Somewhere, hidden at the center of our galaxy, is a massive object known as Sagittarius A*. Almost certainly a black hole with a mass of about 4 million times that of our own sun, the object has exerted its gravitational influence on the Milky Way for billions of years. Yet scientists have never been able to view Sagittarius A*, or any other black hole, directly.
That could soon change, thanks to a configuration of powerful telescopes, collectively known as an "Event Horizon Telescope" (EHT). Researchers hope to use a technique known as very-long-baseline interferometry (VLBI) to combine the power of each individual array in order create a virtual telescope equivalent to the size of the Earth, allowing them to see the event horizon of a black hole for the very first time.
Black holes are regions of space where matter is squeezed together so densely that everything, including light, becomes trapped in its gravitational pull. As a result, it is impossible to see the "hole" itself, since no light can escape it. But theoretically, scientists should be able to use an EHT to see a black hole's event horizon – the gravitational point of no return for anything being sucked inside.
"We hope to see the un-seeable," Sheperd Doeleman, director of this EHT, told NBC. "We want to see something that by its very nature tries to do everything it can not to be seen. It's the ultimate cloaking device."
Over five nights of observation during a 10-day period that began on Wednesday, eight EHT telescopes will begin to record observations of Sagittarius A*, as well as an even larger black hole in the relatively nearby M87 galaxy. The team expects to gather two to three petabytes of data from the five nights of observation, which will take months, if not years, to sift through. But at the end of that period, they hope to emerge victorious with new understanding of how black holes work – and with any luck, they'll have the pictures to prove it.
"It's a very bold and gutsy experiment," theoretical astrophysicist Roger Blandford of Stanford University in Palo Alto, Calif., who is not involved in the project, told Science Magazine. "It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing."
Scientists have known about black holes since Albert Einstein first theorized their existence via his theory of general relativity more than a century ago. But visually proving their existence has proven very difficult. The first problem is that black holes are very small, cosmically speaking; Sagittarius A*, despite having the mass of millions of stars, is only 17 times wider than our sun, making it difficult to view at vast distances.
"That's like trying to image a grapefruit on the surface of the moon," astronomy research professor Gopal Narayanan at the University of Massachusetts Amherst said in a statement.
Second, the vast gravitational pull of black holes pulls in large quantities of gases and dust, which are heated to billions of degrees. At those temperatures, the matter circling the black hole forms a bright plasma, blocking the event horizon from view.
"In a paradox of their own gravity, black holes can be some of the brightest things in the sky," said Dr. Doleman.
To get around these problems, EHT researchers have been growing their telescope network for a decade. This year, the new additions of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the South Pole Telescope in Antarctica to the network will effectively make the EHT 1,000 times stronger than the Hubble Space Telescope. The researchers will also use high-frequency radio waves in order to see past the bright plasma boundary in front of the black hole for an unprecedented view of Sagittarius A*. And what they see could challenge fundamental scientific principles that have never been directly observed in such extreme conditions.
"At the very heart of Einstein's general theory of relativity there is a notion that quantum mechanics and general relativity can be melded, that there is a grand, unified theory of fundamental concepts," said Dr. Narayanan. "The place to study that is at the event horizon of a black hole."
So what do scientists expect Sagittarius A* to look like? If current theories are correct, the black hole should appear as in the data as a bright crescent of light with a dim interior. If it looks different, it could mean that a century of theoretical physics is flawed – but for many scientists, that's where the real excitement of the project lies.
"I would love to be in the collaboration that finds that general relativity is wrong," Avery Broderick, a theorist at the Perimeter Institute for Theoretical Physics, told NBC. "I think every physicist has dreamed that for the past 60 years."