Conceptual computer illustration of an event horizon, theorised to exist around a black hole. Black holes are objects of extreme density, with such strong gravitational attraction that even light cannot escape from their grasp if it comes near enough. The theory of general relativity predicts that a sufficiently compact black hole can deform spacetime. Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation. Hawking radiation is black body radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. It is defined as "the point of no return", the point at which the gravitational pull becomes so great as to make escape impossible. The illustration also evokes the image of the Union Jack.