The Equivalence Principle (EP) is not one of the ``universal'' principles of
physics (like the Action Principle). It is a heuristic hypothesis which was
introduced by Einstein in 1907, and used by him to construct his theory of
General Relativity. In modern language, the (Einsteinian) EP consists in
assuming that the only long-range field with gravitational-strength couplings
to matter is a massless spin-2 field. Modern unification theories, and notably
String Theory, suggest the existence of new fields (in particular, scalar
fields: ``dilaton'' and ``moduli'') with gravitational-strength couplings. In
most cases the couplings of these new fields ``violate'' the EP. If the field
is long-ranged, these EP violations lead to many observable consequences
(variation of ``constants'', non-universality of free fall, relative drift of
atomic clocks,...). The best experimental probe of a possible violation of the
EP is to compare the free-fall acceleration of different materials.Comment: 14 pages, contribution to the ONERA workshop on "Missions spatiales
en physique fondamentale" (Chatillon, 18-19 Jan 2001), to appear in a special
issue of the Comptes Rendus de l'Academie des Sciences (Paris), edited by C.
Borde and P. Toubou
