A reactionless drive or inertial propulsion engine (also reactionless thruster, reactionless engine, and inertia drive) is any form of propulsion not based around expulsion of fuel or reaction mass - 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 use a hypothetical form of thrust that does not require any outside force or net momentum exchange to produce linear motion. While such a device may not necessarily violate the law of conservation of energy, it would appear to violate conservation of momentum, a fundamental principle of all current understandings of physics, and is therefore considered by most physicists impossible to construct outside of science fiction.
All devices that have been claimed to be reactionless drives and tested under controlled conditions have failed and are found to rely on a non-linear effect of the supports they sit on. Some examples include devices that inch along a tabletop or when floating in water, but stop working once they are suspended or in vacuum.
The classic example was the patented Dean drive, named after Norman L. Dean, which gained considerable publicity in the 1950s and 1960s, particularly in the columns of John W. Campbell, editor of Astounding Science Fiction magazine. Authors in the science-fiction genre have continued to make considerable use of the concept. For example, the Basestar in Battlestar Galactica, impeller-driven ships of the Honorverse and some ships in Larry Niven's Ringworld all use reactionless drives.
There are hundreds of such devices, but the most common is the oscillation thruster, which uses friction to transfer momentum to the device. Misconceptions as to how these devices work have led to people believing they are building reactionless drives - when in reality they are not. Another class of similar devices uses interaction with the air in a similar manner. As such they are not economic breakthroughs, as wheels and propellors are far more efficient ways of moving a vehicle in reaction to air or ground.
In most cases the devices in question are supported by a single inventor. Often, some people involved in the creation and promotion of the device (namely the inventor) blame "Big Science" for the failure of the idea to take off.
A subtle problem with energy occurs, specifically the energy becomes reference frame dependent and for energy to be conserved, a favoured reference frame becomes necessary in which energy is conserved, whereas in all other frames it is violated. This is not compatible with relativity theory.
Furthermore, if a true reactionless drive was activated, it would use its energy relative to the favoured reference frame. It's very unlikely that the Earth's surface would be that frame, and the Earth's surface is moving relative to the Earth's axis, the Earth is moving relative to the Sun at about 30km/s etc. The net upshot is that it is highly unlikely that for a reasonable energy input, that much change in velocity will occur, as a given input of energy doesn't change the velocity much at such high speeds.
This would, at best, make such a device impractical.
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 crucial 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:
Electrostatic anti-gravity relies on oddly placed capacitors and a high-voltage source. Claims of electrostatic anti-gravity are explained by the Biefeld–Brown effect, which is distinctly not anti-gravity. Believers of this form of anti-gravity agree that the Biefeld–Brown effect is in action, but say that it does not account for all the upward force. This area of research has not been fully exhausted, but all verified experiments fail to show any new physics. Supposed "Electrostatic anti-gravity"-machines have been tested in vacuums, even in an episode of the Mythbusters, and it has been observed that they do not generate any force when placed in a vacuum.
At first glance, a device called the electrodynamic tether might appear to constitute a reactionless drive because it does not expel reaction mass. But it is still not reactionless, as electromagnetic fields can carry energy and momentum.
The fundamental scientific problem is one of momentum transfer. If there is no momentum transfer, the postulated device is classified as a "reactionless" drive and labelled a fraud. If there is a mechanism for momentum transfer, then the device is classed as a reaction drive and is therefore by definition not a "reactionless" drive. Tethers do have a mechanism for momentum transfer and although they do not expel reaction mass like a rocket, they do transfer momentum and hence can not be a true reactionless drive.