About Black Holes

GRAVITATIONAL LENSE

Part I

Part II

Part III

Part IV

Part V

Part I

Another consequence of the influence of Einstein's gravitation on light is that gravitational masses can alter the direction of light and cause lensing effects. The adjacent Hubble Space Telescope image shows a spectacular example of such a Gravitational Lens. The arc-like pattern spread across the picture like a spider web is an illusion caused by the gravitational field of the cluster. The arcs are actually distorted images of very distant galaxies that are being imaged by the gravitational lens: The cluster is so massive and compact that light rays passing through it are deflected by its enormous gravitational field, much as an optical lens bends light to form an image. The figure also reveals multiple imaging, a rarer lensing event that happens when the distortion is large enough to produce more than one image of the same galaxy.

A gravitational field may cause an extended source to appear not only multiply imaged but also greatly distorted. There is at least one feature that each of the images will maintain, however, that is the same as the original source: red- or blueshift corrected surface brightness. Any radiative process preserves the specific intensity along the beam. When gravity is involved, power along the beam is not conserved, it grows or shrinks in accordance with the red- or blueshift. What is conserved is the "corrected surface brightness" B_c = B_r / (1 - R_S/r), where B_r is the measured surface brightness at r.

For example, if an observer originally saw an unlensed circular source with constant surface brightness, a gravitational field could cause the observer to see multiple, elongated, images. Each image would have, however, the same corrected surface brightness (B_c) as the original unlensed source.

The net flux that reaches the observer from any single image of the source can be either more or less than the original unlensed flux of the source. Each image will undergo an amplification A, with A not constrained to be greater than unity. This means that when considered together, the images of a source seen near a large gravitational field can have more or less flux than the same source seen without the intervening gravitational field. Essentially, there are two types of amplifications a source can be seen to undergo: time distortion induced amplifications A_time, related directly to the slowing of time in a gravitational field that causes photons to change both their energy (red- or blueshift) and the perceived arrival rate (and hence the source's perceived power integrated over all wavelengths), and amplifications in the apparent angular size of the source, A_angular. The total amplification will be designated A_total = A_time * A_angular. In the convention used here, all amplifications will be greater than zero.