1 Tests of the weak equivalence principle
In 1907 Einstein realized that gravity and acceleration are equivalent, thus starting him on the path that led to the general theory of relativity. The weak equivalence principle - which states that test bodies fall with the same acceleration independent of their internal structure or composition - can be tested by measuring the accelerations, a1 and a2, of two bodies made of different materials in the same gravitational field. It is customary to plot η = (a1 – a2)/2(a1= + a2), and this figure shows how the upper limit on η has decreased over time since the first experiments by Baron von Eötvös. Most of the tests have been performed with torsion balances (red arrows), apart from free-fall experiments in which the bodies are dropped in a tower (green region), and lunar laser-ranging experiments (LLR; blue region) that measure the accelerations of the Earth and the Moon in the gravitational field of the Sun. The green region indicates a period when many experiments were performed in search of a "fifth force".