Testing the Universality of Free Fall

There's an interesting debate in Nature about atom interferometry tests of gravitational redshifts and what they imply about the university of free fall and the Einstein equivalence principle. See the preprint Atom gravimeters and gravitational redshift and the reply. See also the preprint Gravitational Redshift, Equivalence Principle, and Matter Waves. One of the authors in this controversy is Nobel prize winner and Department of Energy secretary Steven Chu.