Anti-gravity machine

An anti-gravity machine is a machine that allows one to conquer the force of gravity and fly. In common use, the term refers to devices that operate contra the known laws of physics, require less energy than is physically possible, or utilise strange phenomena far outside the realm of Newtonian physics, rather than well-known technologies such as aircraft and hot air/helium balloons. Hypothetical power plants that might drive flying saucers often rely on some kind of anti-gravity. Many anti-gravity machines would also qualify as perpetual motion machines: if you can genuinely shield something from gravity, this allows one to generate perpetual motion (raise something "for free" using anti-gravity then drop it, doing work as it falls, and repeat).

Research in anti-gravity has often been spurred by new scientific discoveries, such as general relativity and superconductors. It is unclear whether an anti-gravity device would necessarily violate the laws of physics, and physicists debate the extent to which quantum mechanics and general relativity allow exotic forms of locomotion such as wormholes and for faster-than-light travel. But as yet, there is no practical way of shielding an object from gravity or levitating an object without inputting a much greater amount of energy than the object gains in gravitational potential.

There is evidence that the US government, the US military, and agencies including NASA have occasionally investigated unorthodox antigravity machines. There is however no evidence that they've found anything.

The US patent office is supposed to automatically reject anti-gravity machines, perpetual motion machines, and other inventions that are obviously contrary to the laws of physics, but occasionally things slip through their net. This invariably adds to the suspicion of conspiracy theorists who think the government already has secret technology powering its UFOs and black helicopters.

Gravity shields
This is one of the simplest ideas: what if there was a material that blocks gravity? Anything behind or above the shield would float!

This has featured in science fiction, such as H.G. Wells' "The first Men in the Moon". But there are no obvious candidates in the real world. Today most interest (at least from eccentric home researchers) is on shields with some kind of electrical property. Some people have proposed (materials with a permanent electrostatic charge), or capacitors, and there is also interest in superconductors; these are discussed below.

Superconductivity
Eugene Podkletnov (a Russian physicist) and Boris Volfson (of Huntington, Indiana) have both suggested that superconductors can somehow shield an object from gravity and allow it to fly. Superconductors are clever things, and appear to offer a kind of perpetual motion machine in that they allow motion without resistance, but that does not include flight.

Eugene Podkletnov in 1992 proposed blocking gravity with a spinning superconducting disk, claiming a reduction of 0.5-2% in weight, in a paper published in Physica C. He wrote a more substantial paper that was initially accepted by Journal of Physics-D and due for publication in 1996, but when journalists leaked the story ahead of publication, Podkletnov was rejected both by his employer, the Institute of Materials Science in the University of Tampere, Finland, and his credited co-author, and as a result he withdrew the paper. There were nonetheless various attempts to develop his research, and in 1999, NASA engaged a Columbus, Ohio company Superconductive Components Inc to try and produce a prototype, despite most scientists feeling it was nonsense. Boeing also conducted research, according to press reports in 2014. In 2020, it was announced that Podkletnov was still working in the same area.

Other electric solutions
An "electric rocket" using capacitors has also been proposed, but is considered implausible (as well as risking fatal electric shocks).

Gyroscopes
Gyroscopes are clever little things that spin and always point in the same direction, resisting attempts to turn them; if a force is applied, it produces a highly unintuitive movement at a 90 degree angle, which suggests magic to the naive. They also appear to exhibit an upward "anti-gravity" force: if supported on one side they appear to hover in space. This is not a mysterious force or method of anti-gravity; all their behavior is a consequence of torque around the vertical axis and can be explained perfectly well by Newtonian physics.

Nonetheless, a number of people have claimed that a spinning gyroscope reduces its weight somehow, in a way not explained by current physics. The engineer Eric Laithwaite claims to have discovered strange behavior in gyroscopes, and Hideo Hayasaka and Sakae Takeuchi of Tokohu University reported a weight loss of 0.005% in a 1989 paper.

Laithwaite was Professor of Electrical Engineering at the distinguished Imperial College, London, Professor of Applied Electricity at the Royal Institution, and a pioneer in the development of the and maglev trains. But he had unorthodox views on gyroscopes, claiming to detect a weight loss in experiments. He set out his views to British children in the televised Royal Institution Christmas Lectures in 1974, despite the Royal Institute refusing to publish his reports. It later emerged that he had used engineering approximations in his calculations, and a more accurate calculation revealed there was no discrepancy in his measurements and no weight loss. His continuing defence of his ideas reportedly harmed his career: he left the Royal Institution in 1975 and was refused a Fellowship of the Royal Society.

Likewise, scientists suggested that errors in Hayasaka and Takeuchi's experimental procedure were probably to blame, and attempts by others to repeat it did not find anything. As yet there is no evidence that gyroscopes can fly.

Unequal pressure
It has been suggested that the atomic nature of gas can be exploited, similar to in Maxwell's demon or the If one could let air molecules travel downwards through a membrane but prevent them from going upwards, this would naturally lead to the membrane and anything attached to it rising.

A similar mechanism has been advanced using variations in quantum energy density, particularly the zero-point energy of empty space. It is well known that the can force two plates together, but this doesn't get you far. A variation on this, the dynamical Casimir effect, produces a force by accelerating a mirror through empty space. This has been suggested as a form of propulsion, and it appears theoretically possible, but in practice would provide a tiny amount of thrust.

The EmDrive, partly developed by NASA's Eagleworks, uses differing pressures from electromagnetism in a shaped cavity, where the pressure in one direction is supposedly different to that in the other direction, to supposedly provide a method of propulsion that seems to go against the laws of physics. There have been demonstrations, although skepticism is still strong. Such a device would emit no matter, not even photons: this is in contrast to a range of drives that emit electrically-charged ions or subatomic particles and are well within the bounds of physics.

Fifth force
A force in addition to strong nuclear, weak nuclear, electromagnetism and gravity has been postulated; it may be attractive or repulsive. It's not terribly clear how such a force could be used, unless it could be turned on and off — and if one can control the fifth force, why not gravity?

US government
The US military and government agencies have pursued a lot of crazy ideas (MKULTRA, Project Stargate, etc), often in conjunction with private contractors. In the 1950s there seemed to be particular interest in anti-gravity, with physicist Louis Witten conducting research at the Research Institute for Advanced Studies for aviation firm Glenn L. Martin Company (later part of Lockheed Martin).

NASA's from 1996-2002 investigated a variety of outlandish ideas for space travel.

Gravity Research Foundation
Founded by Roger Babson in 1948, this was one of the most serious attempts at defying gravity, although it later moved into more conventional scientific research.

Rumors
There are rumors of other research efforts. A secret base at Area 51 was allegedly disassembling UFOs for the US government and figuring out how they flew; we can safely assume that is nonsense.

There is talk about Edward Teller, the slightly sinister physicist best known for the hydrogen bomb and considered one of the influences for Stanley Kubrick's Dr Strangelove. He published a paper on the electrical fields in rapidly rotating insulators, and their relationship to gravity via general relativity. Because of this (which sounds like some of the anti-gravity devices already mentioned) and because of his status as a prominent figure in Cold War physics, it is rumored he was involved in anti-gravity research, although discussion seems restricted to more fringe sites such as ExoPolitics, so it might be taken with a pinch of salt.

Tricks and mistakes
Inventors of anti-gravity machines often fall into two categories: cranks who deceive themselves and fraudsters who deceive others. It is easy to convince oneself that one is changing the weight of an object, particularly since demonstrated effects tend to be very small: things like vibration or magnetic fields can affect the performance of scales leading to incorrect readings even if great care is taken. This is often suggested as an explanation in anomalous gyroscope experiments. In theory, even slightly reducing an object's weight would be impressive, but in practice small changes in measurement make it more difficult to control for sources of error that affect the result. As with perpetual motion machines, energy can be added to a system either surreptitiously or accidentally.

In fiction

 * HG Wells invented Cavorite which blocks gravity in The First Men in the Moon
 * James Blish wrote about the in several science fiction books