Gravitation, Equivalence Principle, and Quantum Mechanics

Gravitation, according to General Relativity, is an attribute of space-time's geometry and hence not a force in the Newtonian sense. This is a consequence of Einstein's equivalence principle, which so far passed all experimental tests with high precision. However, the search for possible violations continues, for they, as the case may be, are expected to point towards more fundamental theoretical extensions of General Relativity. In particular, it is expected that useful insights are gained by studying the interaction between gravitational fields and genuine quantum-mechanical systems, like atoms or molecules. But this raises some fundamental issues, for General Relativity and Quantum Theory rest on partially incompatible sets of concepts. This article tries to explain in an elementary fashion how these issues recently entered modern research in atom interferometry. A shorter and editorially adapted version of this paper will appear in "Spektrum der Wissenschaft" -- the German edition of "Scientific American".