Steve Connor: The black hole we can almost see

Time slows down as you enter the 'singularity' of a hole, theorists say

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Black holes are the weirdest of things. They describe one of the strangest of cosmological phenomena – a volume of space from which no light escapes. Indeed the scientific theorists tell us nothing ever escapes from the depths of a black hole. Not even a government memory stick.

For many decades the very concept of a black hole was as fantastical as the idea that we could ever see one, which was thought to be nigh-on impossible given that they represent the ultimate in photographic underexposure. Because they trap light, it seemed as if black holes would for ever remain as mysterious as the physics that led to their theoretical conception.

But now, painstaking observations by astronomers have begun to illuminate the darkest recesses of the known universe. The latest study took 16 years of data gathering by a team of German astronomers using the European Southern Observatory's flagship telescopes sited on a bone-dry mountain top in the heart of Chile's Atacama desert.

By watching the motions of 28 stars in the centre of our galaxy, the astronomers were able to study the supermassive black hole known as "Sagittarius A star" lying at the heart of the Milky Way. Although the black hole itself is invisible, its immense gravitational influence is detectable from instruments on Earth because this is the force that influences the movements of the nearest stars within the galactic centre.

The astronomers measured the positions of the stars with phenomenal precision. If a star moved at an angle of just 300 microarcseconds, its change in position was duly logged. To give some idea what this means, it's equivalent to being able to see a £1 coin from a distance of about 6,200 miles.

"Undoubtedly, the most spectacular aspect of our long-term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist," extolled Reinhard Genzel of the Max-Planck-Institute for Extraterrestrial Physics in Garching near Munich. "The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."

This, of course, highlights another aspect of black holes. They are very heavy – or massive, to use the more appropriate term. They are thought to result from the collapse of stars at the end of their nuclear-powered life, resulting in an object so small and dense that the escape velocity necessary for anything to leave its gravitational pull is actually greater than the speed of light – which is why nothing can escape, because nothing can travel faster than light. We can thank the American theoretical physicist John Wheeler, who died earlier this year, for coming up with the term black hole. Until then, the notion of light being trapped by a super-dense object was generally described as a "frozen star". Wheeler's black holes have a far more sinister ring to them.

This is probably why they have been the mainstay of so many science-fiction stories, from Star Trek and Dr Who, to the film Contact, starring Jodie Foster, who was sent into a swirling mass of matter influenced by black hole physics.

The other great aspect of black holes, of course, is that spooky things happen if you ever come too close to them. The theorists tell us that time slows down as you fall into the "singularity" of a black hole, and you can actually travel through time if two black holes merge to form a "wormhole" in space.

The journey to discover black holes has only just begun. We may not be far away from witnessing the truly weird and wonderful forces of nature beyond the immediate confines of our little Solar System at the edge of the Milky Way.

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