What Is Gravity? Testing the Equivalence Principle with Quantum Bound Systems and the Transition to the Classical Limit

Gravity is the force we experience most familiarly in everyday life. From the viewpoint of particle physics, however, it is a highly peculiar force, and that very peculiarity is what makes it so fascinating.

In this talk, I will first outline the appeal and characteristic features of gravity. I will then introduce our attempt to test experimentally, within the framework of quantum theory, the equivalence of inertial and gravitational mass (the equivalence principle) on which general relativity is founded.

The experiment uses a system in which cold neutrons are quantum-mechanically bound in the gravitational field. By measuring the time evolution of this quantum bound system, we evaluate the ratio of inertial to gravitational mass in a quantum system. We also observe how the quantum system transitions to a classical one, seeking clues to understanding the meaning of the procedure of quantization. I will also report on the neutron imaging sensor we are developing for this purpose and on the current status of our experiments at Los Alamos National Laboratory (LANL) in the United States and the Paul Scherrer Institute (PSI) in Switzerland.