Black holes are the most extreme astrophysical phenomena in the universe. And there’s a good chance they’re the best known, too. Not everyone has heard of neutron stars, planetary nebulae or brown dwarfs, but most people have come across the idea of a black hole, even if it was only in a science-fiction film. Part of the fascination lies in the way black holes seem to break – or at least twist beyond recognition – all the common-sense laws of physics. Looked at more closely, though, they’re really just an inevitable consequence of these laws.
As a theoretical concept, black holes first made their appearance when scientists were thinking about gravity, particularly in the context of large objects such as stars. Gravity is a force that acts between particles of matter, tending to pull them closer and closer together. In everyday objects in the world around us, the force of gravity is counterbalanced by other physical effects such as electrostatic and nuclear forces – with the result that things maintain a finite size rather than collapsing down to a point. The same is true of stars like the Sun, where gravity is counteracted by fluid pressure. Even in an ultra-compact star like a white dwarf, where a mass similar to that of the Sun is squashed down to a planet-sized volume, there are other more exotic forces that come into play to stop them collapsing forever.
SUPERMASSIVE BLACK HOLES CAN ACCELERATE PARTICLES ALMOST TO LIGHT SPEED
By the middle of the 20th century, however, scientists knew there was an upper
