Posted by Uncle Al on May 15, 2002 at 19:50
Everything falls identically in vacuum no matter what its composition. GmM/r^2 = ma exactly. The Equivalence Principle is measured true to one in ten trillion. It is the foundation of General Relativity and the wellspring of spacetime curvature.
A shoe ignores the owner of a foot, but it does care about the fit. The proper test of spacetime geometry is test mass geometry, not composition.
It could all be wrong (OK - at least incomplete), and nobody has ever looked in the right place to find the error.