At the centre of a galaxy called Messier 87, about 55 million light-years away, about which all of the matter of the galaxy orbits, there lies a monster: a supermassive black hole. With about 6.5 billion times the mass of the sun, the black hole at the centre of M87 is so dense that its escape velocity, or the velocity needed to escape the object’s gravity, is more than the speed of light. Accordingly, not even photons of light can escape once they wander too close. But don’t let the name “black hole” fool you. “In kind of a paradox of nature, black holes, which do not allow light to escape, are some of the brightest objects in the universe,” says Shep Doeleman, a senior research fellow with the Harvard-Smithsonian Center for Astrophysics and the Director of the Event Horizon Telescope (EHT) Project, an international effort to directly image a supermassive black hole with telescopes around the world. Today, the EHT project, including eight observatories and over 60 scientific institutions across more than 20 countries, released the first image of a black hole. “This is the first time I’ve seen this image right now,” says France Córdova, the Director of the National Science Foundation (NSF), at a press conference at the National Press Club. “And it did bring tears to my eyes. This is a very big deal.” Six scientific papers were also published today in the Astrophysical Journal, detailing the first direct observations of a black hole.
Although light cannot escape a black hole itself, a kind of border surrounds every black hole, known as the event horizon. Any matter that wanders beyond the event horizon is consumed by the black hole, but as gasses pile up just outside of the event horizon, they are heated to hundreds of billions of degrees, emitting an enormous amount of radiation across the galaxy. The event horizon around the M87 black hole is about 1.5 light-days across, or about 40 billion kilometres, roughly the same size as our solar system. “What one expects if you see a supermassive black hole at the centre of a galaxy, and we think that they exist at the centres of most galaxies, is that the intense gravity draws the gas in the vicinity toward the black hole, and it heats up,” Doeleman says. “You’re trying to compress a lot of gas into the smallest volume you can imagine … and all that very, very hot gas emits [light].”
The observations of the black hole at the centre of M87 reveal that it rotates clockwise. At the bottom of the image, where the ring of light is brighter, the rotation of the black hole is moving toward us, while the part of the ring at the top of the image is moving away. Taking a picture of the burning gas surrounding a black hole’s event horizon, which astronomers call the black hole’s “shadow” or its “silhouette,” has proven no easy task. The M87 black hole is at the centre of the galaxy, veiled behind bright stars and great swaths of gas and dust. To catch the photons of light that manage to escape the gravity well of the supermassive black hole, drawn in toward the event horizon before flying 55 million light-years through M87 and across intergalactic space to Earth, astronomers have linked some of the most powerful radio telescopes ever built to, in a sense, construct a telescope the size of Earth.
“There is a special field called Very Long Baseline Interferometry, in which you tie together radio dishes around the world, and you get extremely high magnifications,” Doeleman says. Radio astronomy observatories, from the South Pole Telescope to the Greenland Telescope, have contributed or will contribute observations to EHT. “With the VLBI technique, where you make the whole Earth a telescope, you need to link dishes on either side of the Earth together using a network of atomic clocks, and that’s what we do.” The Event Horizon Telescope collected the data for the first black hole image in 2017. By using atomic clocks to align the observations in time, and supercomputers to compile the petabytes of data, scientists can effectively achieve the resolution of an Earth-sized telescope—but not the light collecting capability, so the technique can only be used to observe very bright objects. VLBI can only collect radio waves on the surfaces of the dishes, which are constantly rotating with the Earth, keeping an eye on the centre of M87. “You can think of these telescopes as little bits of silver on an Earth-sized mirror, and as they move around they trace out strands of reflectivity, and so you wind up weaving together, or spinning, an Earth-sized telescope—almost building a web the way a spider does,” Doeleman says. The telescopes collect extremely high frequency (EHF) radio waves, nearly infrared light on the electromagnetic spectrum, with a wavelength of 1.3 millimetres. The frequency is “just perfect” to make the expansive journey from the edges of a black hole to our radio dishes, Doeleman says. The observatories generally turn toward M87 at night, and during the months of March and April, when atmospheric water vapours are at their lowest levels. The Event Horizon Telescope has also been observing Sagittarius A*, the supermassive black hole at the centre of our own galaxy, the Milky Way. Sagittarius A* (pronounced “Sagittarius A-star”) is a much less active supermassive black hole than the one at the centre of M87. Positioned about 26,000 light-years away, Sagittarius A* is small enough that it appears about the same size in the sky as the much farther M87.
In addition to the glowing event horizon around the M87 black hole, the object is ejecting jets of material from its poles out into space. “You get these jets of relativistic particles because of course, it’s very, very energetic, that can stream out for tens of thousands of light-years,” Doeleman says. “They can go all the way across the whole galaxy, and it’s that liberation of energy on a galactic scale that can change the way a whole galaxy looks.” The energy of the jets streaming from a supermassive black hole is determined by how much matter the black hole is consuming as well as its rotation, magnetic field and other properties. “The jets are carrying the equivalent of 10 billion supernovae in energy,” says Sera Markoff, a member of the EHT science council and a professor at the University of Amsterdam, at the press conference. “These bizarre sinkholes in the fabric of space-time have a lot of consequences on their own,” Markoff says. When a black hole is spewing out enormous amounts of energy, it prevents the gasses around the event horizon from forming new stars, stymying the growth of galaxies.
At the centre of a black hole, according to Einstein’s general theory of relativity, is a point of singularity where all the matter of the object is condensed into a volume so small that the density is essentially infinite. At this point, the known laws of physics are believed to break down. Closer to the event horizon, however, scientists will probe the shape of the black hole’s silhouette to test laws of relativity. “I have to admit that I was a little stunned that it matched so closely the predictions we had made,” says Avery Broderick, an astrophysicist with EHT and an associate professor at the University of Waterloo, at the press conference. “It’s gratifying but also a little upsetting.” The shape of the light around the black hole, known as the photon ring where light orbits the centre, serves as the most intensive test of Einstein’s theories of gravity ever conducted. “One of the reasons you see that ring of light is that that’s the orbit at which photons are constrained to move in a circle around the black hole,” Doeleman says. “It’s really extraordinary—you take an object like a photon that is travelling as fast as anything can go in the universe, the fastest you can move, and then you realize there’s an object called a black hole that will make that light ray bend in a complete circle. And that’s essentially what you’re seeing. … And if you go through Einstein’s equations, that’s a very special orbit.” Seeing the ring around a black hole, its shadow silhouetted against the cosmos has confirmed that the theoretical physics laid down more than 100 years ago still hold true “in one of the most extreme laboratories that the universe provides for us. I think it speaks to the human spirit, frankly, that we’re able to pull it off,” Doeleman says.
Credit: Jay Bennett for Smithsonian.Com, 10 April 2019.
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