The Eöt-Wash Group:
Laboratory Tests of Gravitational and sub-Gravitational Physics
Null Experiments
There are many different ways to test a theory quantitatively. From a statistical point of view, the most powerful thing to do is design an experiment where you measure a quantity whose value is equal to zero if the theory is correct. Then, if there is a deviation, the deviation is inherently large compared to the predicted measurement. At the opposite extreme, we could imagine trying to look for a deviation from theory that would result in a 0.001% change in the measurement. This would not be easy!
The Eöt-Wash torsion balance tests of the equivalence principle are null experiments. If the equivalence principle is not violated, and if the local gravitational field is flat, then the forces on the test bodies of our pendulum must be parallel. Then, there is no torque if the equivalence principle is correct. Any torque must be due to some imperfection in the construction of the experiment or to an equivalence principle violation. To distinguish these, we must estimate the effects of various kinds of EP violations and then make other effects, such as nonuniformities in the gravitational field, small compared to expected "real" signals.
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