Ripples on a Cosmic Sea: The Search for Gravitational Waves

  • David Blair &
  • George McNamara
Allen and Unwin/Addison-Wesley: 1998. Pp.179 Aus$17.95£7.99,(pbk)

Spin a nuclear submarine about its short axis until it is near to breaking and the general theory of relativity says it will emit gravitational radiation of about 10−24 watts. This is not much; an ant walking up a wall uses 10−7 watts.

Ripple detector: artist's impression of the Laser Interferometer Gravitational-Wave Observatory, taken from the LIGO website (http://www.ligo.caltech.edu).

The comparison is found in Ripples on a Cosmic Sea, and shows why, to outsiders, the search for gravitational waves seems almost crazy. Nevertheless, the US National Science Foundation has funded a joint project between the California Institute of Technology and the Massachusetts Institute of Technology that uses two huge interferometers to look for such shivers in space-time. The interferometers are like Michelson-Morley experiments but with arms 4 kilometres long. Similar but smaller devices are being built in Europe and Japan, and David Blair's group is part of an Australian collaboration that has made a start on another, near Perth.

The interferometers are only the latest stage in a story that started in the 1960s when Joseph Weber of the University of Maryland put together the first resonant detector. He hung bars of aluminium weighing a couple of tons inside vacuum chambers, insulating them from all known influences. When two bars separated by thousands of miles ‘rang’ in coincidence, Weber argued that gravitational waves were a putative cause of the disturbance; the source could be cosmic catastrophes such as supernovae.

By the early 1970s, Weber's claims had become so forceful that others built similar devices, but by the mid-1970s most people thought Weber was mistaken. Blair and George McNamara provide a colourful description of a confrontation between Weber and Richard Garwin at a conference in the 1970s that might have come to blows had the chairman not stepped in. Weber continues to press his claims, but most of the rest of the field moved on long ago.

The next generation of detectors were Weber bars cooled to liquid-helium temperatures and below. Blair himself runs such a device, although his bar is niobium rather than aluminium. Such attempts to detect gravitational waves are dealt with in the last third of the book. The first section is a short history of science, taking us through Newton to general relativity and the curvature of space, and in the middle is a section about potential sources of gravitational waves. The ‘nuclear submarine problem’ makes it impossible to generate detectable fluxes on Earth, so we must look to cosmic sources. The huge curvatures and energies associated with cavorting neutron stars and black holes make them the current favourites.

Astronomers Joseph Taylor and Russell Hulse studied the decay of a pair of orbiting pulsars over 20 years. The slowdown of 70 microseconds per year in an orbit of seven-and-three-quarter hours fits well with the predicted loss of energy through gravitational radiation. The 1993 Nobel prize for physics was awarded to Taylor and Hulse for this first indirect confirmation of the existence of the waves, but direct observation is still the holy grail.

Towards the end of their orbital decay (in about 300 million years for the Taylor-Hulse pair), neutron stars nearly touch and circle hundreds of times per second. The final inspiral should produce a characteristic ‘chirrup’ of gravitational radiation of enormous power, about as bright as 100,000 galaxies. But even these immense fluxes will bend space so little that they will be on the edge of detectability by the most advanced interferometers now being designed. Colliding black holes should be even more fun, and there ought also to be a just-detectable background of gravitational radiation left over from the Big Bang.

Ripples on a Cosmic Sea is currently without competitors. However, parts of the book are so relentlessly ‘popular’ as to be patronizing. The most informative chapters are those dealing with pulsar sources, and those describing detectors and their inventors. Chapter 12, on laser interferometry, is especially good. It presents important ideas clearly, not shirking complicated new techniques. It also mentions the more operatic organizational upheavals that have attended the US programme and the dirty dealings associated with the funding of different national projects. These chapters provide a nice account of sources and technology and a good thumbnail sketch of the field's history. The book could then be passed on to a young niece or nephew.

Those who get a taste for the science of gravitational radiation detection from the book might want to borrow a library copy of Peter Saulson's Principles of Interferometric Gravitational Radiation Detectors, which includes a short section on resonant bars, or Blair's edited collection of technical essays The Detection of Gravitational Waves. In both books the equations are supplemented with good clear writing, but you'll need to be rich to buy them.