AJ Kell, of Balclutha Primary School, asks :-

Why cannot light escape a black hole?

Jörg Frauendiener, a mathematical physicist at the University of Otago, responded.

The answer to this question has been given already several hundred years ago by Pierre Simon de Laplace, in fact long before the notion of a black hole was around.

His explanation follows from the fact that light travels at a very high but finite speed, namely roughly at 300,000 km/s and goes as follows. Imagine yourself standing on the surface of the earth and throwing a stone upwards in the air. We all know that it will go higher up if you throw it harder but that it will slow down and eventually fall back. But it is simple to imagine that if you throw the stone hard enough so that it gets very fast initially then it will never come back. In fact, you will have to throw the stone with an initial velocity greater than about 11 km/s if you don't want it to come back.

This initial escape velocity of 11 km/s has to be overcome by rocket scientists who construct rockets and satellites leaving the earth for outer space.

Imagine now a much more massive planet or star. If one wants to leave the gravitational attraction of such a star behind one needs to have a much larger escape velocity. And if this star is so massive that the necessary escape velocity is larger than the speed of light then not even light is fast enough to get away from it. Since nothing can move faster than the speed of light this also means that nothing can escape the pull of such a huge massive object. Since no light gets away from it, it will `look' like a very dark black hole in the sky.