Professor of Physics History at CalTech, Dr James Woodward has been investigating an idea called the Mach Effect, which is supposed to be based on Mach's Principle and which is claimed to allow a means for propellantless propulsion without violating Conservation of Momentum. The idea is that changing the internal energy of an object - electric charge distribution, for example - would cause its mass to change, and therefore oscillating this property in conjunction with mechanical oscillation could lead to an asymmetric acceleration force. Justification for non-violation of Conservation of Momentum is offered by way of Mach's Principle, claiming that the apparent change in momentum of the object is offset by an infinitesimal change in the momentum of rest of the universe. Woodward has been publishing results from his own experiments attempting to measure such an effect, however the results are so weak that they border on being inconclusive. A new paper has recently been published outlining a new experimental setup for more accurate measurement of any possible Mach Effect, postulating that use of magnetic fields in place of a mechanical oscillator should improve the means to observe and detect mass fluctuations, if any indeed occur: http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=13020.0;attach=484587 From p.78: Recognizing that the Mach Effect conjecture itself may not necessarily be valid, what are the opinions on this proposed improvement in the experimental setup and its claims of greater accuracy? Is magnetic field induced acceleration the best possible method to expose or discern whether or not mass fluctuations are occurring? If not, why not?