Reactionless drive

These two also stand out as they both received much publicity from their promoters and the popular press in their day and both were eventually rejected when proven to not produce any reactionless drive forces.

[2] More recently, the EmDrive was taken seriously enough to be tested by a handful of physics labs, but similarly proved to not produce a reactionless drive force.

[9] Eric Laithwaite, the "Father of Maglev", received a US patent for his own propulsion system, which was claimed to create a linear thrust through gyroscopic and inertial forces.

[2] David M. Burns, formerly a NASA engineer at the Marshall Space Flight Center in Alabama, theorized a potential spacecraft propulsion drive that could possibly exploit the known mass-altering effects that occur at near the speed of light.

He wrote a paper published in 2019 by NASA in which he describes it as "A new concept for in-space propulsion is proposed in which propellant is not ejected from the engine, but instead is captured to create a nearly infinite specific impulse".

Because there is no well-defined "center of mass" in curved spacetime, general relativity allows a stationary object to, in a sense, "change its position" in a counter-intuitive manner, without violating conservation of momentum.