Introduction

Abstract

We live in exciting times, from a scientific viewpoint, for both experimental and observational science now allow us to investigate the fundamental nature of the Universe with hitherto undreamed of accuracy. Bounds can be set, and competing theories can then be tested and evaluated. Nowhere has this progress been more opportune than in the search for the basic structure of space-time and the nature of gravity. Here, considerations of the cosmos at ultra-large scales (e.g. investigations into the cosmological constant, cosmic and galactic dynamics, dark matter, etc) have prompted research into the ultra-small scale—and vice versa—through attempts to quantize gravity, to unify it with the other three known forces, or to resolve outstanding difficulties with the Standard Model of particle physics (e.g. its apparent complexity, CP symmetry, the various unexplained mass scales, etc). New and subtle forces of nature continue to arise naturally out of this theoretical endeavour, and the Equivalence Principle (EP)—the principle of the equivalence of inertial and gravitational mass—is uniquely placed as a sensitive probe of these same putative forces. Therefore, rigorous experimental testing to the highest accuracy possible of the EP, and thereby of General Relativity, is vital to our future understanding of the physical world.Experimentation on the so-called Weak EP, or the equivalence of free-fall, for different small test bodies falling in the same gravitational field is not a new idea, of course. After two millennia, during which the erroneous views of Aristotle held sway, actual tests of the EP began in the 17th century with those of Galileo: his well-known experiment of dropping a lead musket ball and an iron cannonball (apparently) from the leaning tower of Pisa, for example. Such tests have continued from that time to the present day: firstly with Newton (using pendulums), and subsequently with Eötvös, Dicke, Braginsky, Adelberger, and others—using predominantly torsion balances in various ways—with increasingly greater sophistication, elegance and sensitivity. However, the inescapable micro-seismicity of the Earth, drifting gravity gradients of the locality, and a limited available driving acceleration for the experiment, now make further progress in testing the EP on the Earth extraordinarily difficult—notwithstanding the very great technical advances that have been made. For this reason the greatest potential future gains in measurement accuracy for the EP are likely to be realized through employing the exceptionally quiet and tranquil environment that may be found in space.It is noteworthy that the title of the Symposium `Testing the Equivalence Principle in Space' was and still is an aspiration for some EP experiments; and many of the experimental papers (which dominated the Symposium) were concerned with ground-based measurements of the EP. Nevertheless, it is worth underlining the fact that advances in Space Science such as (indirectly) `Lunar Laser Ranging', or (directly) `drag-free' control of spacecraft, mean that today Testing the Equivalence Principle in Space lies squarely within the bounds of practical, technical possibility.The International Symposium on Testing the Equivalence Principle in Space was held over four days (4–7 September 2000) and was organised jointly by the University of Stanford in the USA and the University of Strathclyde in Glasgow, Scotland. The venue for the meeting was Ross Priory on the southern shores of Loch Lomond, this residential conference centre—dating back originally to 1693, and located in 173 acres of grounds with impressive views over the loch and the highlands of Scotland—being owned by the University of Strathclyde. The relative isolation and the splendid tranquillity of the setting certainly helped to draw the delegates together, and to focus discussions in a truly creative and a constructively positive way. 42 delegates attended the meeting.Whilst the Symposium was called at rather short notice, and therefore was not fully comprehensive in terms of the range of either experiments or theory covered, it nevertheless conveyed fully the excitement of this burgeoning field of scientific endeavour. I hope you will find the Proceedings of interest.Finally, I should like to thank Dr Paul Worden of Stanford for assisting with the US side of the organisation for the Symposium, Dr Rüdeger Reinhard of ESTEC for assistingwith the programme, and Mrs Elsie MacVarish for her invaluable assistance with the local organisation. The Symposium was supported by NASA, ESA and PPARC. N A LockerbieGuest Editor