The mysteries of black holes.
Ablack hole is a region of space where matter has become so densely packed that its gravity will let nothing escape – including light. Time comes to a stop at its edge, and its centre may hide a point of infinitely small volume and infinite density, where all laws of physics break down.
Possessing the terrifying power to devour anything unlucky enough to pass within their grasp, it is little surprise that these cosmic chasms have become a staple of sci-fi since the term ‘black hole’ was coined in 1964.
Almost 50 years before this discovery, Karl Schwarzchild’s solutions to Einstein’s theory of general relativity had predicted how a black hole could form. In 1916, Schwarzchild predicted that if matter could be drawn tightly enough together, it could suffer a cataclysmic collapse to an infinitesimal point called a singularity, a bottomless pit in the fabric of space-time.
He also provided a useful distance from the centre of the black hole you would not want to go beyond, the Schwarzchild radius. This marks the boundary where the speed required to escape the gravitational pull of a black hole is equal to the speed of light. Beyond this point, matter and light are forever trapped. This is why we are not able to actually see inside a black hole.
Given that they emit nothing, not even light, how do we know that black holes even exist? The answer is that we have observed their effects on other celestial bodies and material that we can see.
The light fantastic
One of the brightest clues is the accretion disc; a flattened band of gas, dust and other debris from a star that has come close to the black hole but not quite fallen in. The particles within the accretion disc are accelerated to tremendous speeds by the black hole’s gravity, in the process releasing heat, X-rays and gamma rays, which can be seen by dedicated observatories. By also measuring the orbit of the star around the hidden object, scientists can infer the latter’s mass and size, and thereby confirm it is indeed a black hole. Using this technique, tens of stellar-mass black holes have been found. Similarly, many hundreds of supermassive black holes, which sit at the heart of most galaxies, have been discovered by observing the fast orbits of stars and gas, and seeing stars being literally torn apart by the strong gravitational field.
Black holes are so dense that they suck up any material that passes too close; even light can’t escape their clutches
Radiation emanating from accretion discs is one way we can infer that a black hole exists