[1][2] The project ended in 2002, when the Advanced Space Transportation Program was reorganized and all speculative research (less than Technology readiness level 3) was cancelled.
[2] Experiments tested Woodward’s theory[9][10] of inducing transient inertia by electromagnetic fields.
[1][2] A possible torsion-like effect in the coupling between electromagnetism and spacetime,[12] which may ultimately be useful for propulsion, was sought in experiments.
[2] Other theories listed in Millis's final assessment as unresolved are Abraham–Minkowski electromagnetic momentum, interpreting inertia and gravity quantum vacuum effects, and the Podkletnov force beam.
A third research avenue that might ultimately prove useful for spacecraft propulsion is the coupling of fundamental forces on sub-atomic scales.
The differential sail is a speculative drive, based on the possibility of inducing differences in the pressure of vacuum fluctuations on either side of a sail-like structure — with the pressure being somehow reduced on the forward surface of the sail, but pushing as normal on the aft surface — and thus propel a vehicle forward.
[2][13][14] The Casimir effect was investigated experimentally and analytically under the Breakthrough Propulsion Physics project.
[1] After funding ended, program manager Marc G. Millis was supported by NASA to complete documentation of results.