IN CENTRAL Washington State, far from any town, two white pipelines stretch across the desert. At first glance, they look as though they might carry gas or oil. In fact their cargo is laser light. To some, these pipes are part of the most exciting scientific instrument ever built. To others, they are a giant folly: a $400 million lab that won鈥檛 tell us a thing. Even enthusiasts think the pipes probably won鈥檛 detect anything until they are refitted several years hence using as yet untested technology. Should we be outraged?
The instrument, called LIGO, is the latest and the largest attempt to find gravitational waves. According to Einstein鈥檚 general theory of relativity, any violent event such as the birth of a black hole or the collision of two dense stars should send out ripples in space-time, very like sound vibrations in air, which squeeze and stretch everything they pass though. We should be able to 鈥渉ear鈥� these gravity ripples 鈥� if only we build ourselves a good enough ear.
The sensitivity required is almost unimaginable. Even the most violent cosmic explosions are predicted to squeeze and stretch space-time by less than a thousandth of the diameter of a proton per kilometre. So you need an incredibly sensitive device to be able to measure them. Physicists have already tried suspending big metal bars to see if they are set ringing by passing waves. But so far without success.
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LIGO is different. Laser beams sent down two pipes are bounced back and forth by mirrors before being recombined. A change in the lengths of the pipes changes how the light waves interfere, so when a gravity wave passes, squashing one pipe and stretching the other, the point of overlap dims or brightens. The longer the arms, the stronger the effect. That鈥檚 why LIGO is a giant. Its two interferometers, one in Washington the other in Louisiana, each has arms 4 kilometres long. They鈥檝e been running since January, and by the end of this year it鈥檚 just possible that they will have actually seen a gravity wave.
Just possible. In its present form, even this costly instrument is probably still too crude to see anything. A refit planned for 2006 should make the instrument vastly more sensitive, according to LIGO鈥檚 chief theorist, Kip Thorne. But one critic accuses Thorne of 鈥渃asually imagining an improvement of a factor of 100 in a detector already pushed to the limits of current technology鈥�. Instead of jumping straight to this 4-kilometre monster, the LIGO team could have developed better technology first on a smaller machine. They might have found that it would lead to a completely different way of doing things.
Why plan to build an instrument that you don鈥檛 expect to work, in the hope you can later rebuild it so it will? One answer is that you can get experience with running a big machine, and meanwhile test out new techniques on smaller ones. For example, the team at another detector in Germany, called GEO 600, have found a new way to amplify the gravity-wave signal using an resonant cavity 鈥� a technology that could be adopted in LIGO II.
So it鈥檚 a gamble: spend some extra cash for a chance of getting there several years sooner, provided the technologies all work together. On the face of it, that doesn鈥檛 sound like a good bet if being a little more patient means we could get the same results for less.
To some extent, this echoes every debate about big science. In 1991 the Superconducting Supercollider, a giant 86-kilometre-long particle accelerator, was canned. Though it promised a profound advance in particle physics by hunting for the Higgs boson and other exotic new particles, $11 billion seemed too steep. By 2006 a European accelerator called the Large Hadron Collider should be answering many of the same questions for just $1 billion.
That鈥檚 still a fair wad of cash, but unless you abandon science altogether you have to cough up at some point. So when is the time right? There is no hard and fast formula for assessing the value of fundamental science. If you try to balance it with how much other science could be done with the money, what鈥檚 the exchange rate? How many new planets are worth a 3-1 bet on one gravity wave? As big science depends directly on persuading politicians, a lot depends on the skill of the advocate. LIGO has been lucky to have the highly articulate Thorne, whose entertaining talks may be one of the main reasons LIGO got the cash.
LIGO鈥檚 unusually chancy nature may have been hidden by such razzle-dazzle. But in one sense the showmanship is justified: even among big science projects, LIGO鈥檚 goal is breathtaking. If LIGO and its kin one day succeed, we鈥檒l see an entirely new kind of radiation, waves not of electricity and magnetism but of gravity. There is something uniquely mysterious about gravity, because of its profound link with the nature of space and time. To see gravity travelling would be a tremendous achievement. And the sources of gravity waves are fascinating too: black holes, neutron stars, the big bang.
It鈥檚 too late now for any debate to affect LIGO. But the same issues apply to another, far larger device. LISA, a gravity-wave project backed by NASA, would be an interferometer not 4 kilometres across, but 5 million kilometres 鈥� a group of spacecraft flying around the Sun in precise formation. It would search for long-wavelength gravity waves from the first instant of the big bang and from giant colliding black holes. Even more than LIGO, LISA will be pricey and dicey. Will it be worth the money?