Where would science fiction be without its black holes? Mysterious and menacing, they have often provided a quasi-factual lift to an otherwise feeble plotline.
Disney used black holes in an implausible 1978 film of the same name. The writers of the television series Star Trek relied on the concept to get in and out of tricky situations with a bit of time travel. On one occasion, they even employed a black hole to trap the Voyager crew in a seemingly inescapable prison of darkness. Kevin J Anderson, the author of the Star Wars novel Jedi Search, opens the story with a frightening description of a cluster of black holes known as "the Maw". And the scriptwriters of the film Contact sent Jodie Foster into a swirling mass of matter, which from the special effects was evidently inspired by what little we know of black-hole physics, which isn't much.
The man we can thank for all this is John Archibald Wheeler, who died on Sunday at the age of 96. Wheeler, a physics professor at Princeton University, coined the term in 1969 to explain a phenomenon that had actually been seen as a theoretical possibility many decades earlier.
Wheeler was one of the last collaborators of Albert Einstein, the father of relativity, and as a physicist had worked on the top-secret Manhattan Project to build a nuclear bomb. His former colleague and friend Kip Thorne, professor of theoretical physics at the California Institute of Technology, said Wheeler was a giant among scientists.
"Johnny Wheeler probed far beyond the frontiers of human knowledge, asking questions that later generations of physicists would take up and solve," Mr Thorne said. And there are not many subjects that lie much further beyond the frontiers of human knowledge than black holes.
As every science-fiction buff can tell you, the reason why black holes are black is because no light escapes from them. In the simplest scientific terms, that is because they are so massive and so dense that their intense gravitational field prevents anything getting out, even light itself.
Before Wheeler came up with the catchy name, the phenomenon of light being interminably trapped in this way was generally known as a "frozen star". Even then, scientists had a fairly good idea that at the end of their nuclear-powered lives, some stars would fall in upon themselves under the influence of their own gravity.
One of easiest ways to explain the concept of trapped light is to imagine throwing an alarm clock into the air. No matter how hard you can throw it, it will always fall back to the ground because of the gravitational attraction the clock has to the more massive Earth. But, put a rocket under the clock so it can be launched above a certain speed – called the escape velocity – and it will be able to go into space, or perhaps in orbit around the Earth. The Earth's escape velocity is 11.2 km per second (about 25,000 mph).
The point about the escape velocity is that it depends on the size and the density of the planet or object in question. The Moon's escape velocity is only 2.4 km per second, because it is smaller than the Earth, as well as being less dense.
But shrink the mass of the Earth down to the size of a marble – making it extremely dense – and weird things begin to happen. At this density, the escape velocity of the Earth becomes greater than the speed of light. It effectively becomes a black hole because nothing, not even a beam of light, can escape the gravitational field of the marble-sized Earth.
When stars at the end of their life implode to become super-dense objects, they too become so dense that the escape velocity exceeds the speed of light. When that happens, they become stellar black holes. They are typically 10 times the mass of the Sun but with diameters of just 60 km or so; in other words, 10 times heavier than the Sun yet able to fit into a hole roughly the size of Greater London.
So there we have it: black holes explained. If only it were that simple. As Wheeler would have pointed out, there is far more to them than that simplistic description. One of the intriguing aspects of black holes, for instance, is that some of the strangest perturbations of time and space occur within and around them, which is why so many theoretical physicists, Wheeler included, found them so fascinating.
Take that alarm clock, for instance. Instead of trying to launch it into space, try throwing it into a black hole to see what happens. This is not such a daft proposition. If you could actually conduct this experiment, it would help to explain a lot about Einstein's general theory of relativity.
The crucial point about that theory is that massive objects with intense gravity fields create distortions in space and time. As the clock falls into the black hole, an observer outside the hole sees the hands of the clock moving progressively slower. Eventually, as the clock approaches the outer boundary of the black hole, the time appears to stop altogether and the clock's movement grinds to an apparent halt; it appears to be literally frozen in time.
But, if you were able to follow the clock into the black hole, time would not appear to slow in this way, and neither would its movement stop. The hand of the clock would appear to move perfectly normally, and instead of appearing to halt, the clock itself would continue to move to the centre of the black hole, although you and the clock would not survive.
That only goes to show that things can appear differently depending when whether you are moving or standing still, the principle of relativity.
One of the great science fiction myths of black holes is that they act as massive suction pumps that vacuum up huge chunks of the Universe. In fact, you have to get pretty close to a black hole to come under the influences of its super-dense gravity field. If the Sun became a black hole – which it will never do – the Earth would just carry on orbiting it as normal, albeit in complete darkness.
Wheeler was one of the first to realise that the critical boundary where it becomes impossible to escape from a black hole is the "event horizon". Everything inside that boundary is doomed to end being crushed at the centre of the black hole, a spooky point known as "singularity".
Just to make matters even more bizarre, the event horizon can be seen as both static and as moving outwards at the speed of light. It's rather like the Red Queen in Lewis Carroll's Through the Looking-Glass: she has to run as fast as she can just to stay in the same place.
If that is not odd enough, things get even more outlandish when theoreticians such as John
Wheeler toyed around with the idea of black holes merging. The concept that Wheeler dabbled in is known as a wormhole, that favourite element in any self-respecting science-fiction fantasy about time travel.
A wormhole is like a corridor connecting two black holes and it may be a way of distorting Einstein's space-time continuum. In other words it may be a way of moving from one place in time to another: time travel. They could even be used to travel from one universe to another; that is if "multiverses" really do exist, as some cosmologists believe they do.
But of course it is all theory; no one has actually seen a wormhole. Indeed, no one has actually seen a black hole directly, although there is important indirect evidence that they do exist, particularly the "supermassive" black holes that are understood to exist at the centre of galaxies.
Intriguingly, a new atom-smashing machine being built at Cern, the European nuclear research centre near Geneva, may create the conditions for mini-black holes to form. Theywould be sub-atomic in size and bear little resemblance to the sort of black holes that swallow up stars and planets.
Professor Stephen Hawking at Cambridge, who first postulated a theory whereby black holes eventually disappear through a process of evaporating radiation, insisted that the experiment at Cern is safe. "Particle collisions at energies far greater than those at [Cern] occur all the time in cosmic rays, and nothing terrible happens," Professor Hawking said in an email exchange with TheIndependent. "But, according to some theories of the structure of space-time, the collision might create a mini black hole.
"If that happened, then by a process I discovered, the black hole would radiate particles, and disappear... If we observed this, it would open a whole new area of physics, and I would get a Nobel Prize. But I'm not holding my breath."
So there is more to black holes than just an empty, dark space in space. For science fiction writers they can rescue a limp script and provide much-needed action. For scientists, however, they can offer insights into the weird and wondrous world of physics.