We have light waves, radio waves and, according to Einstein, we should have gravitational waves. Dan Falk reports on a multi-million-pound project that may finally prove the theory right

NO FORCE in the universe is more familiar than gravity. It keeps our feet firmly on the ground, it keeps the moon and planets in their respective orbits, and it keeps galaxies and clusters of galaxies bound in close-knit cosmic families. Some 300 years after a falling apple supposedly triggered Isaac Newton's first insights into gravity, you'd think we'd have it all figured out. And, to be sure, we do know quite a few of the details. Eighty years ago, Albert Einstein formulated the modern description of gravity - the general theory of relativity - and most of its many predictions were confirmed experimentally in the decades that followed.

But one key prediction of relativity remains untested. According to Einstein, a massive object, under certain conditions, should emit gravitational waves. These waves, a fall-out from the equations of general relativity, should be traversing the universe at the speed of light, emanating from any spot where massive objects are throwing their weight around. Because they're so weak, however, gravitational waves have so far eluded detection.

That may change in the early years of the next decade, when a number of gravitational-wave observatories begin operation. The largest of these projects consists of a pair of detectors now under construction in the US. The project is known as LIGO (Laser Interferometer Gravitational Wave Observatory) and, if successful, will open a new window on the universe.

But what, exactly, is a gravitational wave? The best way to picture one is by analogy. Suppose you're standing by the edge of a pond. You lean over, put your hand in the water, and move it back and forth. The result is a series of waves that spreads out in a circular pattern. Just over a century ago, scientists found that electromagnetic waves work in a similar way: if you take an electric charge and move it back and forth (technically, you're accelerating it) then electromagnetic waves radiate outward in a similar pattern. Radio waves are one example of electromagnetic radiation; light is another. And, according to general relativity, an accelerating mass produces gravitational waves in just the same fashion.

"In Einstein's language, gravity is associated with a warpage of space- time," explains Kip Throne, a physicist at the California Institute of Technology. "So these gravitational waves are in fact a warpage of space- time." Gravitational waves, Thorne says, are like ripples in the very fabric of the universe, stretching and shrinking space itself as they pass by.

So why hasn't anyone seen these ripples? The answer hinges on gravity's inherent weakness. Gravity, in fact, is weaker than the electromagnetic force by a factor of 10 to the power 39 (that's a one followed by 39 noughts). So while the motion of each of the planets around the sun, for example, would theoretically produce gravitational waves, the effects would be far too small to detect. Instead, the LIGO project will be on the lookout for gravitational waves from some of the most energetic phenomena in the universe - objects such as rapidly-revolving pairs of neutron stars, colliding black holes, and supernova explosions.

Though gravitational waves still await experimental detection, physicists already have good reason to believe they exist. Starting in the mid-1970s, two American astrophysicists, Joseph Taylor and Russell Hulse, made careful observations of a star system called a binary pulsar, which is a pair of small, dense stars revolving rapidly around one another. According to general relativity, the pair should radiate energy in the form of gravitational waves, and this, in turn, should cause the two stars to slow down in their orbits.

Over the years, measurements showed that the pair were indeed losing energy, and at precisely the rate predicted by the theory. Taylor and Hulse shared the 1993 Nobel Prize for Physics for their work.

"It gave unequivocal proof - in my mind, at least - that gravitational waves exist, and that they have the properties predicted by general relativity," says Stan Whitcomb, a physicist at Caltech who is the detector group leader for the LIGO project. "We know that the waves are out there."

But even detecting the gravitational waves from powerful cosmic sources such as binary pulsars will be a tremendous challenge. That's why the LIGO project, from the beginning, has been about size: even compared to the largest detectors being planned in other countries, the American project, administered by Caltech and the Massachusetts Institute of Technology, is a giant. The $360m (pounds 225m) project is funded by the US National Science Foundation.

Each of the LIGO detectors is being built in the shape of a giant "L" - two long, vacuum-filled tubes, 4km (2.5 miles) in length, meeting at a right angle. Quartz weights, each of 10kg (22lb) will be suspended at the end of each arm, and at the "elbow". Powerful lasers will send a beam of light down the length of each arm of the "L", reflecting off mirrors mounted on the weights.

Using an interferometer, the two laser beams are later combined into one. When a gravitational wave passes by, one of the detector's arms will be momentarily stretched, while the other will shrink. That change in length will be very slight - about one hundred times smaller than the width of an atomic nucleus. That should be enough, however, to pull the two laser beams out of phase and register a distinctive interference pattern at the spot where the laser beams merge.

By using two separate detectors - one in the state of Washington and one in Louisiana - any false readings at either location should be screened out.

The gravitational-wave detectors of the 21st century will show us a greatly enriched view of the cosmos. But, as with any new observing scheme, there will almost certainly be surprises. "It's not an instrument for the precise study of things that we already know about," says Whitcomb, "but a survey instrument to see things that we've never encountered in the past - signals that perhaps we've not expected at all." For it's worth pointing out that some people have suggested that rather than using radio or light waves to make their presence known, alien intelligences might signal their advanced state by communicating with gravitational, rather than electromagnetic waves. First, though, you have to catch your wave.

Dan Falk is a science journalist based in Toronto, Canada.