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https://en.wikipedia.org/wiki/Breakthrough_Propulsion_Physics_Program

The Breakthrough Propulsion Physics Program (BPP) is a research program which was funded from 1996 through 2002 by NASA, in the hope of studying various proposals for "revolutionary" methods of spacecraft propulsion which would require breakthroughs in physics before they could be realized, hence the name. Specific proposals studied under the aegis of the BPP included the diametric drive, the pitch drive, the bias drive, the Alcubierre drive, the disjunction drive, and the differential sail.

Diametric drive

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 Jamie 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. However, positive and negative gravity on a single spacecraft involves balanced forces within a structure, and would not result in acceleration.

Additionally, it was argued that stability issues might arise.

Disjunction drive

The "Disjunction Drive": 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."

Pitch drive and bias drive

One proposed method of achieving a "diametric drive", or possibly a "disjunction drive", which was allegedly studied in the BPP was called the pitch drive. This has been described as involving a hypothetical "disjoint field" which, it was claimed, would eliminate the need for the field to be generated on the spacecraft itself.

One specific proposal for such a pitch drive was allegedly called the bias drive. According to this proposal, if it were possible to locally alter the value of the gravitational constant G in front of and behind the craft, one could create a bias drive. It might help to point out here that while the gravitational constant is indeed a fundamental physical constant in our current gold standard theory of gravitation, general relativity, its best known competitor, the Brans-Dicke theory of gravitation, does in a sense allow for a locally varying gravitational constant, so the notion of a locally varying gravitational constant has been seriously discussed in mainstream physics. It has been claimed that one problem with the concept of a bias drive was that it might create a singularity in the field's gradient located inside the vehicle, but this "objection" appears to be as speculative as the proposal itself.

Alcubierre drive

The Alcubierre drive, also called the warp drive, is a proposal, originally due to the physicist Miguel Alcubierre, which consists of a toy model of a Lorentzian spacetime with properties somewhat reminiscent of the fictional "warp drive" from the science fiction series Star Trek. In the semipopular literature, this proposal has often been described as having the status of an exact solution of the Einstein field equation, but this characterization is wildly misleading. In fact, every possible spacetime is an exact solution given a certain configuration of energy. It has also been claimed that the Alcubierre drive is grounded in a well-established physical effect, the Casimir effect, which is currently understood in terms of quantum field theory, but this is also rather misleading.

Differential sail

The differential sail was another speculative proposal, which appealed to the zero-point energy field. As the Heisenberg uncertainty principle implies that there is no such thing as an exact amount of energy in an exact location, vacuum fluctuations are known to lead to discernible effects such as the Casimir effect. The differential sail apparently was a speculation that it might be possible to use the cosmic background radiation to create some kind of light pressure gradient which could be used to push on some kind of sail.

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Published in July 2009.

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