Scientists believe they will soon be able to detect 'gravity waves'

Scientists believe they will soon be able to detect 'gravity waves', confirming what Einstein predicted nearly a century ago - and opening up vast areas of space never seen before. Steve Connor reports

They come from the furthest depths of space and are born out of some of the most violent events imaginable - from the explosions of stars to the collisions of black holes. Yet they are one of the most elusive phenomena in the Universe, so elusive that there is every chance that they have passed straight through your body without your realising it.

Albert Einstein was the first to predict the existence of gravitational waves when he formulated his general theory of relativity in 1916 but no one has ever actually detected one. They are supposed to be generated by the high-speed movements of exceedingly massive objects, yet proof of their existence has been largely theoretical.

That could be about to change with the official switch-on of one of the weirdest experiments in Europe. It is called the Gravitational Wave Detector and is a joint effort by British and German scientists to finally capture the essence of what Einstein's theory predicted nearly a century ago.

The instrument in Hanover will, from this week, run continuously for 18 months in the hope of finally capturing the essence of a gravitational wave - a ripple or a distortion in that relativistic entity known as "space-time".

If the experiment succeeds it will do more than prove Einstein right. It will also provide a new insight into the hidden fabric of the Universe - such as the mysterious "dark matter" that makes up more than 95 per cent of the cosmos yet which cannot be seen using ordinary telescopes.

"Conventional astronomy on the whole looks at what comes from the outside of stars," says Professor Sheila Rowan of Glasgow University. "Because of this, we don't really know what happens inside stars. But being able to detect gravitational waves allows us to look at what is coming from the inside of the darkest regions of space. It is a way of seeing the inside of objects."

The detector in Germany will work in close collaboration with three other machines in the US, which together form a global instrument that should for the first time be sensitive enough to measure gravitational waves. It takes the combined efforts of at least four gravity detectors to be sure that a positive result is genuine and not a false alarm - which was sadly the case with one premature announcement of a "discovery" in the late 1960s.

A machine that can detect and measure gravity waves will act as a new kind of telescope that can see into the darkest reaches of the universe, normally inaccessible via the telescopes used in conventional light and radio astronomy. Gravitational wave astronomy should be able, for instance, to amass precise information on the distribution of neutron stars and black holes as well as on the detailed course of cosmic catastrophes such as the collapse and explosion of stars or the merger of two compact stars into a black hole.

Even the gravitational remnants of the big bang - the event 13 billion years ago that created the Universe - may be detectable as the waves cross deep space.

"The first step towards gravitational wave astronomy has been taken, at last allowing us to observe the 96 per cent of our universe which has been hidden from us up to now," says Professor Karsten Danzmann, head of the International Centre for Gravitational Physics at the University of Hanover.

"If there is a supernova in our vicinity during the next couple of months, our chances of detecting and measuring the resulting gravitational waves are good," Professor Danzmann says. "We are opening a wholly new chapter in the long history of astronomy with the direct observation of the 'dark side' of our universe - black holes, dark matter and the reverberations of the big bang."

Gravitational waves are generated by extremely massive objects as they move through space at incredible speeds. The gravity of such objects distorts space-time and sends ripples through the Universe that stretch and compress all other objects in their path.

But such waves are so weak that they have escaped the best efforts of physicists to detect them over the past 40 years. Even if a supernova were to explode within our own galaxy, the gravity wave it produces would dissipate quickly by the time it reaches Earth.

In fact such a gravity wave would distort the entire distance between the Earth and the Sun by the distance of just one atom, and then only for several hundredths of a second. Since we want to detect gravitational waves from more distant galaxies, the sensitivity has to be a thousand times greater. This means that over a distance of about one kilometre, an instrument would have to detect a distortion of a thousandth of a diameter of an atomic nucleus.

The detector in Germany, called the GEO600, is set to remedy the situation with the help of laser beams running down the two 600m-long arms of the instrument, which are set at right angles to one another. Mirrors at the centre and each end of the two arms carefully bounce two laser beams along each arm. These beams are sent back to the centre where they interfere perfectly with one another in such a way that they cancel each other out - and the light at this point becomes total darkness.

If a gravity wave passes through the instrument it should distort the distance between the mirrors at the ends of the two arms. This distortion should eliminate the "destructive" interference of the laser beams with the result that light shines where there was once darkness. If this happens, then a gravity wave is detected. It is a most exacting technical challenge because the distortion distances involved are fractions of the width of a human hair.

The American budget for its gravitational wave detectors is $365m (£200m) whereas the Europeans had to manage with just €7m (£4.8m) - with Britain's contribution coming through the Particle Physics and Astronomy Research Council.

Many technical challenges face the experimenters, ranging from dust particles contaminating the mirrors, to major earthquakes that would shake the ground enough for the detectors to be shut down until the aftershocks have subsided.

Since its first test run in 2002, the sensitivity of the Hanover instrument has been improved, says Karsten Danzmann. "We could only reach out towards a small fraction of our own galaxy, the Milky Way, in those days. Today, our sensitivity has increased by a factor of 3,000 and we can detect events from distances many times greater than that between us the Andromeda galaxy," he says.

Professor Jim Hough of Glasgow University, who is one of the British scientists involved in the experiment, is optimistic that a gravitational wave will soon be found. "I think the chances of detecting gravitational waves over the next two years are something like 50:50, but as the sensitivity of the instruments is improved, will reach guaranteed detection by 2013," Professor Hough said.

"I hope very much that we'll detect one before I retire in four years time," he said.

Apart from his professional interest, Professor Hough, along with many of his colleagues, have more personal reasons for finding gravitational waves.

Last August the bookmaker Ladbrokes opened the betting on detecting gravity waves by 2010 at 500:1, which led many particle physicists to rush out and place a bet. "To those of us working on gravitational waves this was an opportunity not to be missed and we quickly staked the maximum amount allowed by the bookmakers," Professor Hough says. He manage to place a £25 bet when the odds had slipped to 100:1. Within a few weeks of the book being opened, the odds had shortened to 2:1.

"It would appear that gravitational wave physicists have more confidence in their experiments than Ladbrokes," Professor Hough said.

What is a gravity wave?

Isaac Newton was first to realise that gravity was a force, which assumed that space was something of a "flat" entity.

Albert Einstein's theory of general relativity saw gravity not as a force but as a consequence of the curved geometry of something called space-time - the combination of space and time.

The basic idea is that matter tells space-time how to curve and space-time tells matter how to move. The Newtonian view of gravity is that massive objects attract one another, whereas Einstein's view is that as objects move they warp space and therefore influence each other's path - causing an apparent attraction.

Extremely violent events - such as colliding black holes - give rise to perturbations in space-time that spread in all directions as gravitational waves.

They propagate at the speed of light but their energy dissipates quickly, like ripples on a pond, making them difficult to detect far from their original source.

The stars in July

It's that time of year again - barbies, booze and nights outside when you swear you're seeing two stars where there should only be one. But don't blame the beer. Your eyes might be telling the truth...

Double star systems (even multiple star systems) are the norm in the Galaxy. More than two-thirds of all stars are partnered up, and our Sun is unusual in being a singleton. It's all down to the way that stars are born. They form in nests of dark dust and gas, and - although some stars go off on their own - the majority cling together for the duration.

This month, look towards the Plough (the central seven stars of Ursa Major, the Great Bear) for the classic example of a double star. Just at the "kink" towards the end of the bear's tail, you'll see Mizar and its fainter neighbour, Alcor. The Arabic names have been translated many times, and the latest interpretation is "the horse and rider".

Actually, Mizar and Alcor - despite what most books tell you - aren't a double star system. They travel together through space, but they're not in orbit about each other: at more than 20 light years apart, there's no way they could be. But Mizar and Alcor do have their own companions, too close to the parent star to make out with a telescope.

Now it's time to look overhead. The brilliant star Vega (in Lyra) is reaching the zenith, and next to it (left) you can spot the star Epsilon Lyrae. Through binoculars, you can see that it's a double star. But a small telescope reveals more: it's actually a double-double. Two pairs of stars orbit each other, making it a beautiful quadruple family.

The most stunning double star system in our region of the Galaxy is just a glance away. The glorious constellation of Cygnus, the Swan, is just to the left of Lyra. Bright Deneb marks its tail; Albireo (Arabic for "the hen's beak") marks its head. Albireo is a pair of dazzling blue and golden stars against the blackness of space. The colours? They're a result of the stars' temperatures: blue, extremely hot; yellow, moderate hot like the Sun.

You'll need a small telescope for this one, but you won't be disappointed. As the venerable Victorian astronomer Agnes M Clerke observed: "perhaps the most lovely effect of colour in the heavens".

WHAT'S UP

Jupiter is the king of the night sky. Our Solar System's greatest world is an incredible sight through a small telescope, with its constantly changing cloud patterns, wrapped in stripes around its tangerine-shaped body. And its ancient Great Red Spot - a permanent hurricane - has been joined by a junior pretender to the throne in the shape of a small red spot. It's "only" the size of the Earth. Around 10 July, the two spots will pass one another, and there's a chance the Great Red Spot may swallow up its younger sibling.

Binoculars will show the four largest of Jupiter's family of more than 60 moons. On the night of 2 July they are stretched out in order - Io, Europa, Ganymede and Callisto - to the right of the planet.

The summer constellations are now kicking in, with the stars of the Summer Triangle (Vega, Deneb and Altair) dominating the skies. And on a clear night, the Milky Way - which runs through the Triangle - is a glorious sight.

Heather Couper and Nigel Henbest