World gets first-ever visual of a black hole
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THE first image of a black hole was unveiled by astronomers at coordinated press conferences around the world yesterday.
The Event Horizon Telescope, an array of eight ground-based radio telescopes, was designed to capture images of a black hole, and researchers have now revealed their success, unveiling the first direct visual evidence of a supermassive black hole and its shadow.
The image reveals the black hole at the center of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. It is about 55 million light years from Earth and has a mass 6.5 billion times that of the sun.
The EHT was the result of years of international collaboration and offers scientists a new way to study the most extreme objects in the universe predicted by Albert Einstein’s theory of general relativity.
“We have taken the first picture of a black hole,” said Sheperd Doeleman of Harvard, leader of the team of about 200 scientists from 20 countries. “This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”
Michael Kramer, director at the Max Planck Institute for Radio Astronomy, said: “The history of science will be divided into the time before the image, and the time after the image.”
European Commissioner for Research, Science and Innovation Carlos Moedas called the feat a “huge breakthrough for humanity.”
Black holes are extraordinary cosmic objects of enormous mass but extremely compact in size. Their presence affects their environment in extreme ways, warping space-time and superheating any surrounding material.
“If immersed in a bright region, like a disk of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein’s general relativity that we’ve never seen before,” said EHT Science Council chair Heino Falcke.
“This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these objects and allowed us to measure the enormous mass of M87’s black hole.”
Multiple calibration and imaging methods have revealed a ring-like structure with a dark central region — the black hole’s shadow.
“Once we were sure we had imaged the shadow, we could compare our observations with extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well,” said Paul Ho, an EHT board member.
“This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.”
Creating the EHT was a formidable challenge which required upgrading and connecting a worldwide network of eight pre-existing telescopes deployed at a variety of challenging high-altitude sites. These locations included volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert and the Antarctica.
The construction of the EHT and the observations announced yesterday represent the culmination of decades of work. This global teamwork required close collaboration by researchers around the world.
The project was also supported by the Center for Astronomical Mega-Science of the Chinese Academy of Sciences, co-established by National Astronomical Observatories, Purple Mountain Observatory and Shanghai Astronomical Observatory.
“This picture will tell us many unknown things like the spinning speed of the black hole and the shape of the accretion disc around it,” said Zu Ying, a researcher with Shanghai Jiao Tong University’s School of Physics and Astronomy.
“To some extent, black holes are behind the evolution of stars and the universe. They are born from death,” Zu said. “But some small black holes will still die based on Stephen Hawking’s concept of what is now called ‘Hawking radiation,’ put forward in 1974.”
Hawking predicted black holes would emit thermal radiation and gradually evaporate.
“However, most black holes will exist forever,” Zu said.
Frederic Gueth, one of the EHT project’s lead scientists and deputy director of the Institute for Millimetric Radio Astronomy in Grenoble, France, explains the groundbreaking exploit and the science behind it.
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 — is brilliant in contrast.
As long as they have not been swallowed by the black hole, the material can be detected. The objective, then, is to visualize 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. 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.
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