IMAGINE bizarre shapes鈥攇iant loops, spirals and balls of
wool鈥攎ade of delicate materials, all spinning sedately in space. Not an
interplanetary sculpture park, but an array of detectors that could one day net
one of the greatest prizes in physics: gravity waves. 鈥淲e are seriously
suggesting using such structures to detect cosmic gravitational waves,鈥� says
Robin Tucker of the University of Lancaster.
Gravity waves are ripples produced in the fabric of space-time by a massive
body, such as a black hole, undergoing violent acceleration. As gravity waves
pass by, they alternately stretch and squeeze space, so one way to detect them
would be to put a solid body in their way and look for any periodic shrinkage or
expansion. 鈥淥ur idea is to use 鈥榮lender bodies鈥欌�攖hat is, ones that have a
small cross-section compared with their length,鈥� says Tucker.
The theory of how slender bodies vibrate was developed by the Cosserat
brothers鈥攖wo French engineers鈥攁round 1909. Using their ideas, Tucker
and his Lancaster colleague Charles Wang have come up with vibrating detectors
in various shapes. 鈥淎 loop would be a narrowband detector and a flat spiral a
broadband detector of gravitational waves,鈥� says Tucker. 鈥淏oth would be
directional, but an antenna shaped like a ball of wool would be omnidirectional,
capable of picking up random bursts of gravitational waves expected from the big
产补苍驳.鈥�
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Such antennas would have to be many kilometres across and require very
sensitive motion detectors. This is because gravity waves arriving in the
Earth鈥檚 neighbourhood are likely to be weak. 鈥淎 passing wave is expected to
change the dimensions of a 1-kilometre structure by only a fraction of the
diameter of an atom,鈥� says Tucker.
Measuring such tiny changes is a huge challenge. Tucker and Wang are looking
at several possibilities, including the idea that an uncrewed spacecraft might
float outside the antenna and bounce radio signals off it, detecting any
movement from the Doppler shift in the frequency of the reflected radio waves.
鈥淒etection is a problem for the future,鈥� says Tucker. 鈥淲e recognise that the
technical problems are huge.鈥�
Tucker and Wang say their antennas could be 1 centimetre in cross-section and
made from carbon nanotubes鈥攁 very light, very strong material. The
material might be extruded like toothpaste from a spacecraft and the completed
structure towed to an orbit that鈥檚 permanently in the Earth鈥檚 shadow and so
would minimise confusing vibrations caused by solar heating.
But Tucker and Wang aren鈥檛 the only gravity wave show in town. NASA and the
European Space Agency are cooperating to launch the Laser Interferometric Space
Antenna (LISA)鈥攖hree formation-flying spacecraft deployed in a huge
triangle, its 5-million-kilometre sides formed by laser beams. Passing gravity
waves would reveal themselves by minutely altering the paths of the beams.
Tucker and Wang believe their antennas could fill a 鈥渘iche鈥�, picking up
gravitational waves in the millihertz to hertz range that will be missed by LISA
when it reaches deep space in 2011.