Gravitation, Equivalence Principle, and Quantum Mechanics

TL;DR

This paper explores the intersection of gravitation, the equivalence principle, and quantum mechanics, highlighting experimental tests, potential violations, and the challenges of unifying these fundamental theories through atom interferometry.

Contribution

It discusses recent research issues at the intersection of gravity and quantum mechanics, especially in the context of atom interferometry and fundamental theoretical challenges.

Findings

Experimental tests uphold the equivalence principle with high precision.

Studying quantum systems in gravitational fields may reveal new physics.

Fundamental incompatibilities between General Relativity and Quantum Theory are discussed.

Abstract

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".