equivalenceprincipe

Equivalence principle, sometimes rendered in French as principe d'équivalence, is a fundamental concept in physics that expresses a deep connection between gravity and acceleration. In its broad sense it states that local experiments performed in a freely falling reference frame cannot distinguish between a uniform gravitational field and an equivalent acceleration of the frame. The principle underpins the view that gravity is geometrical in general relativity.

The principle is commonly formulated in several closely related versions. The weak equivalence principle (WEP) asserts that the trajectory of a freely falling test body is independent of its composition and structure, implying the equality of inertial and gravitational masses. Einstein’s equivalence principle (EEP) strengthens this by requiring that local non-gravitational experiments yield results independent of the velocity or position of the freely falling frame, i.e., local Lorentz invariance and local position invariance hold. The strong equivalence principle (SEP) extends these ideas to include gravitational experiments themselves, positing that the outcome of any local experiment, including those involving gravitation, is independent of where and when in the universe it is performed and of the frame’s velocity.

Historically, the concept traces to Galileo’s thought experiments and the experimental work of Eötvös and others, culminating in Einstein’s incorporation into general relativity around 1915. Experimental tests have become increasingly precise: WEP tests reach parts in 10^13–10^14 with torsion balance and lunar laser ranging; space missions like MICROSCOPE have pushed sensitivities to about 10^−15. Gravitational redshift tests (local position invariance) achieve accuracies around 10^−4 to 10^−5 with clock experiments such as Gravity Probe A and Pound-Rebka-type measurements.

The equivalence principle remains a cornerstone of general relativity, constraining alternative theories of gravity that predict violations and guiding experimental searches for new physics.