LIGO, the worldwide network of gravitational wave detectors, has found nothing on its first pass. But that鈥檚 not necessarily a failure 鈥� the negative result means that for the first time, astronomers can put upper limits on the number of violent, space-twisting events happening in our galactic backyard. 鈥淭his shows that LIGO can do real astrophysics,鈥� says team analyst Jolien Creighton from the University of Wisconsin at Milwaukee.
Einstein predicted that objects moving through space would create gravitational waves that disturb the fabric of space-time. These waves would come from neutron stars spiralling in and merging, for example, or the fallout from astrophysical explosions such as supernovae, hums from asymmetrical pulsars, and background static left over from gravitational explosions in the early Universe.
But the waves are so faint that even catastrophic events like these would produce just the faintest whisper back here on Earth. LIGO, which links detectors at Hanford and Livingstone in the US and at Hannover in Germany is the first experiment to stand any chance of detecting them. The detectors work by splitting a laser beam into two parts and sending them along paths at right angles to each other before recombining them. If a gravitational wave ripples past Earth, it will warp space-time in the direction it is coming from, changing the relative lengths of the different paths and altering the interference pattern produced when the beam is recombined.
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How far LIGO can see depends on the strength of the signal, but at its current sensitivity it should be able to see neutron stars or black holes colliding as far away as the large and small magellanic clouds, just beyond our galaxy (see Graphic). The experiment ran for 17 days last August, and found no evidence for gravitational waves. These first results have allowed researchers to calculate that the Milky Way must be home to less than 164 such mergers per year. The negative result also sets a limit of fewer than 1.4 bursts, such as supernovae, in our Galaxy per day.
Unfortunately those numbers don鈥檛 tell astronomers much that is new. They only expect to see between 1 and 100 mergers per hundred thousand galaxies. And we know there is only one supernova in our Galaxy every 30 years or so 鈥� although the burst limit does also apply to other poorly understood explosions such as gamma-ray bursts. The fact that LIGO didn鈥檛 pick up any static from the early Universe enables researchers to say only that the energy carried by gravitational waves must be less than 72.4 times the total amount of energy in the Universe.
Even so, the successful first run is an important proof of principle, made more hopeful by the fact that the machine was running at only a hundredth of its intended sensitivity. Modifications made to the mirrors that channel the laser light in the last few months have led to a tenfold increase for the second run, due to finish next week. The team expects another tenfold increase by 2006, giving LIGO a view of thousands of galaxies. And that will finally bring them within kissing distance of the range at which astronomers expect to see something.
But the only way to be certain of detecting gravitational waves 鈥� if they are out there 鈥� is to stump up more cash. A $100 million upgrade to LIGO should reach well into the expected range of gravitational wave activity by 2012.
