We provide a pedagogical discussion of the nature of the Equivalence Principle as it applies (familiarly) in classical physics and (less familiarly) in quantum physics. In particular we provide an apparently new derivation of the Colella, Overhauser and Werner (COW) formula for the gravitationally induced phase shift expected of a quantum mechanical neutron beam traversing an interferometer placed vertically in the earth's gravitational field, and show that this COW effect is completely compatible with the quantum-mechanical Equivalence Principle. We also derive an analogous phase shift formula for purely classical light waves traversing the same interferometer, and show that the two formulas (for light and neutrons) nicely exhibit wave-particle duality.

ANL Physics Division Colloquium Schedule