1 Introduction

At the time of the birth of general relativity (GR), experimental confirmation was almost a side issue. Einstein did calculate observable effects of general relativity, such as the perihelion advance of Mercury, which he knew to be an unsolved problem, and the deflection of light, which was subsequently verified. But compared to the inner consistency and elegance of the theory, he regarded such empirical questions as almost peripheral. Today, experimental gravitation is a major component of the field, characterized by continuing efforts to test the theory’s predictions, to search for gravitational imprints of high-energy particle interactions, and to detect gravitational waves from astronomical sources.

The modern history of experimental relativity can be divided roughly into four periods: Genesis, Hibernation, a Golden Era, and the Quest for Strong Gravity. The Genesis (1887 - 1919) comprises the period of the two great experiments which were the foundation of relativistic physics - the Michelson-Morley experiment and the Eötvös experiment - and the two immediate confirmations of GR - the deflection of light and the perihelion advance of Mercury. Following this was a period of Hibernation (1920 - 1960) during which theoretical work temporarily outstripped technology and experimental possibilities, and, as a consequence, the field stagnated and was relegated to the backwaters of physics and astronomy.

But beginning around 1960, astronomical discoveries (quasars, pulsars, cosmic background radiation) and new experiments pushed GR to the forefront. Experimental gravitation experienced a Golden Era (1960 - 1980) during which a systematic, world-wide effort took place to understand the observable predictions of GR, to compare and contrast them with the predictions of alternative theories of gravity, and to perform new experiments to test them. The period began with an experiment to confirm the gravitational frequency shift of light (1960) and ended with the reported decrease in the orbital period of the Hulse-Taylor binary pulsar at a rate consistent with the GR prediction of gravity wave energy loss (1979). The results all supported GR, and most alternative theories of gravity fell by the wayside (for a popular review, see [282 ]).

Since 1980, the field has entered what might be termed a Quest for Strong Gravity. Many of the remaining interesting weak-field predictions of the theory are extremely small and difficult to check, in some cases requiring further technological development to bring them into detectable range. The sense of a systematic assault on the weak-field predictions of GR has been supplanted to some extent by an opportunistic approach in which novel and unexpected (and sometimes inexpensive) tests of gravity have arisen from new theoretical ideas or experimental techniques, often from unlikely sources. Examples include the use of laser-cooled atom and ion traps to perform ultra-precise tests of special relativity; the proposal of a “fifth” force, which led to a host of new tests of the weak equivalence principle; and recent ideas of large extra dimensions, which have motived new tests of the inverse square law of gravity at sub-millimeter scales.

Instead, much of the focus has shifted to experiments which can probe the effects of strong gravitational fields. The principal figure of merit that distinguishes strong from weak gravity is the quantity $2 e ~ GM/(Rc )$ , where $G$ is the Newtonian gravitational constant, $M$ is the characteristic mass scale of the phenomenon, $R$ is the characteristic distance scale, and $c$ is the speed of light. Near the event horizon of a non-rotating black hole, or for the expanding observable universe, $e ~ 0.5$ ; for neutron stars, $e ~ 0.2$ . These are the regimes of strong gravity. For the solar system, $- 5 e < 10$ ; this is the regime of weak gravity. At one extreme are the strong gravitational fields associated with Planck-scale physics. Will unification of the forces, or quantization of gravity at this scale leave observable effects accessible by experiment? Dramatically improved tests of the equivalence principle, of the inverse square law, or of local Lorentz invariance are being mounted, to search for or bound the imprinted effects of Planck-scale phenomena. At the other extreme are the strong fields associated with compact objects such as black holes or neutron stars. Astrophysical observations and gravitational wave detectors are being planned to explore and test GR in the strong-field, highly-dynamical regime associated with the formation and dynamics of these objects.

In this Living Review, we shall survey the theoretical frameworks for studying experimental gravitation, summarize the current status of experiments, and attempt to chart the future of the subject. We shall not provide complete references to early work done in this field but instead will refer the reader to the appropriate review articles and monographs, specifically to Theory and Experiment in Gravitational Physics [281 ], hereafter referred to as TEGP. Additional recent reviews in this subject are [276 , 284 , 286 , 71 , 98 , 239 ]. References to TEGP will be by chapter or section, e.g., “TEGP 8.9 [281 ]”.