Astronomers have for the first time observed the event horizon of a black hole - the region where its pull is so strong that nothing, not even light, can escape.
The scientists linked together radio dishes in Hawaii, Arizona and California to create a telescope array called the Event Horizon Telescope (EHT) that can see details 2,000 times finer than the Hubble Space Telescope.
They used them to observe M87, a galaxy around 50 million light years from the Milky Way, which has at its heart a black hole six billion times more massive than our sun.
And, using this array, they saw the glow of matter at the event horizon of the black hole.
"Once objects fall through the event horizon, they're lost forever," says Shep Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Smithsonian Astrophysical Observatory. "It's an exit door from our universe. You walk through that door, you're not coming back."
At the edge of a black hole, the gravitational force is so strong that it pulls in everything from its surroundings. But a 'cosmic traffic jam' can build up, with gas and dust creating a flat pancake of matter known as an accretion disk.
This disk orbits the black hole at nearly the speed of light, steadilyfeeding in superheated material. Eventually, this disk can cause the black hole to spin in the same direction as the orbiting material.
Magnetic fields accelerate hot material along powerful beams above the accretion disk to create high-speed jets, extending for hundreds of thousands of light-years.
Doeleman says it's possible the observations could confirm Einstein's theory of general relativity, by allowing scientists to estimate the black hole's spin through careful measurement of the jet's size as it leaves the black hole.
According to Einstein, a black hole's mass and its spin determine how closely material can orbit before becoming unstable and falling in toward the event horizon.
"Einstein's theories have been verified in low-gravitational field cases, like on Earth or in the solar system," says Doelman. "But they have not been verified precisely in the only place in the universe where Einstein's theories might break down - which is right at the edge of a black hole."