Paul Fawbert, of Maungaraki School, asks :-

What would happen if two black holes got close to each other?

Jörg Hennig, a relativistic applied mathematician at Otago University, responded.

It can indeed easily happen that two black holes are very close. For example, the most common type of black hole appears as the final state of a heavy star, which, at the end of its life, explodes in a tremendous supernova explosion. Thereby the core of the star can be compressed into a black hole. Since many stars are found in pairs called binary stars (two stars orbiting around each other), they could eventually become a binary black hole system—if both went supernova.

For some orbiting objects (like the Earth circling around the Sun) one expects that nothing interesting happens. The gravitational force is compensated by the centrifugal force so that the two objects could go on circling forever. For black holes the situation is quite different. It was Albert Einstein’s clever idea that very heavy objects can cause gravitational waves—little ripples in the space-time continuum. In this way, two orbiting black holes would lose energy via gravitational radiation. Consequently, their orbit cannot be stable. Instead, they come closer and closer to each other until they eventually merge. At the end just one big black hole remains.

Another situation where two black holes get close to each other is the collision of two galaxies (which is indeed occasionally observed). It is expected that most galaxies contain a gigantic, supermassive black hole in their centre. When two galaxies collide, their black holes would attract each other and again merge to an even larger black hole. In this process something spectacular can happen. Typically the loss of energy via gravitational waves in the merger process is not symmetric, i.e. there should be more radiation in some directions than in others. As a consequence, the remaining black hole could gain a very high speed (as fast as 10,000 km per second!) such that it would literally be ejected from the galaxy. This is called a “superkick”.

There might be some very heavy black holes flying around out there. There are many scientists around the world who simulate the orbiting and merger of black holes on supercomputers. An example movie that illustrates the gravitational waves emitted in the inspiral can be found on the webpage of the Albert Einstein Institute (Potsdam, Germany) under the following link, http://numarch.aei.mpg.de/numrel-webpages/movies/bbh08 small.mov and the above-mentioned “superkick” is illustrated on this page of the The Center for Computational Relativity and Gravitation (Rochester, USA), http://ccrg.rit.edu/movies/numerical-relativity/2-black-hole-merger-kick