Testing General Relativity Experimentally

Tests of general relativity are notoriously difficult, due to the small size of general relativistic corrections to Newtonian gravity in the weak field limit. Yet, ingenious experiments continue to be devised and performed, which so far confirm the theory.

General relativity predicts that photons going up (down) in a gravitational field are red-shifted (blue-shifted). Robert Pound and Glen Rebka managed to confirm this tiny (1 part in 1015 ) effect over a drop of only 74 feet. A later test by Robert Vessot et al. verified the shift over a distance of 10,000 km.

General relativity is based on the equivalence principle, which implies that gravitational and inertial masses are the same. Over the years, several groups have tested the equivalence principle to high precision, notably in a recent series of elegant experiments by Eric Adelberger and colleagues.

Some modified theories of gravity, such as Brans-Dicke theory, violate the equivalence principle, at least in its most stringent form, due to the Nordvedt effect. This is where the self-energy of a body contributes to its gravitational mass, but not to its inertial mass. This effect becomes important only for very massive bodies, and thus requires astronomical tests, such as the lunar lasing tests of James Williams et al.

See Physics article: Focus: The Weight of Light