First-ever image of a black hole that stunned the world is Science’s 2019 Breakthrough of the Year
Black holes are very small by cosmic standards and by definition emit no light, which is why until recently few astronomers imagined such an image was even possible
In April 2019, the world saw the first-ever image of a black hole. That’s when an international team of astronomers released a startling close-up image of a black hole’s “shadow,” showing a dark heart surrounded by a ring of light created by photons zipping around it.
Now honoring this feat that was once considered impossible, Science journal has named the Event Horizon Telescope or EHT’s image of a supermassive black hole as its 2019 Breakthrough of the Year.
Black holes have captivated the imaginations of scientists and the public for decades. For astronomers, the image is a validation of decades of work theorizing about objects they could not see.
Using the Event Horizon Telescope, scientists obtained the iconic image of the black hole at the center of the galaxy Messier 87 (M87), outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun
Heino Falcke of Radboud University in Nijmegen, the Netherlands, a member of the team that produced the image, said the first glimpse felt like “looking at the gates of hell.”
“I’m still kind of stunned. I don’t think any of us imagined the iconic image that was produced,” writes astrophysicist Roger Blandford from Stanford University in Palo Alto, California, in the journal.
Such an image was thought impossible, until recently
Massive, ubiquitous and in some cases as big as our Solar System, black holes are immensely dense cosmic objects with gravity so strong that they capture and consume everything surrounding them, including light. Since they reflect no light, black holes often hide in plain sight, perfectly camouflaged against the inky black of the void.
The effect of their gravity on objects around them and, lately, the gravitational waves emitted when they collide reveal their presence. But no one had ever seen one directly—until April.
Until recently, few astronomers imagined that capturing an image of a black hole was impossible because an image of something from which no light can escape would appear completely black.
“When they grow to gargantuan masses, as happens in the centers of galaxies, the swirling mayhem of gas, dust, and stars stirred up by their extreme gravity creates an additional barrier,” say experts.
For scientists, the challenge was how, from thousands or even millions of light-years away, to capture an image of the hot, glowing gas falling into a black hole. An ambitious team of international astronomers and computer scientists has managed to accomplish this.
The international collaboration
By imaging the cloud of hot, brightly glowing gas that surrounds it, the EHT team was able to capture the silhouette of the supermassive black hole that lies at the center of Messier 87 (M87). While massive, it is small by galactic standards at roughly the size of our Solar System.
Despite its name, EHT is not a telescope but a consortium involving more than 200 scientists from across the globe. By 2017, the EHT was a collaboration of eight telescopes in six geographic locations, and more have been added since then.
The scientists describe that a turning point came when EHT enlisted the Atacama Large Millimeter/submillimeter Array (ALMA). Made up of 66 dishes high in the desert mountains of northern Chile, it is by far the largest observatory at millimeter wavelengths.
“Allowing the EHT team to come in, open up the hood, and tinker with the engine of this $1.5 billion US-European-Japanese collaboration took years of persuasion and several rejected designs, but adding ALMA boosted EHT’s sensitivity 10-fold,” says the report.
In April 2017, everything was in place for a major 10-night observing run. ALMA, along with seven other observatories in the US, Mexico, Chile, Spain, and at the South Pole, took repeated night-long exposures of both Sgr A* (the 4-million-solar-mass black hole at the center of the Milky Way Galaxy, known as Sagittarius A*) and M87*.
The EHT observations use a technique called very-long-baseline interferometry (VLBI), which synchronizes telescope facilities around the world and exploits the rotation of our planet to form one huge, Earth-size telescope observing at a wavelength of 1.3 mm.
The team started calibrating and processing the data collected as they worked in parallel data centers in the US and Germany. At the same time, other teams checked the results independently. In April 2019, the team revealed an image of M87* -- the black hole’s silhouette outlined in a ring of light.
Close to the black hole, tremendous gravitation bends photons and particles far from the intended paths. Those that get too close get pulled in, never to escape. They have crossed the path of no return known as the event horizon, explain scientists. The strange environment creates not only a black hole but a surrounding shadow, with escaping photons forming a ring. That ring of light makes its way to Earth, where it was captured by the Earth-spanning observation collaboration.
Why is it important
Scientists now have a new way of studying black holes that they have never had before, and this, according to the team, is just the beginning.
According to experts, learning about mysterious structures enables scientists to test observation methods and theories, such as Einstein’s theory of general relativity.
Albert Einstein’s description of gravity, general relativity, predicts a black hole’s shadow should be perfectly round — a prediction confirmed to within 10% by the M87* image.
“One of the main results of the EHT black hole imaging project is a more direct calculation of a black hole’s mass than ever before. Using the EHT, scientists were able to directly observe and measure the radius of M87*’s event horizon, or its Schwarzschild radius, and compute the black hole’s mass. That estimate was close to the one derived from a method that uses the motion of orbiting stars – thus validating it as a method of mass estimation,” says NASA.
The data, says NASA, also offers some insight into the formation and behavior of black hole structures, such as the accretion disk that feeds matter into the black hole and plasma jets that emanate from its center.
While scientists have hypothesized about how an accretion disk forms, they have not been able to test their theories with direct observations until now.
The best tests of general relativity could come from an image of Sgr A*, which the team hopes to finish in 2020. Researchers have much more precise data on the mass, size, and distance of Sgr A* than of M87*.
“That will immediately improve the quality of constraints [on general relativity],” says team member Feryal Özel of the University of Arizona in Tucson.
EHT has 11 facilities lined up for the next round of observing in 2020, including new dishes in Greenland and Arizona and an upgraded array in France.
“Observations will soon move to even shorter wavelengths, 0.86 millimeters instead of the 1.3 millimeters used so far. Further in the future, the team hopes to add a dozen or so purpose-built dishes scattered across the globe and some in space, to increase the resolution still further. A dish one-third of the way to the Moon, for example, would bring another 20 supermassive black holes into imaging range,” the report says.
Such expansion would also enable EHT to graduate from making still images to ‘movies’ of a black hole “sucking in material and funneling it into high-powered jets” that fire from its poles millions of light-years into space.