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Astronomers Creep Up to the Edge of the Milky Way's Black Hole

Since the 1990s, Ghez's group at UCLA and the European team, led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, have used ever-sharper techniques to resolve the orbits of stars right around the galactic center. Earlier this summer, Genzel's team published a measurement of how general relativity is affecting the light of a star now passing close to the black hole; a similar paper by Ghez's team is now under review. "It's a remarkable moment, in terms of these experiments' ability to start probing how gravity works near a supermassive black hole," Ghez said.

But since last year, the European team has had a unique tool—the power of four giant telescopes working together in a project called GRAVITY. On a typical night, the European Southern Observatory's four 8-meter telescopes on Cerro Paranal, overlooking Chile's Atacama desert, loll in different directions on the sky. GRAVITY pulls them together using a technique called interferometry that combines observations from multiple telescopes to produce artificial images that only a preposterously huge real telescope could make.

To do this in infrared wavelengths— close to what human eyes can perceive—requires blending light in real time to avoid losing crucial information. So on July 22, when Sagittarius A* flared, the light collected by each scope traveled through a Rube Goldberg–like setup of mirrors and fiber-optic cables that traced out a path with a total length that varies no more than 1/1,000th the width of a hair, said Frank Eisenhauer, a physicist at Max Planck in Garching and the leader of GRAVITY. Then, inside a 3-ton freezing toolbox of optical tech, these light waves mixed together, their peaks and troughs combining and canceling to produce position measurements with impossible crispness.

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