Black holes. They are not a rarity to look upon in terror. There are billions of them, of various types. They are scary, perhaps, but only because no one knows, for now, how they form or why they manage to become supermassive, millions or billions of times heavier than the Sun. We know that they are at the center of almost all galaxies.

What is a black hole?

In astrophysics, a black hole is a celestial body with a gravitational field so intense that it does not let matter or electromagnetic radiation escape, that is, from a relativistic point of view, a region of spacetime with such a great curvature that nothing from its interior can escape, not even light since the escape velocity is higher than c, precisely the speed of light.

The black hole is the result of implosions of sufficiently high masses. Gravity dominates over any other force so that a gravitational collapse occurs which tends to concentrate spacetime in a point in the center of the region, where a state of matter of curvature tending to infinity and volume tending to zero is theorized, called “singularity”, with characteristics unknown and extraneous to the laws of general relativity. The limit of the black hole is defined as the event horizon, a region that delimits its observable boundaries in a peculiar way.

Due to the above properties, the black hole is not directly observable. Its presence is revealed only indirectly through its effects on the surrounding space: the gravitational interactions with other celestial bodies and their emissions, the mainly electromagnetic irradiation of matter captured by its force field.

During the decades following the publication of general relativity, the theoretical basis of their existence, numerous observations were collected that could be interpreted, although not always uniquely, as evidence of the presence of black holes, especially in some active galaxies and binary X star systems. The existence of such objects is now definitively demonstrated and new ones with very variable mass are gradually identified, from values of about 5 to billions of solar masses.

A black hole is the exact solution of the field equations of Einstein’s theory of general relativity. The solution was discovered by German Karl Schwarzchild while serving in the army as a volunteer in World War I. His solution provides for the existence of singularities on a sphere of a given radius, which is called the Schwarzchild radius. If the radius of a stellar object is smaller than the Schwarzchild radius, then everything that has mass and even photons must inevitably fall into the central body. When the mass density of this central body exceeds a defined limit, a gravitational collapse is triggered which, if it occurs respecting a spherical symmetry, generates a black hole. The Schwarzschild solution, which makes use of Schwarzschild coordinates and the Schwarzschild metric, leads to a derivation of the Schwarzschild radius, which is the size of the event horizon of a non-rotating black hole.

There are perhaps 10 million to a billion black holes in the Milky Way alone, a galaxy of which the solar system and therefore all of us are part. The same order of magnitude for each of the billions of galaxies in the universe. You do the math.

We have recently been able to develop an observation system capable of “seeing” them. The quotes are necessary because as said we can see them only indirectly. What we do is translate frequencies not visible to the naked eye into colors. We “photographed” a couple of them so far.

Two close encounters

The first is located at the center of Messier 87, abbreviated M87, a huge elliptical galaxy, with a radius of about 150 kiloparsecs (1 parsec = 3.26 light-years, 1 kiloparsec is 1000 parsecs; by calculating 150 kiloparsecs are 490 thousand light-years), much more massive than our Milky Way (from the Latin via lactea, which derives from the Greek γαλακτικός κύκλος (galaktikos kýklos), which translates as “milky circle”; apparent diameter between 100 and 200 thousand light-years; recent simulations suggest that it is surrounded by dark matter extending over a diameter of about two million light-years).

The second is Sagittarius A*, abbreviated Sgr A*, 27 thousand light-years away from Earth, 1000 times less massive than the black hole of M87, 4 million solar masses against perhaps 6.5 billion of M87. While M87 is binge eating in a compulsive and exaggerated way, Sgr A* is on a strict diet, so much less bright. More difficult to observe: only 17 times larger than our Sun and 27,000 light-years away. Solving an image of Sgr A* is like solving the image of an apple on the lunar surface. To make things even more difficult there is the speed of rotation of the plasma at 1000 billion degrees centigrade that surrounds it, 1000 times faster than what happens around M87, which changes its appearance from one minute to the next.

The observations collected in 2017 required two years of work to process the image of M87, while it took five for Sgr A*. The two images are actually very similar. The implication is that, regardless of their size, when you get to the edge of a black hole, gravity commands everything.

When a star dies, because it has run out of nuclear fuel, if it is heavy enough, gravity overcomes the intrinsic resistance of matter and the star collapses catastrophically. The trace left, while the stellar matter continues to fall into the hole that has been generated, towards a completely unknown destiny, is the horizon of events. The remains of the sucked material orbit around it and their energy illuminates the scene. The trajectory of the emitted light is changed by the curvature of space caused by the black hole’s mass. The light emitted behind the black hole is then redirected towards the observer. You don’t see the black hole, but the disc of light that surrounds it. It’s the light, or rather electromagnetic radiation, which can be observed.

This is precisely what the EHT does, which stands for Event Horizon Telescope, a system consisting of 8 radio telescopes (LMT-Large Millimeter Telescope, Mexico; SPT-South Pole Telescope, Antarctica; SMT-Submillimeter Telescope, Mount Graham, Arizona; SMA-SubMillimeter Array, Maunakea, Hawai; ALMA-Atacama Large Millimeter / submillimeter Array, Chile; APEX-Atacama Pathfinder Experiment, Chile; JCMT-James Clerk Maxwell Telescope, Hawai; IRAM 30m, Institute of Millimetric Radio Astronomy, Pico Veleta, Andalusia, Spain) which, working in a coordinated way from the South Pole to Spain, make the Earth a single gigantic virtual telescope, with the required resolution.

Over the course of 10 consecutive nights, they observed and collected Sgr A* data. Billions of gigabytes. Too many even for the internet. More than 1000 ultra-high-capacity memory disks have been physically transported to two data processing centers: the Haystack Observatory near Boston, USA, and the Max Planck Institute for Radio Astronomy in Bonn, Germany.

The two photos are the result of decades of observation and research. A stop on the journey that began in 1918 when the astronomer Harlow Shapley was the first to observe the congregation of stars at the center of the Milky Way. Place in space where powerful radio emissions were then detected, suggesting the presence of a massive but compact object. Very compact.

The two images were made possible by the results of the research aimed at the ability to follow, with very high precision, the path of the stars. Research awarded with the 2020 Nobel Prize in Physics to Roger Penrose, Reinhard Genzel, and Andrea Ghez. Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity” in his 1965 work. Genzel and Ghez “for the discovery of a supermassive compact object at the centre of our galaxy“, Sgr A*, precisely.

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