One of the major principles of Einstein's theory of relativity is that space-time is curved around a massive object such as a black hole.
In a geometry that is curved (as opposed to flat or Euclidean), objects and photons followed curved paths rather than straight lines, and the sum of the angles of a triangle is not equal to 180o.
According to Einstein's theory, the warping of space-time around the Sun causes the observed advance of the perihelion of Mercury and the bending of starlight as it passes close to the Sun.
