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Gravitational Wave Detection for Non-Specialists

According to the general theory of relativity, gravitational waves should be emitted by moving masses. They are, however, very weak and only when huge amounts of mass are converted into energy, as when stars explode or collide, are measurable amounts of gravitational radiation generated.

In the early years of the next century it is planned to launch a group of satellites which ought to be able to detect low frequency waves by measuring the changing distance from one satellite to another. My project, however, is concerned largely with ground-based detectors.

There have been three generations of ground-based detectors. The first generation was designed by Professor Joseph Weber of the University of Maryland. Weber is widely credited with being the pioneer of the field. Many scientists believed that the field would not be where it is now if Weber had not decided to try this `impossible’ experiment against all the odds. When Weber started scientists were still arguing about whether gravitational waves could be detected even in principle.

Weber hung a massive bar, weighing a ton or so, in a vacuum chamber and insulated it from all known forces. He compared the residual vibrations in the bar with the vibrations in similar bars separated by hundreds or thousands of miles. When the separated bars vibrated in coincidence, this was taken to be evidence for the existence of gravitational waves. Weber began to make credible claims of this sort in the late 1960s and early 1970s.

The trouble was that calculations suggested that Weber was seeing far too much gravitational radiation. According to accepted theory, with his calculated sensitivity, he should have seen nothing, and certainly substantially less than one pulse a year, and he was seeing several per day.

A number of groups tried to repeat his experiment and they eventually concluded that Weber was wrong, and that his design of bar was insufficiently sensitive to see the amount of radiation that Weber claimed to see. Weber , however, did not give up and argued back. My early studies - papers 1 to 3 below - turn on this argument.

By 1975 the consensus had clearly turned against Weber but he is still developing theories and finding evidence to support his early claims. More recent studies - papers 5 and 6 - explore the history of Weber’s claims after the consensus had turned against him.

The second generation of gravitational wave detectors are like Weber’s design except that they are cooled to liquid helium temperatures. This design of detector dominated the field from 1975 until the early 1990s. Once more, the consensus is that they have detected nothing. Paper 4, deals with the history of a proto-claim to have detected interesting coincidences between a cooled bar in Italy and another in Australia and the argument that took place with an American group.

The third generation of detectors use a different technique called interferometry. Carefully controlled beams of laser light are directed along long arms and reflected back to the origin. The way the light beams `interfere’ with each other reveals any comparative changes in arm length during the passage of the light. [This is the basis of the famous Michelson-Morley experiment of 1887 - See my book The Golem under main books.]

This technique promises to be more sensitive to the radiation and to be able to see the shape of the pulses of radiation, not just their energy. But it is a much more expensive technique. For example, the American program is costing in excess of $300M and is the largest project ever funded by the US National Science Foundation. [High energy physics is much more expensive - eg the Super-Conducting Super-Collider was to cost $8 billion - but in the US it is largely funded by the Department of Energy.]

The sheer size of the interferometer program has meant that it has come to dominate the field in recent years. I argue in papers 4 and 5 that recent events in the resonant bar field can be fully understood only in the context of the larger program.

The first interferometers should come on air in the first years of the new millenium. A pair of interferometers with 4km arm lengths is being being built in the US, a 3km device is being built in Italy by a French-Italian team, a British-German team is building a 600m device in Hanover, while smaller devices, accompanied by plans to build much larger ones, are to be found in Japan, Australia, and Malaysia.

According to consensual theory, gravitational waves will first be seen by the first generation of these devices around 2003-4, or by the second generation which will be `on-air’ a couple of years later.

Paper 7 deals with the changing management structure of the American Laser Interferometer Gravitational-Wave Observatory (LIGO) as it developed from small science to big science.

Paper 8 deals with the problems of the international collaborations which are integral to this field.