Disjunction drive

The Breakthrough Propulsion Physics Project (BPP) was a research project funded by NASA from 1996-2002 to study various proposals for revolutionary methods of spacecraft propulsion that would require breakthroughs in physics before they could be realized . During its six years of operational funding, this program received a total investment of $1.2 million.

The project addressed a selection of "incremental and affordable" research questions towards the overall goal of fuel-less propulsion, hyper-fast travel, and breakthrough propulsion methods. By the end of the project, 16 of these investigations were completed. The result was that "about a third were found not to be viable, a quarter have clear opportunities for sequels, and the rest remain unresolved."

The diametric drive was a speculative proposal for an "engine" which would create a non-conservative gravitational field with non-zero curl. It was argued that in such circumstances, the side of the field which creates more force on the spacecraft will accelerate the spacecraft in the direction of the force.

One idea for realizing this concept involved hypothetical particles with negative mass, originally proposed by Robert Forward and James Woodward. If one were to construct a block of negative mass, and then attach it to a normal "positive" mass, the negative mass would fall towards the positive as does any mass toward any other. On the other hand, the negative mass would generate "negative gravity", and thus the positive mass (the spaceship itself generally) would fall away from the negative mass. If arranged properly, the distance between the two would not change, while they continued to accelerate forever. It has been argued that stability issues might arise.

The concept is illustrated according to Millis by:

where:

By disjunction is meant the separation of the source of a field from the matter with which it would otherwise interact; viz., under normal physical states. According to a summary of speculative propulsion ideas on NASA's website:

This concept entertains the possibility that the source of a field and that which reacts to a field can be separated. By displacing them in space, the reactant is shifted to a point where the field has a slope, thus producing reaction forces between the source and the reactant. Although existing evidence strongly suggests that the source, reactant, and inertial mass properties are inseparable, any future evidence to the contrary would have revolutionary implication to this propulsion application.

...
Wikipedia