  In this paper we establish quantitative results about the bridges of the
Langevin dynamics and the associated reciprocal processes. They include an
equivalence between gradient estimates for bridge semigroups and couplings,
comparison principles, bounds of the distance between bridges of different
Langevin dynamics, and a Logarithmic Sobolev inequality for bridge measures.
The existence of an invariant measure for the bridges is also discussed and
quantitative bounds for the convergence to the invariant measure are proved .
All results are based on a seemingly new expression of the drift of a bridge in
terms of the reciprocal characteristic, which, roughly speaking, quantifies the
" mean acceleration " of a bridge.
