A reactionless drive (also known by many other names, such as an inertial propulsion engine, reactionless thruster, reactionless engine, bootstrap drive or inertia drive) is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion. The name comes from Newton's Third Law of Motion, usually expressed as: "For every action, there is an equal and opposite reaction". Such a drive would necessarily violate the conservation of momentum, a fundamental principle of all current understandings of physics. In addition it can be shown that conservation of energy is violated.
In spite of their physical impossibility, such devices are a staple of science fiction, particularly for space propulsion, and as with perpetual motion machines have been proposed as working technologies.
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An oscillation thruster (also known as a stiction drive, internal drive or slip-stick drive) proposes to use the motion of internal masses to create a net thrust. These thrusters include either vibrational or rotating masses, in which one portion of the cyclical motion is high-speed, and the other low-speed, or alternately high and low impulse. The actual result is that for some of the motion a high force is generated, enough to overcome friction. However on the "return stroke" the force is not high enough, and any motion occurring in the first portion is not reset. In this way the devices "steal" working mass from their supporting surface, a fact that may not be apparent to casual observation.
Basically, an oscillation thruster works just like walking does, one mass is "thrown" backward, "thrusting" the device forward according to conservation of momentum (like a person taking a step forward), then the mass is more slowly brought forward to its initial position (like the person using their step to pull the rest of their body forward). The thruster is allowed to move forward in the first step because the mass is "thrown" back with large enough force to overcome static friction. The "thruster" doesn't move backward in the second step because static friction provides an outside force that overcomes the step (like the ground does when you're walking).
Although there have been many versions proposed, all oscillation thrusters have the following common components:
A primary feature is that these internal masses go through some sort of cyclic motion where the motion in one direction is quicker than in the return direction.
Inventors of oscillation thrusters extrapolate its behavior to mean that it can work in a vacuum in zero gravity.
One of the most famous proposed reactionless drives was the Dean drive. Although Dean himself gave few indications of how his "reactionless drive" was supposed to work, it appears to be an attempt at an oscillation thruster. Other examples of oscillation thrusters are the Thornson Inertial Engine[1] and Henry Bull’s Impulse Engine of 1935.[2]
Magnetoelectric materials can be electrically polarized by magnetic fields; the quantum vacuum contains randomly fluctuating magnetic fields. A sufficiently small piece of magnetoelectric material can be placed in the quantum vacuum and then rotated 180 degrees. The random magnetic fields can induce a change in electric polarization in the block of material. It has been proposed that this change causes the combined electric and magnetic fields to push the block in one direction while they (the fields themselves, or more technically, the bosons of which these fields are composed) get pushed in the other direction. The trick doesn't work if the magnetoelectric block is too large but curiously it should work if there's a grid of many tiny blocks working together. This mechanism was first suggested in December 2009 and has not been tested. The author of the original paper suggested that the mechanism could be used would be powerful enough to reorient satellites if a material with a magnetoelectric constant ten times higher than the strongest such materials known today could be found.[3]
Reactionless drives are impossible as "breaking the law of conservation of momentum shatters the entire mathematical framework" of physics.[4] These devices by their very nature violate the law of conservation of momentum—if a ship with such a drive existed, then any motion of that ship, unaccompanied by the motion of any other material, would cause the center of gravity of the universe to move with it.
Furthermore, the total energy of the universe becomes reference frame dependent—energy can only be conserved in the presence of a reactionless drive in (at most) one reference frame, because in all other reference frames the change in kinetic energy of the ship differs from the power expended by the ship. Given that these two laws—the conservation of momentum and the principle of relativity—are so well established (and thoroughly verified) in physics, any ordinary mechanical system purporting to violate the laws is generally dismissed outright.[5]
Although the basic tenet of reactionless propulsion is physically impossible, hypothetical cases have been put forward which would create the same effect without evidently violating either Newton's Third Law or conservation of energy:
The Dean drive, invented by Norman L. Dean, is a device intended to be a reactionless thruster. According to Dean his models were able to demonstrate this effect. He received two patents for related devices that are known to be unable to generate a uni-directional force, but he occasionally demonstrated devices that were different.[9][10] Dean's claims of reactionless thrust generation have subsequently been shown to be in error; the thrust generated is understood to be reliant on friction with the surface on which the device is resting.[11][12]
The Gyroscopic Inertial Thruster is a proposed reactionless engine which uses entirely mechanical principles. The concept involves various methods of leverage against the supports of a large gyroscope.[13] Scottish inventor Sandy Kidd, a former RAF radar technician, investigated the possibility without success in the 1980s.[14] He posited that a gyroscope set at various angles could provide a lifting force, defying gravity.[15] In the 1990s, several people sent suggestions to NASA's Space Exploration Outreach Program (SEOP) recommending that NASA study gyroscopic inertial drive, especially the developments of American inventor Robert Cook, and Canadian inventor Roy Thornson.[13] In the '90s and 2000s, enthusiasts attempted building and testing GIT machines.