SHAKE your fist and you shake the Universe. Just by moving the mass of your hand back and forth, you are sending out ripples in space and time-pieces of travelling gravity that distort everything they meet. These subtle waves will race outwards, warping the substance of the Sun after about eight minutes, and then heading on into the vastness of interstellar space.
Einstein said that these gravitational waves should exist, but they have never been detected directly. Later this month, however, two giant detectors, one in the US and the other in northern Germany, will switch on and link up in an unprecedented global search. Before long, the astronomers who have built these instruments hope to be feeling the vibes from violent supernova explosions, merging black holes and colliding knots of nuclear matter. And, because gravitational waves move through the Universe unhindered by matter or energy fields, once we can see them they will give us an entirely new-and possibly shocking-view of the cosmos. 鈥淣ature is much more imaginative than we are,鈥� says David Shoemaker of MIT. 鈥淢ost of what we discover will be things we鈥檝e never thought of.鈥�
To reveal nature鈥檚 imagination, scientists have been hunting gravitational waves for more than 40 years. The first to try was Joseph Weber at the University of Maryland in the 1960s. He set up an aluminium bar about one-and-a-half metres long as an antenna. The idea was that the passage of a wave would set it ringing, and that would be picked up by piezoelectric sensors wrapped around the bar鈥檚 middle.
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Weber did claim to have detected gravitational waves, but many subsequent experiments in the 60s and 70s failed to back him up. The problem is that gravitational waves are subtle. According to Einstein鈥檚 general theory of relativity, space and time form an inseparable continuum called space-time, and what we feel as the force of gravity is actually matter and energy causing space-time to bend. So space-time behaves rather like a rubber sheet, and all masses distort this sheet.
But if a mass is accelerated-like a shaking fist or an orbiting planet-then its effect on space-time fluctuates. The wobble in its gravitational influence spreads out in all directions, moving at the speed of light. Because these waves are actually changes in space-time, they warp any object they meet, alternately squashing and stretching it in different directions.
But space-time is stiffer than rubber. So shaking your fist disturbs it only by an infinitesimal amount. The Earth in its orbit is a stronger source of gravitational waves, but even with more than 1021 tonnes of rock hurtling around the Sun, the ripples produced are tiny. To generate something detectable, you need a lot more matter accelerating far more violently. An exploding star would do, for example, or two colliding black holes. Or maybe two orbiting neutron stars-hugely dense balls of subatomic particles, spiralling slowly in towards a cataclysmic impact.
Such events are rare, and we鈥檙e extremely unlikely to witness one in our neighbourhood any time soon-and just as well, since it could be devastating to our planet. But this means that if we鈥檙e to have any hope of observing gravitational waves, we need a detector capable of seeing such an event at least on the other side of our Galaxy, and preferably far beyond that. It鈥檚 a significant challenge: the signals start small, and as they spread out they fade in strength.
Astrophysicists reckon that to stand a good chance of detecting about one burst of waves per year, they鈥檒l have to be able to spot distortions of about one part in 1021-equivalent to a change in the distance from London to New York by the width of a uranium nucleus. To reach such astonishing sensitivity, physicists and technicians across the world have spent more than 20 years developing a new and very different kind of detector.
In the early 1970s, Rainer Weiss of MIT suggested using laser interferometers. The interferometer has two arms at right angles to each other, and the idea is to compare the lengths of these arms using a laser. A laser beam splits in two where the two arms join, and part of the beam races up each arm (see Graphic). The light bounces off suspended mirrors at the ends of the arms, and returns to the splitter, where the two beams are recombined. Each beam of laser light is coherent, with all peaks and troughs in phase, so when recombined the crests of one may coincide with the troughs of the other, and they cancel each other out.
You can set up your interferometer so that the beams produce this perfect destructive interference. When the beams combine at a light detector, the detector will see nothing. But when a gravitational wave comes by, it briefly alters the length of the light鈥檚 path, stretching one arm and squashing the other. This changes the relative phase of the two beams. They no longer cancel perfectly, and your detector registers some light. Time to crack open the champagne.
The chances of a detection will be increased if you insert an extra 鈥渞ecycling鈥� mirror between the source laser and the interferometer, so the beams bounce back and forth, up and down the arms about a hundred times. This forms a resonant cavity, so a slight change in the arm length has a stronger effect on the interference of the beams. It also means that the number of photons in the system is much larger, allowing these devices to detect yet smaller wobbles, because a given slight change in the interference means a stronger signal at the light detector. 鈥淚f you have enough photons you can measure to any fraction of a wavelength,鈥� says Kenneth Libbrecht of Caltech in Pasadena.
And the longer the arms, the more sensitive the instrument will be. So the new gravity wave detectors are huge. The biggest of them is LIGO, the Laser Interferometer Gravitational-Wave Observatory, which has 4 kilometre-long arms.
LIGO actually comprises two observatories. One is in Livingston, Louisiana, and the other is at Hanford in Washington State, 1900 miles away. They are identical, except that the Hanford observatory also houses an interferometer with 2-kilometre arms. This provides a way to test whether an observation is genuine, as a gravitational wave should stretch a 2-kilometre arm exactly half as much as a 4-kilometre arm, producing a signal half as strong. Most other disturbances would affect both detectors about equally.
But the best way to tell if an observation is valid is because genuine gravitational waves should affect both detectors, despite the distance between them. Any spurious disturbance, such as a ground tremor or glitch in the instrument鈥檚 electronics, will hit only one site.
On 28 December, Livingston and Hanford will run in parallel for the first time. And on the same day, another observatory called GEO 600 is due to switch on in Hanover, Germany. So that date will mark the birth of a new network of interferometers, a global observatory that many astronomers believe will be our first real chance to see Einstein鈥檚 elusive gravitational waves.
It will be much more than just a test of Einstein鈥檚 theories, however. Whenever we have found a new way of looking at the Universe, it has produced surprises. And gravitational waves pass unhindered through dust, stars, galaxies and anything else, so they should allow us to observe events that would normally be hidden if electromagnetic radiation were the only way to look. 鈥淎 lot of the Universe is hidden in the centres of galaxies, and gravitational waves might allow us to see some new phenomena coming from those regions,鈥� says Libbrecht.
All in all, gravitational waves may reveal some of the strangest things in the Universe (see 鈥淥ut of the darkness鈥�). The new instruments will need a lot of tinkering to reach their full sensitivity, but it could be reached within a year.
All over the world
Having a global network of gravitational-wave detectors is vital, not just for checking the validity of any potential signal. It also becomes possible to compare arrival times, and work out where the waves came from. Adding this to information from individual detectors about the waves鈥� strength and wavelength and the way they rise and fall, physicists hope to work out just what produced them.
Aside from LIGO and GEO, there are two other main interferometric observatories. VIRGO, a 3-kilometre giant near Pisa in Italy, is due to come online in 2003. TAMA is a smaller set-up in Japan, with arms 300 metres long, but it is already up and running. AIGO, the Australian International Gravitational Observatory, is also being planned.
Although the GEO observatory is smaller than LIGO, only 600 metres along each arm, its designers believe it will be just as sensitive as LIGO. That鈥檚 because it was designed and built recently, and so incorporates some innovations that LIGO doesn鈥檛 yet have.
GEO uses wires of fused silica-a very pure glass-to suspend its mirrors, whereas LIGO uses steel. In both types of wires, the thermal energy of the atoms will jiggle the mirrors about very slightly, adding noise to the signal. But, unlike steel, the thermal jiggling of the atoms in the silica is confined to a very narrow range of frequencies. That makes it much easier to screen out the noise when you鈥檙e looking for the signature of gravitational waves.
The detector鈥檚 sensitivity will also be increased thanks to a technique invented by Brian Meers at the University of Glasgow. Gravitational waves create extra frequency components in the laser light and GEO is designed to channel these components-which contain all the information about the gravitational-wave signal-into a separate cavity where it is easier to analyse them.
In around 2006, once the global network has been running for a few years, LIGO will be upgraded into a still more powerful instrument. The glass mirrors will be replaced by gigantic sapphires, each weighing forty kilograms-two hundred thousand carats to a jeweller. Sapphire is even more highly resonant than fused silica, so thermal noise should be less of a problem. Many other changes will be introduced-including GEO鈥檚 method of siphoning gravity signals into their own resonant cavity. The new instrument will be so sensitive that the scientists will have to take into account the effect of Heisenberg鈥檚 uncertainty principle on the position of the mirrors.
If LIGO doesn鈥檛 see anything in its first incarnation, astronomers will be disappointed. 鈥淏ut with advanced LIGO, if we were to see nothing, then something would be seriously wrong,鈥� says Shoemaker. The upgraded LIGO will be so sensitive that it ought to spot gravitational waves every day. If it doesn鈥檛, physicists might even have to discard current theories of gravitation.
But after 28 December, cosmology and astronomy may have to change anyway. As Shoemaker says, if we do detect gravity waves, we might start seeing things that we never dreamed existed. Put the champagne on ice: this could be a happy new year.
