Black Holes

Lensing

Einstein predicted that strong gravity has the power to attract not only matter (particles) but also to bend the path of light! His predictions were verified, by detecting a slight deviation of the orbit of Mercury, and a slight bending of starlight by the Sun by very accurate observations of the positions of stars during a total eclipse of the Sun. What would you see if you looked for a black hole?

[NASA/GSFC, GMU, R. Nemiroff]
Here are two computer generated pictures showing how strange things would seem. On the left is a normal star field containing the constellation Orion (notice the three light blue stars of nearly equal brightness that make up Orion's Belt, to the right). On the right is the same star field, with a black hole placed in the center of the frame. The black hole has such strong gravity that light is noticeably bent towards it – causing some very unusual visual distortion. In the distorted frame, every star in the normal frame has at least two bright images – one on each side of the black hole. In fact, near the black hole, you can see the whole sky – light from every direction is bent around and comes back to you. Black holes are thought to be the densest state of matter, and there is indirect evidence for their presence in stellar binary systems and the centers of globular clusters, galaxies, and quasars.
Here are two similar pictures, but they are real rather than computer simulations. In each case, the large gravitational force is created not by a black hole but by a gigantic cluster of galaxies. The first pictures shows the clusters A2218, composed of the horde of bright, undistorted galaxies in the frame (centered to the left). The elongated, streaked galaxies which adorn the frame are distorted images of galaxies located far behind the cluster. As the light from these distant galaxies passes near the cluster, it is bent towards it, and we observe the result.

[NASA/HST, ESA, R. Ellis, J.-P. Kneb, A. Fruchter, the ERO Team]
Cluster CL0024+1654 has a similar pattern of distortion. In this case the golden amber galaxies form the cluster, and the blue butterfly patterns are all multiple images of a distant, ring-shaped forming galaxy in the background.

[NASA/HST, W. Colley, E. Turner, J. Tyson]
The gravitational force near a black hole is so great that it can deflect light by large angles, resulting in bizarre optical effects. We can actually observe stars that lie behind a black hole, because their light is bent towards us. This diagram shows how the light from a distant spiral galaxy can be distorted by a large foreground object (a black hole, or a galaxy cluster). The large blue dot shows its true position, while the two smaller blue dots represent the way it appears to us, after being lensed by the red foreground mass (a black hole or a large galaxy cluster). See how the white light is bent around the large mass, drawn closer by its strong gravitational force.

[NMSU, N. Vogt]