The first photograph of a black hole and its fiery halo, released Wednesday by Event Horizon Telescope astronomers, is the "most direct proof of their existence," one of the project's lead scientists told AFP.
Frederic Gueth, deputy director of the Institute for Millimetric Radio Astronomy (IRAM) in Grenoble, France, talked about the ground-breaking exploit and the science behind it.
How did you do it?
"The Event Horizon Telescope (EHT) marshalled all the millimetric (radio) telescopes on the planet to make the same rigorous observations at exactly the same time.
"By combining the data gathered by all the telescopes - a technique called very long baseline interferometry - we created a virtual antenna the diameter of Earth.
"The millimetric range - measured in thousandths of a metre - turns out to be the best wave length to investigate black holes because the waves pass through the dust clouds that enshroud them. That is not true for infrared."
What do we see in the image?
"By definition, a black hole per se cannot be seen, and never will be.
"But we know that the accretion disk of matter that surrounds a black hole - made up of hot gases we call plasma, along with the debris of stars torn apart by gravity - are brilliant in contrast.
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"As long as they have not been swallowed by the black hole, the material can be detected. The objective, then, is to visualise the black hole by contrast.
"What we see in the image is the shadow of the black hole's rim - known as the event horizon, or the point of no return - set against the luminous accretion disk.
"The event horizon is a bit smaller (in diameter) than the shadow. The black hole itself is within the event horizon.
"Our observations revealed that the supermassive black hole in the galaxy M87 has a mass 6.5 billion times greater than the Sun, and that it turns clockwise."
What's next?
"Because it worked so well in 2017, when the observations were made, we are clearly going to do it again!
"The EHT will continue to evolve in the coming years, notably with the addition of two new telescopes: the NOEMA telescope in the French Alps, and the Greenland Telescope.
"The picture from the M87 galaxy emphatically confirms the models we have of rotating black holes. We are seeing exactly what the models predicted. That is satisfying.
"The challenge now will be to measure the exact density of the matter around a black hole, and to better understand the crucial role of magnetic fields, and how matter within the accretion disk rotates."
Scientists set to reveal first true image of black hole, earlier report
PARIS: The world is finally about to see a black hole - not an artist's impression or a computer-generated likeness, but the real thing.
At six press conferences across the globe scheduled for 1300 GMT on Wednesday, scientists will unveil the first results from the Event Horizon Telescope (EHT), conceived precisely for that purpose.
It has been a long wait.
Of all the forces in the Universe that we cannot see - including dark energy and dark matter - none has frustrated human curiosity as thoroughly as the invisible, star-devouring monsters known as black holes.
Yet, the phenomena are so powerful that nothing nearby - not even light - can escape their gravitational pull.
"Over the years, we accumulated indirect observational evidence," said Paul McNamara, an astrophysicist at the European Space Agency and project scientist for the LISA mission that will track massive black hole mergers from space.
In September 2015, for example, the LIGO gravitational wave detectors in the United States measured two black holes smashing together.
"X-rays, radio-waves, light - they all point to very compact objects, and the gravitational waves confirmed that they really are black holes, even if we have never actually seen one," McNamara told AFP.
Two candidates are vying to be in the first-ever image.
Oddsmakers favour Sagittarius A*, the black hole at the centre of our own spiral galaxy, the Milky Way.
Sag A* has four million times the mass of our Sun, and measures about 24 million kilometres across.
That may sound like a big target, but for the telescope array on Earth some 26,000 light years (245 trillion kilometres) away, it's like trying to photograph a golf ball on the Moon.
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The other candidate is 1,500 times more massive still, ensconced in a faraway elliptical galaxy known as M87.
Comparing the two, distance and size balance out, making it roughly as easy (or hard) to pinpoint either.
Ripples in time-space
A black hole is a celestial object that compresses a huge mass into an extremely small space. The more mass, the larger the black hole.
At the same scale of compression, Earth's mass would fit inside a thimble, while the Sun's would be a mere six kilometres from edge to edge.
There are two types.
Garden-variety black holes - up to 20 times more massive than the Sun - form when the centre of a very big star collapses in on itself.
So-called supermassive black holes are at least a million times bigger than the Sun. Both Sag A* and M87 fall into this category.
The EHT is unlike any star-gazing instrument ever devised.
"Instead of constructing a giant telescope we combined several observatories as if they were fragments of a giant mirror," Michael Bremer, an astronomer at the Institute for Millimetric Radio Astronomy in Grenoble, told AFP.
Eight such radio telescopes scattered across the globe - in Hawaii, Arizona, Spain, Mexico, Chile, and the South Pole - zeroed in Sag A* and M87 on four different days in April 2017.
Each is at least a big as a football pitch. Together, they form a virtual telescope more than 12,000 kilometres across, the diameter of Earth.
Data collected by the far-flung array was to be collated by supercomputers at MIT in Boston and in Bonn, Germany.
"The imaging algorithms we developed fill the gaps of data we are missing in order to reconstruct a picture," the team said on their website.
Astrophysicists not involved in the project, including McNamara, are eagerly - perhaps anxiously - waiting to see if the findings challenge Einstein's theory of general relativity, which has never been tested on this scale.
The LIGO experiments from 2015 detected signature ripples in the curvatures of time-space during the black hole merger.
"Einstein's theory of general relativity says that this is exactly what should happen," McNamara said.
But those were tiny black holes compared with either of the ones under the gaze of the EHT.
"Maybe the ones that are millions of times more massive are different - we just don't know yet," McNamara said.
