Spin foam serves as a model for 4-geometries used 
Spin foam serves as a model for 4-geometries used to dynamically observe the quantum form of this force. Mossbauer spectroscopy of an iron-57 gamma emitter was used to measure Doppler redshift associated with this force in the Pound-Rebka experiment, and two lead balls were used to construct a torsion balance to measure the magnitude of this force in the Cavendish experiment. Conceptualized by wells in spacetime, for 10 points, identify this weakest fundamental force that causes objects in the air to accelerate to Earth at 9.8 meters per second squared.
Spin foam serves as a model for 4-geometries used to dynamically observe the quantum form of this force.
gravity [or gravitational force; accept equivalents]