Black hole. Computer model of the accretion disc around a rotating (Kerr) black hole (centre). The colours indicate the doppler shifting of radiation emitted from the disc, rotating anticlockwise around the hole. The white line separates red shifted (right) and blue shifted (left) areas. Red or blue shifting occurs when a radiation source is moving away from or towards an observer, increas- ing or decreasing its wavelength respectively. The hole's distorted appearance is due to the gravitational effects of its rotation. A black hole is a region where the gravitational field is so strong that light cannot escape. Work done at the University of Utah, USA.