Thus, external observers can never see what occurs inside the event horizon. For a 3 M Sun black hole, the radius of the event horizon is 9 km. The radius of the event horizon is proportional to the mass of the black hole. The presence of mass in the universe causes space to curve. The curvature is strongest near a very dense mass concentration, particularly a black hole.
Spatial curvature causes the paths of light rays to bend.
We observe this effect in the solar system, where light rays passing close to the edge of the Sun bend by a small angle. This was predicted by Einstein's General Theory of Relativity in 1916 and was first confirmed by observations during a solar eclipse in 1919. Thus the hole pulls harder on the near side of an infalling object than the far side. The result is to stretch out the object in the direction towards the black hole and to compress the object in the perpendicular direction. Since any collapsed object above 3 M Sun is too massive to be supported against gravity as a neutron star, the collapsed object in Cygnus X-1 must be a black hole.