space happening at a hyper-accelerated pace, as outside time would appear to speed up as they approach the singularity. Unfortunately, this observer would never be able to communicate this information as there is no escape.
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