Dr. James Woodward’s research has led to the Woodward Effect, a radiation reaction and Wheeler-Feynman like in that it makes the supposition that when an object with mass accelerates, it radiates gravitational waves that interact with the rest of the universe. In particular, these gravitational waves travel from the present accelerating object to the future universe, and may open the door to inertial propulsion.
Woodward has shown that if you accelerate an object with mass in a particular manner that it is possible for the mass of that object to fluctuate. Therefore, if you push on the object when it is less massive and pull on the object when it is more massive, the object will experience a net thrust. This is effect is called the Woodward Effect. Ultimately, these mass fluctuations are caused by the gravitational radiation reaction between the object and the rest of the universe.