Such a star would obviously not be visible on Earth because its light would never reach us, but Mitchell argued that it might be possible to detect the presence of that star by the effect that its gravitational field would have on nearby matter (Hawking 117). The idea being implied was that light could actually be slowed by a strong enough force of gravity. According to the theory of general relativity, space and time together can be regarded as forming a four-dimensional space called spacetime (Hawking 118). This space is not flat but rather distorted and curved by the matter and energy within it. The curvature of spacetime can be observed in the bending of the light or even radio waves that travel close to a strong gravitational field. This field would be so strong around and within a black hole that not even light could escape. The boundary of a black hole is formed by an area called the event horizon, which is formed by light that fails to escape from the black hole but stays hovering around its edges.
To answer the question of whether or not black holes do in fact exist, scientists must rely on indirect evidence, i.e., light that they don't see. Suppose you have found a region of space where you think there might be a black hole. The first thing scientists .
must check for when trying to determine whether or not a black hole exists is to measure how much mass there is in that region. If there is a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there. There are two kinds of systems in which astronomers have found such compact, massive, dark objects: the centers of galaxies (including perhaps our own Milky Way Galaxy), and X-ray-emitting binary systems in our own Galaxy (internet 1). According to a recent review by two prominent astronomists Kormendy and Richstone, eight galaxies have been observed to contain such massive dark objects in their centers.
