This special issue of Classical and Quantum Gravity constitutes the proceedings of the 4th Edoardo Amaldi Conference on Gravitational Waves held in Perth, Western Australia in July 2001. The field of gravitational wave detection has been slowly emerging since the pioneering experiments of Joseph Weber more than 30 years ago alerted the world to the possibility of actual detection of gravitational waves. The funding of kilometre scale interferometers and the planning of million kilometre scale space interferometers has grown the field of gravitational wave detection to a size where major review meetings are essential to obtain an overview of the field.

The early days of gravitational wave detection were characterized by hope: the hope that nature would surprise us with unexpected strong sources of gravitational waves. In fact nature did have a surprise for us, but it was not the enormous population of colliding black holes for which we had hoped. Instead it was the Hulse-Taylor binary pulsar. This proved the existence of gravitational waves and defined a class of source which has become a major target for detection. More than this, it gave us the knowledge that a broad spectrum of gravitational waves definitely exists. Today we know that this spectrum awaits us like an unexplored continent, like Australia before European settlement. We know that the Great South Land exists, but its rivers and mountains, deserts and plains wait to be explored. We create and perfect our detectors with the knowledge and anticipation that we are privileged to be the first human beings to explore this unknown spectrum.

Edoardo Amaldi led the development of gravitational wave research in Italy. He forged the first links between the gravitational wave community and the high-energy physics community which led to the long term hosting of the Explorer gravitational wave detector at CERN. His links also stretched across the Atlantic, leading to close collaboration between the first cryogenic resonant mass gravitational wave detectors at Stanford, Louisiana and Rome, and then later to the Niobe detector at Perth. Today, high-energy physics and gravitational wave physics are understood to be deeply entwined, providing alternative and complementary means of addressing fundamental questions in cosmology. The international research links and collaborations which Amaldi supported have broadened and today are represented by GWIC, the Gravitational Wave International Committee, which is now the official sponsor of the Amaldi Conferences. The Amaldi Conferences (with emphasis on terrestrial detectors) along with the LISA Symposia (which emphasize space interferometers, and which have also been published by this journal) have allowed the breadth of progress in the field to be presented while specialized workshops (such as the data analysis workshops) cater for specialized subgroups.

Like radio in its pioneering days, the field of gravitational wave detection has been extraordinarily innovative. Detectors are complex instruments which combine mechanical, optical, photonic, digital, laser and vacuum systems, all of which are critical components and all of which must exceed the prior state of the art by an enormous margin. Every subsystem has required the development or discovery of major innovative solutions. The innovations that have arisen in this field are broad and extensive. While some of them can be found in these proceedings, their applications are unlikely to be discussed. I believe that it is worth listing a few of them to give ammunition to those who need to answer the perennial question 'what is its use to the average pedestrian?'. Useful and economically valuable technologies that have arisen directly or indirectly from gravitational wave research are

• Improved SQUID magnetometers: brain scanning and many other applications
• Low noise microwave oscillators: improved radars
• Sapphire clocks: flywheel oscillators for atomic clocks
• Low outgassing vacuum materials: high performance vacuum systems
• Ultra-precision mirrors and coatings: improved optical systems
• High efficiency lasers: numerous present and future applications from materials cutting to medicine and metrology
• Improved vibration isolation: electron microscopy, atomic force microscopy and semiconductor manufacture etc
• Low acoustic loss materials: metrology systems
• Gravity gradiometers: mineral exploration
• High performance position sensors: accelerometers, tilt sensors, seismometers
• Laser stabilization: metrology systems, time standards
• Data analysis: weak signal detection
• Very low optical absorption materials: high laser power applications

Amaldi 4 was the second major international conference hosted by the UWA Gravitational Wave Research Group. The first was the 5th Marcel Grossmann Meeting in 1988, when gravitational wave research was still a subfield within the context of relativistic astrophysics. Both in 1988 and in 2001, the organization was a major effort and a substantial distraction from research. However, in both cases, I believe the benefits outweighed the costs. In the case of Amaldi 4, I believe that it is no coincidence that in the same year we achieved the first substantial level of funding for our gravitational wave research in Australia. After struggling for many years with inadequate funding, the Australian Consortium is now able to play a substantial role in the development of gravitational wave technology through its high optical power test facility at Gingin. Funding announced after the conference allows us to properly staff this facility and to become a partner in Advanced LIGO.

Amaldi 4 brought additional benefits to Australia. Besides ACIGA's Gingin facility, the Gravity Discovery Centre Foundation (a public charitable foundation) is constructing a public education facility. The site, in beautiful and unusual Australian bushland, has been set aside for the future long baseline gravitational wave detector AIGO. It combines clear skies, extremely high botanical biodiversity, Aboriginal cultural significance and, of course, the Australian link to the worldwide effort to detect gravitational waves. This set of ingredients is ideal for the promotion of a scientific understanding of our place in the universe, in the broad context of astronomy, our diverse cultures and our fragile planet in an awesome universe. It is also an ideal opportunity for demonstrating how fundamental science and the search for knowledge inevitably lead to discoveries of benefit to humanity. During the excursion, the conference participants saw the first stage of the Gravity Discovery Centre under construction. The Amaldi conference, which included two public lectures and a reception in which potential donors could meet with conference participants, brought wider public recognition of the Discovery Centre project which itself led to sufficient significant donations for the next stage of the discovery centre to be able to begin construction in 2002.

Australian gravitational wave research for many years consisted only of the development of the Niobe resonant mass detector at UWA, and it was pleasing that at Amaldi 4 we were able to present the lowest noise temperature ever observed in a resonant mass detector. However, of greater significance was the opportunity for the Australian Consortium to present its broad research effort. The vision of AIGO has allowed many universities across Australia to pool their diverse expertise to create a truly national research effort. Amaldi 4 allowed the entire Australian programme to be fully presented to an international audience. For example, the Adelaide laser group presented exciting results on high power lasers, the ANU group presented definitive results on configurations, the UWA group presented important new vibration isolation concepts, and the ANU data analysis group presented interesting results demonstrating the need for a Southern Hemisphere detector. Concurrently with Amaldi 4, the Australian Conference on General Relativity and Gravitation was held and much Australian theory work was presented.

Acknowledgments

The success of Amaldi 4 was due to many people. The organizers thank in particular Barry Barish for his consistent support and Seiji Kawamura for so graciously giving precedence to the Australian bid for the conference. Thanks to the International Union of Pure and Applied Physics, the West Australian Government and the University of WA for sponsorship. Thanks to Alan Robson, Deputy Vice Chancellor of UWA for hosting a reception, and John deLaeter and Rob Jewkes for their support. Thanks to Debbie Greenwood for her brilliant and dedicated organization, and to Ron Burman for his efficient treasurership. Thanks to Yarra Korczynskyj for organizing the excursion and David McClelland for his chairmanship of the Scientific Organizing Committee. Finally, thanks to the session convenors and the participants who were responsible for the excellent science presented at the conference, and to Andrew Wray of IOP for his efficient management of the refereeing process for the articles in this volume.

D G Blair
University of Western Australia

Amaldi Conference Proceedings and LISA Symposia Proceedings

Proceedings of the 1st Edoardo Amaldi Conference
Villa Tuscolana, Frascati, Rome, Italy (14-17 June 1994)
Published in 1995 Gravitational Wave Experiments ed E Coccia, G Pizzella and F Ronga (Singapore: World Scientific)

Proceedings of the 2nd Edoardo Amaldi Conference
CERN, Switzerland (1-4 July 1997)
Published in 1999 Gravitational Waves ed E Coccia, G Pizzella and G Veneziano (Singapore: World Scientific)

Proceedings of the 3rd Edoardo Amaldi Conference
California Institute of Technology, Pasadena, California (12-16 July 1999)
Published in 2000 Gravitational Waves (AIP Conference Proceedings volume 523) ed S Meshkov (New York: AIP)

Proceedings of the 4th Edoardo Amaldi Conference
University of Western Australia, Perth, Western Australia (8-13 July 2001)
Published in 2002 Class. Quantum Grav. 19 1227-2050

Proceedings of the 1st International LISA Symposium
Rutherford Appleton Laboratory, Chilton, UK (9-12 July 1996)
Published in 1997 Class. Quantum Grav. 14 1397-1585

Proceedings of the 2nd International LISA Symposium
California Institute of Technology, Pasadena, California (6-9 July 1998)
Published in 1998 Laser Interferometer Space Antenna (AIP Conference Proceedings volume 456) ed W M Folkner (New York: AIP)

Proceedings of the 3rd International LISA Symposium
Max-Planck-Institut für Gravitationsphysik, Golm, Germany (11-14 July 2000)
Published in 2001 Class. Quantum Grav. 18 3965-4164