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  • Interferometry Could ’See’ Black Holes

Photonics Spectra
Feb 2001
Michael D. Wheeler

BOULDER, Colo. -- Few phenomena in the universe are as mysterious or as elusive as black holes, stellar remnants so massive that nothing, even light, can escape their gravitational pull. Now a research team from the University of Colorado and NASA's Marshall Space Flight Center of Huntsville, Ala., has proposed a novel approach that could finally make imaging black holes a reality.

For years, the prospect of imaging the material falling into a black hole has seemed a daunting, if not impossible, task that would require a telescope with at least a million times the resolution of the Hubble Space Telescope. Webster C. Cash, a professor at the university, and his colleagues theorized that an x-ray interferometer, which combines the signals from smaller telescopes to give them the resolution of one large instrument, would be more realistic and affordable.

There are compelling reasons to look to x-rays to understand black holes. As material approaches a black hole's event horizon, within which nothing can escape, its orbital velocity approaches the speed of light. Friction superheats the spiraling material to millions of degrees, causing it to emit in the x-ray band with a very high surface brightness that enables imaging with a smaller aperture.

To make the technique work, however, the team had to compensate for a serious problem: The wavelength of an x-ray is about a thousand times shorter than visible light, making x-ray telescopes difficult to build. Surface irregularities that are too small to affect visible light easily scatter x-rays, and conventional x-ray telescopes feature a quasi-cylindrical optical surface that is very difficult to figure and polish.

The researchers sidestepped this problem with an interferometer design -- which they described in the Sept. 14, 2000, issue of Nature -- that uses four flat mirrors. In the process, they devised a way to mix the x-ray wavefronts without using a beamsplitter.

Looking for holes

Because x-rays cannot penetrate Earth's atmosphere, up to 33 spacecraft carrying the optical mirrors, flying in a set formation and focused on a spacecraft equipped with the x-ray detector, must be deployed. As a precursor to this plan, NASA is investigating a Pathfinder mission in which the x-ray optics would be housed on a single spacecraft.

The intense gravity of a black hole causes distortions of space-time that are imprinted on the emitted x-rays, so the final interferometer could answer many mysteries of the universe. "Watching the behavior of matter in the ultimate gravitational field will doubtless give us new information about nature in its extremes and the exact nature of black holes," Cash said. "Imaging black holes will give us a unique, new probe of the nature of space-time itself, and check general relativity in a new way.

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