Washington DC
FORGET multimillion-dollar spacecraft and elaborate underground labs. The
simplest way to detect gravity waves directly could be to listen to flutters in
the regular bursts of radio waves from distant pulsars.
Any accelerating mass is believed to create ripples in space-time called
gravity waves. They were predicted by Einstein as part of his general theory of
relativity, and people have been trying to pick up these weak and elusive
undulations ever since. If we could detect them, they should reveal valuable
information about the structure of the Universe, and we鈥檇 even be able to 鈥渟ee鈥�
mysterious dark matter that is only detectable gravitationally.
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So scientists are dreaming up ever more ambitious and expensive experiments
to detect the waves. Gravity-wave hunters are currently gearing up to study data
from NASA鈥檚 $371 million Laser Interferometer Gravitational Observatory,
which is scheduled to go online by the end of March. LIGO, which is
ground-based, will compare the arrival times of beams of laser light sent down
perpendicular arms and bounced back to a detector. Gravity waves squeeze and
stretch space as they pass through it, so the relative length of the arms should
be affected by a passing gravity wave, and the time taken by the laser beams to
traverse the arms would reveal this.
But there may be no need for a costly detector to confirm the existence of
gravity waves if Andrea Lommen and her colleagues at the University of
California, Berkeley, are on the right track. They鈥檙e looking instead at
millisecond pulsars鈥攊ncredibly dense neutron stars that spin dozens of
times a second, sending regular radio pulses towards Earth.
The timing of the pulses varies by less than 1 part in 1015, making them
more accurate than many atomic clocks. Gravity waves travelling through space
between Earth and the pulsar should disrupt the precise timing of the pulses and
thereby betray their presence to observers.
It would take incredibly massive objects to create gravity waves powerful
enough for us to detect, but the merging of two black holes should do the trick
(see Graphic). Lommen has analysed 17 years鈥� worth of pulses from two
30-millisecond pulsars that lie thousands of light years away from Earth.
There鈥檚 no evidence of any gravitational radiation in the data, but that may
simply be because black-hole mergers are very rare.FIG-23262701.jpg
Sterl Pinney, an astronomer at Caltech in Pasadena, who has chaired NASA鈥檚
committee for LISA, a space-based gravity detector due to be launched in 2006,
agrees that Lommen鈥檚 method should eventually spot gravity waves. But he says we
still need to build detectors because they will be able to pick up gravity waves
with higher frequencies. Such waves might come from pairs of orbiting stars that
are about to merge, and these events are thought to be much more common than the
merger of a pair of black holes.