One Step Closer to Black Hole’s Event Horizon

An international team of astrophysicists has the first time measured the black hole’s ‘point of no return’ – the closest distance that matter can approach before being irretrievably pulled into the black hole.

This artist’s conception shows the region immediately surrounding a supermassive black hole. The black hole is orbited by a thick disk of hot gas. The center of the disk glows white-hot, while the edge of the disk is shown in dark silhouette. Magnetic fields channel some material into a jet-like outflow – the greenish wisps that extend to upper right and lower left. A dotted line marks the innermost stable circular orbit, which is the closest distance that material can orbit before becoming unstable and plunging into the black hole (Chris Fach / Perimeter Institute & University of Waterloo)

“Once objects fall through the event horizon, they’re lost forever,” said Dr Sheperd Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Harvard-Smithsonian Center for Astrophysics, who led the study published in the Science Express. “It’s an exit door from our Universe. You walk through that door, you’re not coming back.”

The team examined the supermassive black hole at the center of a giant elliptical galaxy called Messier 87, which is located about 50 million light-years from Earth. That black hole is 6 billion times more massive than the Sun. It’s surrounded by an accretion disk of gas swirling toward the black hole’s maw. Although the black hole is invisible, the accretion disk is hot enough to glow.

“Even though this black hole is far away, it’s so big that its apparent size on the sky is about the same as the black hole at the center of the Milky Way,” said co-author Dr Jonathan Weintroub of the Harvard-Smithsonian Center for Astrophysics. “That makes it an ideal target for study.”

According to Einstein’s theory of general relativity, a black hole’s mass and spin determine how close material can orbit before becoming unstable and falling in toward the event horizon. The team was able to measure this innermost stable orbit and found that it’s only 5.5 times the size of the black hole’s event horizon. This size suggests that the accretion disk is spinning in the same direction as the black hole.

The observations were made by linking together radio telescopes in Hawaii, Arizona and California to create a virtual telescope called the Event Horizon Telescope. The telescope is capable of seeing details 2,000 times finer than the Hubble Space Telescope.

The team plans to expand its telescope array, adding radio dishes in Chile, Europe, Mexico, Greenland, and the South Pole, in order to obtain even more detailed pictures of black holes in the future.

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Bibliographic information: Sheperd S. Doeleman et al. Jet-Launching Structure Resolved Near the Supermassive Black Hole in M87. Science, published online September 27 2012; doi: 10.1126/science.1224768