For the solar system, ϵ < 10 −5 this is the regime of weak gravity.Īn alternative view of “strong” gravity comes from the world of particle physics. Near the event horizon of a non-rotating black hole, or for the expanding observable universe, ϵ ∼ 1 for neutron stars, ϵ ∼ 0.2. To one steeped in general relativity, the principal figure of merit that distinguishes strong from weak gravity is the quantity ϵ ∼ GM/Rc 2, 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. ![]() Much like modern art, the term “strong” means different things to different people. Since that time, the field has entered what might be termed a Quest for Strong Gravity. The results all supported general relativity, and most alternative theories of gravity fell by the wayside (for a popular review, see ). 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 general relativistic prediction of gravitational-wave energy loss (1979). New technologies - atomic clocks, radar and laser ranging, space probes, cryogenic capabilities, to mention only a few - played a central role in this golden era. Experimental gravitation experienced a Golden Era (1960–1980) during which a systematic, world-wide effort took place to understand the observable predictions of general relativity, to compare and contrast them with the predictions of alternative theories of gravity, and to perform new experiments to test them. 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.īut beginning around 1960, astronomical discoveries (quasars, pulsars, cosmic background radiation) and new experiments pushed general relativity to the forefront. 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 general relativity - the deflection of light and the perihelion advance of Mercury. The modern history of experimental relativity can be divided roughly into four periods: Genesis, Hibernation, a Golden Era, and the Quest for Strong Gravity. He famously stated that if the measurements of light deflection disagreed with the theory he would “feel sorry for the dear Lord, for the theory is correct!”.īy contrast, today experimental gravitation is a major component of the field, characterized by continuing efforts to test the theory’s predictions, both in the solar system and in the astronomical world, to detect gravitational waves from astronomical sources, and to search for possible gravitational imprints of phenomena originating in the quantum, high-energy or cosmological realms. But compared to the inner consistency and elegance of the theory, he regarded such empirical questions as almost secondary. ![]() ![]() Admittedly, 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. When general relativity was born 100 years ago, experimental confirmation was almost a side issue. Current and future tests of relativity will center on strong gravity and gravitational waves. Gravitational wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Einstein’s equivalence principle (EEP) is well supported by experiments such as the Eötvös experiment, tests of local Lorentz invariance and clock experiments. ![]() The status of experimental tests of general relativity and of theoretical frameworks for analyzing them is reviewed and updated.
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