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Issue 3.07

The Astrophysics Spectator

April 12, 2006

This badly-delayed edition of The Astrophysics Spectator adds a new simulator to the site: the Schwarzschild lens simulator. This simulator shows the appearance of the sky from the gravitational lens of a zero-angular-momentum black hole.

Our preconception of a black hole's appearance is inevitably colored by the movies, which often depict the black hole as a colorful whirlpool. But in nature a black hole would have a very different appearance. The black hole candidates we see in binary star systems and at the centers of galaxies would appear as a dark region at the center of an accretion disk. More striking would be the appearance of a black hole isolated in space, because it would be invisible except for its distortion of the sky.

As with a star, the gravitational field of a black hole bends the path of passing starlight. But while the effects of a star's gravitational field are mild, so that the “gravitational lens” of a star is only effective when the star is far from us, the effects of a black hole's gravitational field are dramatic, and its gravitational lens is apparent when one is very near the black hole. Starlight that passes near a black hole can orbit the black hole several times. This means that we see not only the starlight from stars beyond the black hole, but also the starlight from the stars behind us. The black hole acts like a crystal, reflecting the full sky onto an infinite series of annuli, so that the “black hole” is better thought of as a “black crystal.”

Next Issue: The next issue of The Astrophysics Spectator is scheduled for publication on April 26.

Jim Brainerd

General Relativity

Schwarzschild Lens Simulator. This simulator shows the effect of a Schwarzschild black hole on the appearance of the sky. The black hole acts like a crystal, creating an infinitude of images of the whole sky that lie on annuli encircling the black hole. The simulator permits the reader to rotate the sky relative to the black hole. The black hole has its strongest effect on starlight when a star is directly behind it; the surface area of the star's image is very large, wrapping completely around the black hole. A star behind the observer also has an image that wraps around the black hole, but the image's width on the sky is very narrow. While the simulator calculates the four largest images that the black hole creates of a source, only two are generally visible. (continue)

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