Abstract
The Einstein equivalence principle is the foundation for general relativity and all metric theories of gravity. Of its three tenets—the equality of acceleration of test bodies, or weak equivalence principle; the validity of Lorentz invariance in local freely falling frames; and the position invariance of local physical laws—the weak equivalence principle has played the most important role historically, and continues to be a focus of intense theoretical and experimental investigation. From the probably apocryphal 16th century demonstrations by Galileo at Pisa's leaning tower to the sensitive torsion-balance measurements of today (both pictured on the cover of this issue), this principle, dubbed WEP, has been crucial to the development of gravitation theory. The universality of the rate of acceleration of all types of matter in a gravitational field can be taken as evidence that gravitation is fundamentally determined by the geometry, or metric, of spacetime. Newton began his magnum opus 'The Principia' with a discussion of WEP and his experiments to verify it, while Einstein took WEP for granted in his construction of general relativity, never once referring to the epochal experiments by Baron Eötvös.
The classic 1964 experiment of Roll, Krotkov and Dicke ushered in the modern era of high-precision tests, and the search for a 'fifth force' during the late 1980s (instigated, ironically, by purported anomalies in Eötvös's old data) caused the enterprise to pivot from pure tests of the foundation of GR to searches for new physics beyond the standard model of the non-gravitational interactions.
Today, the next generation of experimental tests of WEP are being prepared for launch or are being developed, with the goal of reaching unprecedented levels of sensitivity, in search of signatures of interactions inspired by string theory, extra dimensions and other concepts from the world of high-energy physics. At the same time observations continue using lunar laser ranging and binary pulsar timing to test a stronger version of WEP, in order to verify whether gravitational mass/energy itself falls with the same acceleration as normal matter.
This focus issue brings together a set of invited papers to explore the many aspects of testing WEP. An introductory article laying out the theoretical context is followed by articles on current laboratory experiments. Four articles describe the latest results from lunar laser ranging and binary pulsar timing, while two articles discuss progress toward testing the free fall of antihydrogen. The final four articles address future experiments to be carried out in space on orbiting or sub-orbital platforms.
We hope that readers will take away from these articles both the centrality of this principle to gravitational physics and the rich and wide-ranging experimental activity that is being carried out to test it.
C C Speake and C M Will Guest Editors