Lab tuned to gravity's 'ripples'
By Jonathan Amos
Science reporter, BBC News
One of the great scientific experiments of our age is now fully
underway.
A German/UK team has put the giant GEO 600 gravitational wave
detector in a continuous observational mode.
The Hanover lab is trying to detect the ripples created in the fabric
of space-time that sweep out from merging black holes or exploding
stars.
Success would confirm fundamental physical theories and open a new
window on the Universe, enabling scientists to probe the moment of
creation itself.
GEO 600 is working alongside a US project known as Ligo (Laser
Interferometer Gravitational Wave Observatory). It may also be joined
in the hunt by an Italian lab within a year.
A confirmed detection would require the super-sensitive equipment at
more than one of these widely spaced facilities to record an event
simultaneously.
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A NEW VIEW ON THE COSMOS
- Gravitational waves are an inevitable consequence of the Theory of
General Relativity
- They describe the gravity force as distortions made by matter in
the fabric of space-time
- Any moving mass will produce gravitational waves and they propagate
at the speed of light
- Detectable sources should include exploding stars merging black
holes and neutron stars
- GEO 600 fires a laser into L-shaped tunnels to detect their very
weak signal
- Compelling independent corroboration would come from a spacecraft
that can see the burst of gamma-ray radiation expected to accompany
the cataclysmic events that produce gravitational waves.
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"If there is a supernova in our vicinity during the next couple of
months, our chances of detecting and measuring the resulting
gravitational waves are good," said Professor Karsten Danzmann, head
of the International Centre for Gravitational Physics, which is
jointly run by the Max Planck Society and the University of Hanover.
"The first step towards gravitational wave astronomy has been taken."
Researchers are extremely confident they now have the technology to
detect gravitational waves.
Observatories such as GEO 600 bounce lasers down long tunnels, hoping
to pick up the fantastically small disturbances the waves should
generate as they pass through the Earth.
Unlike electromagnetic waves - the light seen by traditional
telescopes - gravitational waves are extremely weak. If one were to
pass through the Earth it would alternately stretch its space in one
dimension while squashing it in another; but the changes are tiny.
Laser interferometers are looking for disturbances in their
experimental set-ups that are equivalent to mere fractions of the
diameter of a proton, one of the particles that make up the nucleus
of an atom.
Getting GEO 600 to approach this level of sensitivity has been an
immense challenge.
"There's more to come from GEO 600; I think we're still about a
factor of three away from the design sensitivity over part of the
frequency range. But the sensitivity we have makes it very worthwhile
stopping improvement to run for an extended period," said Professor
Jim Hough, from the Institute for Gravitational Research at Glasgow
University, UK.
"I think the most likely event for us to detect at the moment are
coalescing black holes. I'm extremely confident," he told BBC News.
A detection would be a final test of Albert Einstein's General Theory
of Relativity.
It would also usher in a new type of astronomy - one that is not
dependent on the observation of light. This is necessary because most
of the cosmos is "dark"; the majority of its matter cannot be seen
with traditional telescopes.
The new approach would also give scientists the opportunity to probe
the Universe's earliest moments, by observing the remnant
gravitational waves from the Big Bang that should still pervade all
space.
This, however, will require the super-sensitive laser technology to
be launched on satellites high above the Earth. Just such as project,
known as Lisa (Laser Interferometer Space Antenna), is being
developed currently by the US and European space agencies.
Even before then, spacecraft may have an important role to play in
corroborating ground-based observations of gravitational waves.
Satellites such as Nasa's Swift telescope can see the high-energy
radiation bursts that are produced when there is an extreme event of
the type that might also produce detectable gravitational waves.
A Swift alert would tell the GEO 600 team to take particular note of
any anomalous signal in its data.
"It's very exciting that short gamma-ray bursts are probably due to
the same sorts of sources," explained Professor Hough. "Immediately
there's a gamma-ray burst, there's huge interest in what's happening
in the gravitational wave detectors."
HOW GEO 600 HOPES TO CATCH GRAVITATIONAL WAVES
- Two coalescing black holes circling each other [1] are expected to
emit gravitational waves that move out at the speed of light
- At GEO 600, a high-powered laser [2] is fired at a 'beam splitter' -
a semi-transparent mirror - which divides the beam down two vacuumed
tunnels
- Mirrors [a+b] at the far ends bounce the light back; more mirrors
[c+d] extend the measuring distance, and yet more [e+f] are used to
recycle the power and enhance the signal
- The light paths from the separate arms are recombined and sent to a
photodetector. If a gravitational wave has passed though the
observatory, it will have changed the length of the arms and the
signal should be evident when analysed by computer
