A hole in space seems to make no sense at all, yet scientists are convinced that these prisons of light are for real, even though they have never really been seen and the only evidence for their existence is circumstantial.
But astronomers have now got close to staring a black hole in the face. With the help of an array of 10 radio telescopes in America, they have pictured the void at the centre of our own Milky Way galaxy 26 million light years away, where a supermassive black hole sits invisibly like the transparent eye of a hurricane.
This particular black hole is estimated to have a mass equivalent to four million Suns and yet the latest measurements, published in the journal Nature, suggest it occupies a volume with a radius less than the distance between the Earth and the Sun.
This is less than half the size previously estimated, indicating that astronomers are close to defining the crucial outer boundary of one of the most elusive phenomena in cosmology - one that has mystified scientists for decades. "We're getting tantalisingly close to being able to see an unmistakable signature that would provide the first concrete proof of a supermassive black hole at a galaxy's centre," said Zhi-Qiang Shen, of Shanghai Astronomical Observatory in China, one of the leaders of the study.
No light escapes from black holes, which is why they are so invisible. They can however be detected by the radiowaves emitted from their periphery as they gobble up any surrounding matter that falls within range.
The first sign of a black hole within our own galaxy came in February 1974, when two American astronomers, Bruce Balick and Robert Brown, detected a powerful source of radiowaves emanating from the constellation of Sagittarius. Balick and Brown calculated that, whatever the cause of the radiation, the source was coming from the dead centre of the galaxy. They suspected a black hole at the heart of the Milky Way and the race began for astronomers to capture the first image of this radio source, which they called Sagittarius A* (pronounced "A-star").
"Black holes are perhaps the most exotic objects to impinge on the cosmic consciousness," explains astronomer Christopher Reynolds of the University of Maryland, writing in Nature. "They are formed when matter such as that from a dying massive star collapses in calamitously under its own gravity, forming a region of space in which the gravitational field is so strong that it swallows all matter and radiation that come near it."
One way of looking at black holes is how they distort space and time. Imagine the space-time continuum as a rubber sheet. An object the size of the Sun would act like a heavy ball placed on a trampoline, causing a minor indentation. Heavier objects, such as cannonballs, would create further indentations in the space-time continuum but something as heavy and dense as a black hole would cause such a steep dent that it would be like a bottomless pit from which nothing could escape once it fell in.
>This view of black holes comes with the benefit of Einstein's theories of relativity. But the concept actually predated his pioneering work. In fact, black holes, like many cosmological phenomena, were predicted long before scientists began to construct the sort of instruments that could detect them.
Indeed, an English clergyman and scholar called John Michell predicted in 1784 that some stars might be so big, and hence so heavy or massive, that they would create a gravitational field strong enough to prevent light from escaping. If something was 500 times bigger than the Sun, the Rev Michell wrote, "all light emitted from such a body would be made to return towards it, by its own proper gravity".
Such predictions were based on what was known at the time from Isaac Newton's work on gravity. It was not until after Albert Einstein formulated his general theory of relativity that further work could be done on the theory of black holes - although no one actually called them by that name until 1967.
Using Einstein's theory, Karl Schwarzschild, a German physicist, discovered that relativity equations led to the predicted existence of an object so dense that other objects would fall into it and never come out again.
Schwarzschild talked about a "magic sphere" around such an object where gravity was so powerful that nothing within that sphere could escape. Furthermore, all matter within the sphere would be crushed to a point of infinite density occupying virtually no space. This point is known as the "singularity" and every black hole is believed to have a singularity at its centre.
J Robert Oppenheimer, the father of the American atom bomb, calculated that a black holes was the ultimate end-product of a star's lifecycle, the point when it collapsed in on itself and the resulting ultra-dense material gave rise to a singularity.
But the real turning point came in 1967, when the American astrophysicist, John Wheeler, actually coined the term "black hole" - and launched a wave of popular fascination with these gravity-defying voids.
In 1971, the first experimental evidence from space for the existence of black holes came with data captured by the American Uhuru satellite. Its instruments detected a source of X-rays coming from a star that appeared to be orbiting an invisible companion that was estimated to be five times the mass of the Sun.
This was the first of several contenders for the "smaller" kind of black hole caused by the collapse of a stellar objects. But in more recent years scientists have been chasing much, much bigger black holes.
These black holes are supermassive affairs, like the one at the centre of our own galaxy which is estimated to weigh in at about 4 million times the mass of the Sun.
But astronomers believe there are even bigger ones, 10 billion times the mass of the Sun, at the heart of every galaxy, said the cosmologist Marcus Chown, author of The Universe Next Door. "No one knows how they form. No one knows why they are at the centre of galaxies. It's even possible they were there first and seeded the formation of galaxies such as the Milky Way."
Such is the mystery surrounding black holes that a small minority of scientists still cannot quite bring themselves to believe in them. "The truth is we don't absolutely know for sure that black holes exist. No one has actually seen a black hole, " explained Mr Chown.
This is why the latest study is so important, because scientists are getting so tantalising close to taking that first image of a black hole in all its mysterious splendour. But what would "nothing" look like? How can we take an image of something that swallows up all matter and radiation?
Professor Reynolds said that we may not be able to see a black hole itself, but we should be able to see the boundary or "event horizon" beyond which all matter is swallowed up. "What is needed is a more discerning test than simply detecting something massive and compact; we need to find the event horizon, the defining property of a black hole," he said.
"As physical phenomena go, event horizons are tricky to observe... High-resolution imaging, however, does provide a compelling way to search for an event horizon. If a black hole is surrounded by an almost spherical distribution of radiation matter ... a sufficiently high-resolution image should reveal a shadow around it.
"This dark circle is caused by radiation from sources behind the black hole that are being swallowed by the event horizon. Surrounding this shadow would be a bright ring - the result of the strong deflection by the black hole's gravitational field of those light rays that do scrape past it."
Fred Lo, director of the US National Radio Astronomy Observatory, which runs the array of telescopes that collected the latest data, said that, with a slightly higher resolution, telescopes should soon be able to see this shadow of a black hole.
"The extremely strong gravitational pull of a black hole has several effects that would produce a distinctive 'shadow' that we think we could see if we can image details about half as small as those in our latest images," Dr Lo said. "Seeing that shadow would be the final proof that a supermassive black hole is at the centre of our galaxy."
Mr Chown said that the best way to get the final elusive proof of the existence of supermassive black holes is to observe the one that is closest to us. "The proof will be to see a bright ring with a dark region inside it - presumably, the bright ring is matter super-heated as it falls into the black hole and the dark region is the black hole," Mr Chown said. "Fred Lo and his people have come the closest yet to getting that proof."
Black holes are so strange that they may defy the laws of physics as we know them, for instance by creating "wormholes" in space. When we are finally able to see black holes with our own eyes, we may have also found gateways to other universes.
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