More than 100 years ago, Albert Einstein explained gravity with the theory of general relativity. Till today the theory of relativity is used for example for the satellite navigation system GPS. Einstein's theory has postulated unbelievable objects: Black Holes. Their gravity is so strong that not even light is able to escape. If something falls in a Black Hole, it will never leave it again. Later, observations with telescopes confirmed that Black Holes are reality.
But how can we investigate Black Holes? They are very far away from Earth and very difficult to observe. However, stars which orbit a Black Hole are directly observable. From the orbits of the stars it is possible to derive the properties of the Black Hole, like its mass. Hence, from the point of view of a theoretical physicist, trajectories (so-called geodesics) are particularly interesting. It is possible to model the trajectories of objects orbiting Black Holes for various physical theories. In the course of this, the nature of Black Holes can be investigated.
Till today, gravity still holds a lot of unsolved mysteries for scientists. Einstein's theory of relativity does not match the quantum mechanics well. Hence, alternative theories of gravity are under investigation. In many of these theories a higher dimensional world is assumed. Furthermore, most alternative theories are very complex. Thus, only numerical models can be generated. The study of Black Holes in these theories helps to understand gravity.
Trajectories around Black Holes and their event horizons.
(Image credit: Dr. Grunau)