Shortly after Einstein proposed his general theory of relativity, a German physicist called Karl Schwarzschild found one of the first and most important solutions to Einstein鈥檚 field equations. Now known as the Schwarzschild solution, it describes the geometry of space-time around extremely dense stars 鈥� and it has some very strange features.
For a start, right at the centre of such bodies, the curvature of space-time becomes infinite 鈥� forming a feature called a singularity. An even stranger feature is an invisible spherical surface, known as the event horizon, surrounding the singularity. Nothing, not even light, can escape the event horizon. You can almost think of the Schwarzschild singularity as a hole in the fabric of space-time.
In the 1960s, the New Zealand mathematician Roy Kerr discovered a more general class of solutions to Einstein鈥檚 field equations. These describe dense objects that are spinning, and they are even more bizarre than Schwarzschild鈥檚 solution.
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The objects that Schwarzschild and Kerr鈥檚 solutions describe are known as black holes. Although no black holes have been seen directly, there is overwhelming evidence that they exist. They are normally detected through the effect they have on nearby astrophysical bodies such as stars or gas.
鈥淣o black holes have been seen directly yet, though there is overwhelming evidence that they exist鈥�
The smallest black holes can be found paired up with normal stars. As a star orbits the black hole, it slowly sheds some of its material and emits X-rays. The first such black hole to be observed was Cygnus X-1, and there are now a number of well-measured X-ray binaries with black holes of about 10 times the mass of the sun.
Evidence for much larger black holes came in the 1960s when a number of very bright and distant objects were observed in the sky. Known as quasars, they arise from the havoc black holes seem to create at the cores of galaxies. Gas at the centre of a galaxy forms a swirling disc as it is sucked into the black hole. Such is the strength of the black hole鈥檚 pull that the swirling gas emits copious amounts of energy that can be seen many billions of light years away. Current estimates place these black holes at between a million and a billion times the mass of the sun. As a result, they are called supermassive black holes.
The evidence now points to there being a supermassive black hole at the centre of every galaxy, including our own. Indeed, observations of the orbits of stars near the centre of the Milky Way show that they are moving in very tightly bound orbits. These can be understood if the space-time they live in is deeply distorted by the presence of a supermassive black hole with more than 4 million times the mass of the sun.
Despite their names, British physicist Stephen Hawking has pointed out that black holes may not be completely black. He argues that, near the event horizon, the quantum creation of particles and antiparticles may lead to a very faint glow. This glow, which has become known as Hawking radiation, has not been detected yet because it is so faint. Yet, over time, Hawking radiation would be enough to remove all the energy and mass from a black hole, causing all black holes to eventually evaporate and disappear.
Read more: Instant Expert: General relativity