Rivals on the trail of elusive ripples

Gravity's Shadow
March 4, 2005

Astronomical discoveries come in two forms. The first type is the lucky one. In 1781, William Herschel was testing a new telescope mirror outside his Bath town house when one of the "fixed stars" moved. Herschel had accidentally discovered the planet Uranus, and, in a trice, the solar system doubled in size. The second type of discovery is the long slog. Take stellar parallax. In 1543, Nicolaus Copernicus suggested that the Earth was orbiting the Sun. Consequently, nearby stars should swing backwards and forwards every year. Unfortunately, the nearest star to the Sun is more than a quarter of a million times farther away than Earth. The annular parallactic "swing" is 0.0002°. It took 300 years to find it and required great skill and the most sophisticated instrumentation.

Gravitational waves are today's equivalent of the stellar parallax. Albert Einstein predicted their existence in 1916 as part of his theory of general relativity. They move through space at the velocity of light and bathe Earth with their energy. Unfortunately, the energy is minute, and today's detectors are just too insensitive. The word "just" is important. All detectors are searching for signals among noise. The contented scientist is confronted with a large, obvious, easily measurable signal that swamps the noise. The gravitational-wave hunter is not so lucky.

Gravitational waves are the ripples in the geometry of space-time generated by the acceleration of large masses. The process is analogous to the production of radio waves by the acceleration of electric charges. The snag is that electric charges are both positive and negative; mass is only positive. The conservation of momentum ensures that the acceleration of one mass is compensated by the recoil of another. A net flux requires a shape change. This happens in close binary stars, and these emit a continuous flux. Sharp bursts of gravitational waves are produced by supernovae explosions and by the anisotropic formation of black holes. A third category, a cosmic whistle, occurs when very close binary stars merge.

Unfortunately, even the strongest and closest source will produce a gravitational wave at Earth that alters the dimensions of space-time by less than one part in 10²², and this, to put it mildly, is extremely difficult to detect.

Harry Collins is a professor of sociology at Cardiff University. He has spent many years peeping over the "playground wall" at scientists doing their experiments. His sociological field work concerns the methodology of science and the interactions between scientists in a specific research group and scientists in competing research groups.

In 1972, Collins turned his attention towards the hunt for gravitational waves, and Gravity's Shadow is the account of more than 30 years of endeavour, interviews and conference attendances. One of the story's main characters is the American physicist Joseph Weber. Weber started his quest in the late 1960s using a massive 1,400kg aluminium cylinder suspended in a vacuum tank. The bar resonated at 1,660Hz, and the plan was to monitor continuously the level of excitation of its lowest vibrational mode. The detector was sensitive to length changes of only 1 part in 10¹6, for millisecond pulses. After many months of work, Weber thought that he had detected one gravitational wave per day, these emanating from the direction of the galactic centre. Unfortunately at a sensitivity of 1 part in 10¹6, the causative mass loss would have to be so extreme that the fringes of the galaxy should be moving outwards at a detectable rate.

Weber then decided to search for coincident events in two bars - one at the University of Maryland, the other at the Argonne National Laboratory in Illinois. More events were detected. But other scientists could not confirm the claims and a lively debate ensued. The consensus in 2005 is that these gravitational waves have not yet been found.

Collins discusses the early debates in detail and then traces the race towards ever-increasing detector sensitivity. Large bars were replaced by large spheres; the thermal noise at room temperature was diminished by cooling the apparatus to a few milli-Kelvin above absolute zero. Bandwidths were boosted by a factor of 100. Overall sensitivity jumped 1,000-fold.

Still the detection statistics were controversial. Then interferometers were introduced. The US's Ligo (Laser Interferometer Gravitational-wave Observatory) had two 4km orthogonal horizontal arms. A passing gravitational wave should make one very slightly longer than the other.

Other interferometers were built in Italy, Germany and Japan. The sensitivity was boosted to close to 1 part in 10²¹. Theorists suggested that another factor of 10 might clinch detection. Laser power was increased, fused silica supports replaced the metal ones and seismic isolation was improved. The results were still controversial. The next step is to go into space. Lisa (the Laser Interferometer Space Antenna) is planned, with launch scheduled for 2013.

Collins has presented us with an enthralling investigation into the way in which big science advances. The hunt for gravitational waves has much in common with the hunt for stellar parallax. Both were at the frontiers of signal detection, had results that were difficult to interpret and brought out the best in science and in scientists. Data had to be checked, observations repeated, sensitivities improved and theoreticians encouraged.

The whole provides a perfect case study in the sociology of science. Collins delves into the discussions between rival groups. Team approaches are compared with individual efforts, young scientists with old, European with North American and Australian. The brash are weighed against the cautious. The drama of the scientific meeting is investigated. Why are some scientists more likely to be believed than others? Who decides who is doing well? Should the hunters keep publishing their lack of success?

A key topic is the funding flow. How do gravity-wave scientists keep the money coming in? And, in applying for funds, is it better to try to improve on the familiar and tested or to ditch the old and embrace a new technique? What exactly are the funding councils looking for? We are reminded that for every gravitational-wave researcher forecasting that success is just around the corner, a host of scientists in other fields is trying to convince the funding agency that all gravitational wave searches should be put on ice until the instrumentation has improved and the money diverted to other causes.

What is greatly appealing about Gravity's Shadow is that the conclusions can so easily be applied to other scientific endeavours. We see the tantalising relationship between the character of a scientist and their progress. There are people who lead by example and people who bully; people who live for science and their laboratory, and others who are more rounded characters. Some do not mind the external monitoring of their endeavours, while others work best when left alone.

With gravitational waves, the background is anguish. Science is always easier when you are measuring something and producing results. Searching unsuccessfully is never fun. Each year they look; each year they do not find. The annual reports are depressingly similar: promises are made, sensitivity is increased, the goal is approached - but the date of success is difficult to predict.

David Hughes is professor of astronomy, Sheffield University.

Gravity's Shadow: The Search for Gravitational Waves

Author - Harry Collins
Publisher - University of Chicago Press
Pages - 870
Price - £70.00 and £.50
ISBN - 0 266 11377 9 and 11378 7

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