A man who is not usually very good with dates, I assure you that if someone asked him what he was doing on April 10, 2019… “Then yes, I would remember.”
Like all lovers of astronomy and science, that day the subject was glued to the computer screen to attend the presentation of the first real image of a black hole. Until that moment, humanity had theorized for decades about its existenceHundreds of indirect evidence had been collected, mass, velocity, and event horizon data had been analyzed, thousands of artistic representations had been published… but we had never photographed a real black hole.
Then came the great EHT (Event Horizon Telescope) project, a fruitful international collaboration made up of eight radio telescopes spread all over the world that, thanks to interferometryit becomes a gigantic peephole with a virtual size of the Earth itself.
Thus came the famous photograph of the black hole at the center of Messier 87, a vast galaxy located in the Virgo cluster. Located 55 million light years from usthat colossal black hole M87* shone like a ring in Tolkien’s novels, with a mass of 6,500 suns.
However, and although this image from 2019 has gone down in history, there were still many issues to be resolved, especially since it shows no trace of the jets of matter that should accompany a black hole.
In that image, the EHT radio telescopes were not operating at an adequate wavelength to capture them, the frequency needs to be lowered (and therefore increase the wavelength) to be able to visualize them (3.5 mm instead of 1.3 mm).
“The matter that falls in a spiral towards the black hole forms the so-called matter accretion disk”, explains the physicist, mathematician and well-known popularizer Francis Villatoro. “This matter accelerates and gains a lot of energy, with which its atoms separate into their nuclei and their electrons, giving rise to a plasma. Both nuclei and electrons are electrically charged, so they behave like electrons in an electrical wire and generate a magnetic field.
These magnetic field lines in the disk are closed, they form “horseshoes” that go out of the disk and return to it (this also happens on the surface of the Sun). But the lines near the inner edge of the disk cannot be closed and remain as open lines. heading towards the event horizon of the black hole. When the black hole rotates very fast, the empty space between the horizon and the disk also rotates; the open lines of the magnetic field are forced to rotate in a spiral, escaping upwards (perpendicular to the disk) without reaching the horizon. These magnetic field lines drag the plasma from the disk and shoot it out producing the relativistic jet of matter.
As is often the case when trying to understand the Cosmos, there are still many unanswered questions, but the amount of knowledge we are gaining from studying a single black hole is fascinating. The great M87* has become the greatest source of discoveries in black hole physics and observation through interferometry has been revealed as a marvelous technique for studying one of the most exciting phenomena in the Universe.