At the center of our Milky Way galaxy, near the border of the constellations Sagittarius and Scorpius, exists a supermassive black hole. Dubbed Sagittarius A* (Sgr A* for short), this all-consuming region of spacetime spans a distance of more than 27 million miles and is estimated to have a light-sucking core 4 million times the mass of the sun. Because of its relatively close distance to Earth, a separation of "only" 26,000 light-years, it's also one of the few black holes to have been observed influencing the flow of matter nearby.
For the first time, we may soon possess an "image" of this celestial monster.
"To be precise, we are not going to take a direct photograph of the black hole at our galaxy’s heart," Manchester University astronomer Tom Muxlow told the Guardian in 2017. "We are actually going to take a picture of its shadow. It will be an image of its silhouette sliding against the background glow of radiation of the heart of the Milky Way. That photograph will reveal the contours of a black hole for the first time."
Despite its supermassive size, Sgr A* is far enough away from us to present a massive challenge for any one telescope to capture. According to Nature, it would require something with a resolution more than 1,000 times better than the Hubble Space Telescope to pull off. Instead, astronomers decided to create something bigger –– much bigger.
In April 2018, astronomers synchronized a global network of radio telescopes to observe the immediate environment of Sgr A*. Together, like the fictional robot character Voltron, they combined to form the Event Horizon Telescope (EHT), a virtual planet-sized observatory capable of capturing unprecedented detail over great distances.
"Instead of building a telescope so big that it would probably collapse under its own weight, we combined eight observatories like the pieces of a giant mirror," Michael Bremer, an astronomer at the International Research Institute for Radio Astronomy (IRAM) and a project manager for the Event Horizon Telescope, is quoted as saying at the time. "This gave us a virtual telescope as big as Earth — about 10,000 kilometres (6,200 miles) in diameter."
It takes a village (of telescopes)
Over several days, locked to each other using the exceptional precision of atomic clocks, the radio telescopes captured an enormous amount of data on Sgr A*.
According to European Southern Observatory, its Atacama Large Millimeter/submillimeter Array (ALMA), a participating partner in the Event Horizon Telescope, alone recorded over a petabyte (1 million gigabytes) of information on the black hole. Too large to send over the Internet, the physical hard drives were sent via plane and input into computing clusters (called a correlator) located at the MIT Haystack Observatory in Cambridge, Massachusetts, and the Max Planck Institute for Radio Astronomy in Bonn, Germany.
And then the researchers waited. The first obstacle on the road to processing a first image involved the eighth participating radio telescope stationed in Antarctica. Because no flights are possible from February to October, the final data set captured by the South Pole Telescope was literally placed in cold storage. On Dec. 13, 2017, it finally arrived at the Haystack Observatory.
"After the disks have warmed up, they will be loaded into playback drives and processed with data from the other 7 EHT stations to complete the Earth-sized virtual telescope that links dishes from the South Pole, to Hawaii, Mexico, Chile, Arizona, and Spain," the team announced in December 2017. "It should take about 3 weeks to complete the comparison of recordings, and after that the final analysis of the 2017 EHT data can begin!"
That final analysis stretched through all of 2018, with the 200-strong research team carefully studying the collected data and accounting for any error sources (turbulence in the Earth's atmosphere, random noise, spurious signals, etc.) that might degrade the event horizon image. They also had to develop and test new algorithms to convert the data into "maps of radio emissions on the sky."
As Shep Doeleman, director of the EHT, said in a May 2018 update, the process has been so labor-intensive that astronomers have taken to calling it the "ultimate in delayed gratification."
The good news is the wait may soon be over. In recent weeks, the Event Horizon Telescope team has been hinting in updates that the first observations from the 2017 measurements are nearly ready for release. Are we ready to stare at the ominous shadow of one of the universe's most fascinating and terrifying creations?
Could this black hole photo prove Einstein wrong?
So what might we expect to see once the number-crunching is complete? Interestingly, some scientists believe it may look something like the black hole that director Christopher Nolan carefully crafted for his film "Interstellar." Unlike the visually-pleasing splendor of that particular example, however, it's expected that the light created by the event horizon would be more asymmetrical than uniform.
In addition to its shadow, the researchers are also hopeful to image the black hole's event horizon or the point at which nothing, not even light, can escape its gaping maw. It's here in these observations, according to Robert Laing of the European Southern Observatory, that Einstein's theory of general relativity will face one of its greatest tests.
"We want to see whether the idea of a black hole having an event horizon is actually right and whether the quantitative predictions of what its shadow should look like are correct," Laing told The Guardian. "If general relativity is wrong in some way, you should eventually be able to see deviations from its predictions in the shape of the shadow and behavior of our galaxy’s great black hole."
Whatever image is finally revealed, it's likely to only deepen the questions and awe surrounding these mysterious astronomical phenomena. The sheer engineering alone that has given rise to this historical moment is reason enough to celebrate.
“It is the challenge of doing something, that has never been attempted before," astrophysicist and EHT science chair Heino Falcke said. "It is the start of an adventurous journey towards a black hole."
Editor's note: This article has been updated with new information since it was first published in January 2018.