In physical cosmology, astronomy and celestial mechanics, anti-gravity is the idea of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, nor to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift.
Instead, anti-gravity requires that the fundamental causes of the force of gravity be made either not present or not applicable to the place or object through some kind of technological intervention. Anti-gravity is a recurring concept in science fiction, particularly in the context of spacecraft propulsion. The concept was first introduced formally as "Cavorite" in H. G. Wells' The First Men in the Moon, and has been a favorite item of imaginative work since that day.
In the first mathematically accurate description of gravity, Newton's law of universal gravitation, gravity was an external force transmitted by unknown means. However in the early part of the 20th century Newton's model was replaced by the more general and complete description known as general relativity. In general relativity, gravity is not a force in the traditional sense of the word, but the result of the geometry of space itself. These geometrical solutions always cause attractive "forces". Under general relativity, anti-gravity is highly unlikely, except under contrived circumstances that are regarded as unlikely or impossible. The term "anti-gravity" is also sometimes used to refer to hypothetical reactionless propulsion drives based on certain solutions to general relativity, although these do not oppose gravity as such.
There are more recent theories that add to general relativity or replace it outright, and some of these appear to allow anti-gravity-like solutions. However, according to the current widely accepted physical theories, verified in experiments, and according to the major directions of physical research, it is considered highly unlikely that anti-gravity is possible.
Empirical claims and commercial efforts
Anti-gravity devices are a common invention in the "alt" field, often requiring a completely new physics framework in order to work. However there have also been a number of commercial attempts to build such devices as well, and a small number of reports of anti-gravity-like effects in the scientific literature. As of 2007 none of them are widely accepted by the physics community.
Gyroscopes produce a force when twisted that operates "out of plane" and can appear to lift themselves against gravity. Although this force is well understood to be illusory, even under Newtonian models, it has nevertheless generated numerous claims of anti-gravity devices and any number of patented devices. None of these devices have ever been demonstrated to work under controlled conditions.
Perhaps the best known example is a series of patents issued to Henry William Wallace, an engineer at GE Aerospace in Valley Forge, Pennsylvania, and GE Re-Entry Systems in Philadelphia. He constructed devices that rapidly spun disks of brass, a material made up largely of elements with a total half-integer nuclear spin. He claimed that by rapidly rotating a disk of such material, the nuclear spin became aligned, and as a result created a "gravitomagnetic" field in a fashion similar to the magnetic field created by the Barnett effect.
Conventional effects that mimic anti-gravity effects
- Magnetic levitation suspends an object against gravity by use of electromagnetic forces. While visually impressive, gravitation itself functions normally in such devices. Various alleged anti-gravity devices may in reality work by electromagnetism.
- A tidal force causes objects to move along diverging paths near a massive body (such as a planet or star), producing effects that seem like repulsion or disruptive forces when observed locally. This is not anti-gravity. In Newtonian mechanics, the tidal force is the effect of the larger object's gravitational force being different at the differing locations of the diverging bodies. Likewise, in Einsteinian gravity, the tidal force is the effect of the diverging bodies following different paths in the negatively curved spacetime around the larger body.
- Large amounts of normal matter can be used to produce a gravitational field that compensates for the effects of another gravitational field, though the entire assembly will still be attracted to the source of the larger field. Physicist Robert L. Forward proposed using lumps of degenerate matter to locally compensate for the tidal forces near a neutron star.
- Ionocraft, or sometimes referred to as "Lifters" have been claimed to defy gravity, but in fact they use accelerated ions which have been stripped from the air around them to produce thrust. The thrust produced by one of these toys is not enough to lift its own power supply. Specifically, a special type of electrohydrodynamic thruster uses the Biefeld–Brown effect to hover.
Antigravity can annul gravity within a local frame; it does so by the exercise of equal force presence. It operates only with reference to material gravity, and it is not the action of mind. The gravity-resistant phenomenon of a gyroscope is a fair illustration of the effect of antigravity but of no value to illustrate the cause of antigravity. 9:3.3
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