Steve Connor: The hunt is on for gravitational waves

Lab Notes

Monday 20 September 2010 00:00 BST

Gravitational waves are one of the great mysteries of science.

They travel at the speed of light through the Universe, are described as "ripples in the fabric of space time" and are thought to be produced during great cosmic events, such as when two black holes collide. Yet they are quite invisible to detection.

Albert Einstein predicted their existence a century ago and they are fundamental to his theory of general relativity. Even though we haven't been able to detect gravitational waves, there is nothing in the theory of cosmology – well almost nothing – that says they don't exist.

Astronomers say that if we could detect gravitational waves it would not only be a wonderful fulfilment of Einstein's classic theory, it would have practical implications in that their detection would suddenly light up some of the darkest recesses of the Universe. It would give us a radically different view of space, based on something other than the detection of electromagnetic radiation (such as light and X-rays). It would literally allow us to "see" inside black holes, a place where gravity is so strong nothing, not even light can escape.

Last week at the British Science Festival, Alberto Vecchio, an astronomer at Birmingham University, gave a vivid description of what it would be like if we could detect gravitational waves. "Imagine you are in a pitch-dark room where an orchestra is playing, but your ears are covered. It would be terribly uninteresting as nothing seems to be happening. Then all of a sudden, you start hearing. There is a completely new view of that room. Every time we have looked at a different band of the sky we have discovered completely new things. The sky will be different," Dr Vecchio said.

"Imagine pointing a telescope at colliding black holes. You would see absolutely nothing because black holes are, by definition, black. If you were to point a gravitational-wave observatory, it would be the most spectacular event," he said.

Dr Vecchio is one of about 800 scientists around the world who are part of an ambitious international attempt to detect gravitational waves using half a dozen different machines dotted around the world, from the United States to Australia. All of these machines are in the process of being upgraded to make them even more sensitive to these elusive ripples in the fabric of space-time.

One of the upgrades is well under way to completion. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US – a pair of detectors in Washington State and Louisiana – will in 2015 begin its search for the phenomenon that Einstein himself thought we would never be able to detect.

LIGO uses two laser beams, each 4km long, arranged in an "L" shape. When gravitational waves pass through the Earth they cause minute distortions to everything in their path, except the laser light of the detectors – which is why these machines should be able to detect the presence of gravitational waves. To do this, however, the detectors have to be able to monitor the relative movements of their laser beams, movements so tiny they are equivalent to about one thousandth of the diameter of a hydrogen atom – an incredibly difficult feat.

Scientists are as certain as they can be that gravitational waves exist and that these set of detectors will be able to see them. There is always a chance – a slight one – that they are wrong, but if this proves to be the case then a null result would in fact have important implications for everything we know that is based on Einstein's theory. It would mean we'd have to re-write the laws of physics – or some of them anyway.

However, that said, no one in this business believes that these detectors will fail in the goal set for them. But there is one fly in the ointment, as least for the British astronomers who have already contributed £8.9m to the £130m upgrade of LIGO. The forthcoming spending cuts to the science budget could mean that these astronomers are not given the funds to actually collect and analyse the data as it comes from the detectors. "If the comprehensive spending review is very harsh, we may be in trouble," Dr Vecchio said.

The detection of gravitational waves is not going to have any direct and immediate benefits for the taxpayers who are funding the science. It will not, for instance, produce the immediate benefits of a new clinical research centre. But even in a time of financial stringency, there is still an argument that we should not divert all of the science budget to research with immediate practical benefits. Some things are just too important and too damn interesting to abandon. The search for the ripples in the fabric of space-time is one of them.

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