As distinct from conventional electrodynamics in which the advanced potential solution of Maxwell's equations is rejected on causal grounds, absorber theory allows the possibility of a mixture of advanced and retarded radiation, dependent on cosmological boundary conditions. In a recent experiment Partridge attempted to detect advanced effects by introducing a local absorber, but it was maintained by Pegg that, because a static absorber was used, only a null result was possible. The authors give the theory and a brief outline of an experiment which uses a time-asymmetric chopper absorber to alter the boundary conditions and thus the ratio of the advanced to retarded components in the mixture, provided this is non-zero initially, leading to possibly detectable effects.