  This white paper is submitted as part of Snowmass2013 (subgroup CF2). The
extraordinary sensitivity of torsion-balances can be used to search for the
ultra-feeble forces suggested by attempts to unify gravity with the other
fundamental interactions. The motivation, the results and their implications as
well as the future prospects of this work are summarized. The experiments
include tests of the universality of free fall (weak equivalence principle),
probes of the short-distance behavior of gravity (inverse-square law tests for
extra dimensions and exchange forces from new meV scale bosons), and
Planck-scale tests of Lorentz invariance (preferred-frame effects,
non-commutative geometries).
