The Confrontation between General Relativity and Experiment: A Centenary Perspective

Abstract

We review the experimental evidence for Einstein’s special and general relativity. A variety of high precision null experiments verify the weak equivalence principle and local Lorentz invariance, while gravitational redshift and other clock experiments support local position invariance. Together these results confirm the Einstein Equivalence Principle which underlies the concept that gravitation is synonymous with spacetime geometry, and must be described by a metric theory. Solar system experiments that test the weak-field, post-Newtonian limit of metric theories strongly favor general relativity. The Binary Pulsar provides tests of gravitational-wave damping and of strong-field general relativity. Recently discovered binary pulsar systems may provide additional tests. Future and ongoing experiments, such as the Gravity Probe B Gyroscope Experiment, satellite tests of the Equivalence principle, and tests of gravity at short distance to look for extra spatial dimensions could constrain extensions of general relativity. Laser interferometric gravitational-wave observatories on Earth and in space may provide new tests of gravitational theory via detailed measurements of the properties of gravitational waves. During the late 1960s, it was frequently said that “the field of general relativity is a theorist’s paradise and an experimentalist’s purgatory”. To be sure, there were some experiments: Irwin Shapiro, then at MIT, had just measured the relativistic retardation of radar waves passing the Sun (an effect that now bears his name), Robert Dicke of Princeton was claiming that the Sun was flattened in an amount that would mess up general relativity’s success with Mercury’s perihelion advance, and Joseph Weber of the University of Maryland was just about to announce (40 years prematurely, as we now know) the detection of gravitational waves. Nevertheless the field was dominated by theory and by theorists. The field circa 1970 seemed to reflect Einstein’s own attitudes: although he was not ignorant of experiment, and indeed had a keen insight into the workings of the physical world, he felt that the bottom line was the theory. As he once famously said, if experiment were to contradict the theory, he would have “felt sorry for the dear Lord”. Since that time the field has been completely transformed, and today at the centenary of Einstein’s annus mirabilis, experiment is a central, and in some ways dominant component of gravitational physics. To illustrate this, one needs only to cite the first regular article of the 15 June 2004 issue of Physical Review D: the author list of this “general relativity” paper fills an entire page, and the institution list fills most of another. This was one of the papers reporting results from the first science run of the LIGO laser interferometer gravitational-wave observatories, but it brings to mind papers in high-energy physics, not general relativity! The breadth of current experiments, ranging from tests of classic general relativistic effects such as