  Einstein's happiest thought was his leap from the observation that a falling
person feels no gravity to the realization that gravity might be equivalent to
acceleration. It affects all bodies in the same way because it is a property of
spacetime -- its curvature -- not a force propagating through spacetime (like
electromagnetic or nuclear forces). When expressed in a way that is manifestly
independent of the choice of coordinates, this idea became General Relativity.
But the ground for what is now known as the "equivalence principle" was laid
long before Einstein, affording a fascinating example of the growth of a
scientific idea through the continuous interplay between theory and experiment.
That story continues through the present era, with improved techniques and
measurements, and in the new environment of space. Theoretically, equivalence
is now understood to rank with Lorentz invariance as one of nature's most
fundamental principles. Violations of equivalence are generically predicted by
attempts to unify gravity with "non-geometrical" fundamental interactions, but
there is no consensus on the form that these violations will take. Experimental
tests of equivalence thus face the challenge of striving simultaneously for the
highest possible sensitivity and diversity in test materials.
