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SPOTTING the minuscule forces generated by a passing gravity wave could
become easier thanks to a new measurement technique developed by researchers in
Italy. Teams in the US, Germany and Italy are building huge detectors in the
hope that they will pick up the faint gravitational echoes of cataclysmic events
in deep space. The new work could make these giant instruments 10 times more
sensitive.

Gravity wave detectors are giant L-shaped devices with arms as long as 4
kilometres. Laser beams pass through vacuum tubes along the arms and bounce back
and forth between mirrors at either end, providing a very precise measurement of
the length of the arm.

Any gravity wave that passed through would squeeze and stretch the arms by
about 0.1 per cent of the width of an atom. Thermal vibrations in the mirrors
can easily swamp these tiny signals, so researchers need ways to cut down this
thermal noise.

In 1999, Michel Pinard and his colleagues at the 脡cole Normale
Sup茅rieure in Paris perfected what they thought was an ideal technique
for doing this. They bounced light off the mirror鈥檚 surface, and by varying the
tiny force that this exerts on the surface they could cancel out thermal
vibrations鈥攊n effect, cooling it down.

Unfortunately, Pinard and his team found that the technique dampens all
vibrations at that frequency, even those caused by the gravity waves. Now David
Vitali of the University of Camerino in Italy and his team have come up with a
way round this snag.

Their trick is to switch off the laser cooling mechanism and 鈥渓isten鈥� briefly
for gravity waves as the mirror heats up. They then switch the cooling back on
again. The result is a constant cycle of cooling and heating. 鈥淭his should allow
detection of impulses that are very brief compared with the length of the
cycle,鈥� says Vitali.

Physicists are uneasy about such solutions. Experiments at constant
temperature are easier to reproduce and the results easier to interpret. But
Vitali鈥檚 team has worked out the physics behind the non-equilibrium situation so
that researchers can make sense of their results. They have developed a quantum
mechanical description of how the laser cooling works to predict what
experimenters might see under these conditions.

Pinard believes this is a significant advance. 鈥淚 think that the work of
Vitali is important for improving the sensitivity of measurements of impulsive
weak forces,鈥� he says.