Anti-gravity is the idea of creating a place or object that is free from the force of gravity. It does not refer to countering the gravitational force by an opposing force of a different nature, as a helium balloon does; 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 imaginary technology 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 encoded in 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 numerous newer theories that add onto 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.
Some science fiction stories postulate the existence of a substance partially or completely opaque to gravity. Placing this substance underneath an object reduces or eliminates its weight, allowing it to float away from the Earth's surface with a relatively small expenditure of energy. Under Newtonian gravitation, where gravity is a force being transmitted from point to point, this approach made sense; the gravitational field would be blocked by an appropriate shield in the same way a magnetic field could be blocked by diamagnetic substances.
There are strong reasons to believe that no such substance can exist. Consider the results of placing such a substance under one-half of a wheel on a shaft. The side of the wheel under the substance would have no weight, while the other side would. This would cause the wheel to continually "fall" toward the side under the plate. This motion could be harnessed to produce power for free, a clear violation of the first law of thermodynamics. More generally, it follows from Gauss's law that static inverse-square fields (such as Earth's gravitational field) cannot be blocked (magnetism is static, but is inverse-cube). Under general relativity, the entire concept is something of a non-sequitur.
In 1948 successful businessman Roger Babson (founder of Babson College) formed the Gravity Research Foundation to study ways to reduce the effects of gravity. Their efforts were initially somewhat "crankish", but they held occasional conferences that drew such people as Clarence Birdseye of frozen-food fame and Igor Sikorsky, inventor of the helicopter. Over time the Foundation turned its attention away from trying to control gravity, to simply better understanding it. The Foundation disappeared some time after Babson's death in 1967. However it continues to run an essay award, offering prizes of up to $5,000. As of 2007 it is still administered out of Wellesley, Massachusetts by George Rideout, Jr., son of the foundation's original director. Recent winners include California astrophysicist George F. Smoot, who later won the 2006 Nobel Prize in physics.
Near the end of WWII the Nazi war effort was turning to ever more outlandish projects to provide a weapon that could turn the tide. One of these was the so-called Nazi Bell, located in an underground mine, with a ring of heavy duty testing pillars directly above. The device is reported to have been made of two stacked counter-rotating cylinders of mercury with bismuth alloy cores exposed to a high radio-frequency field. The scientists working on the device were trying to create a magnetic dipole based on the first incarnation of Albert Einstein's never-completed Unified Field Theory. Remnants of the testing rig and nearby power station solely used to power it remain to this day, however the Bell itself is gone. In an interview with John Dering, americanantigravity.com claims that SARA, a California-based defense contractor, successfully replicated the Nazi Bell on a small scale but subsequently shelved the project when major sources for funding failed to materialize. However the site also claims that it is likely 'Black Ops' in the US took the Nazi Bell in Operation Paperclip and recently used knowledge gained from it to create a triangular anti-gravity based craft known as the TR-3B. This craft works on a mercury plasma accelerated in a toroid and exposed to high radio-frequency fields but manages to negate only 89% of its mass and gravitational force. It is worthy to note that while a gravitational dipole (a bump in the gravity well) is said to be behind these effects it is unexplained as to why mass is also cancelled - but by the same token it has never been explained in conventional physics why gravitational mass and inertial mass are equal.
General relativity was introduced in the 1910s, but development of the theory was greatly slowed by a lack of suitable mathematical tools. Some of these were introduced in the 1950s, and by the 1960s a flowering of general relativity was underway that later became known as the golden age of general relativity. Although it appeared that anti-gravity was outlawed under general relativity, there were a number of efforts to study potential solutions that allowed anti-gravity-type effects.
It is claimed the US Air Force also ran a study effort throughout the 1950s and into the 1960s. Former Lieutenant Colonel Ansel Talbert wrote two series of newspaper articles claiming that most of the major aviation firms had started gravity control propulsion research in the 1950s. However there is little outside confirmation of these stories, and since they take place in the midst of the policy by press release era, it is not clear how much weight these stories should be given.
It is known that there were serious efforts underway at the Glenn L. Martin Company, who formed the Research Institute for Advance Study. Major newspapers announced the contract that had been made between Burkhard Heim and the Glenn L. Martin Company. Other private sector efforts to master the understanding of gravitation was the creation of the Institute for Field Physics, University of North Carolina at Chapel Hill, in 1956 by Gravity Research Foundation trustee, Agnew H. Bahnson.
Military support for anti-gravity projects was terminated by the Mansfield Amendment of 1973, which restricted Department of Defense spending to only the areas of scientific research with explicit military applications. The Mansfield Amendment was passed specifically to end long-running projects that had little to show for their efforts.
Under general relativity, gravity is the result of following a geometry caused by local mass-energy. Although the equations cannot produce a "negative geometry" normally, it is possible to do so using a "negative mass" (negative energy). Interestingly, the same equations do not, of themselves, rule out the existence of negative mass.
Both general relativity and Newtonian gravity appear to predict that negative mass would produce a repulsive gravitational field. In particular, Sir Hermann Bondi proposed in 1957 a form of negative gravitational mass that could comply with the strong equivalence principle of general relativity theory and the Newtonian laws of conservation of linear momentum and energy. Bondi's proof yielded singularity free solutions for the relativity equations. In July 1988, Robert L. Forward presented a paper at the AIAA/ASME/SAE/ASEE 24TH Joint Propulsion Conference that proposed a Bondi negative gravitational mass propulsion system.
Every point mass attracts every other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses:
Negative mass also seems to suffer from problems similar to the gravity shield. Forward pointed out that a negative mass will fall toward "normal" matter as normal, while normal mass will fall away from the negative matter. Forward noted that two similar masses, one positive and one negative, placed near each other will therefore accelerate in the direction of the line between them, away from the negative mass. Notice that because the negative mass acquires negative kinetic energy, the total energy of the accelerating masses remains at zero.
The Standard Model of particle physics, which describes all presently known forms of matter, does not permit negative mass. Cosmological dark matter (and possibly dark energy) may consist of particles outside the Standard Model whose nature is unknown; however their mass is ostensibly known - since they were postulated from their gravitational effects, which are positive..
Under general relativity any form of energy couples with spacetime to create the geometries that cause gravity. A longstanding question was whether or not these same equations applied to antimatter. The issue was considered solved in 1957 with the development of CPT symmetry, which demonstrated that antimatter follows the same laws of physics as "normal" matter, and therefore has positive energy content and also causes (and reacts to) gravity like normal matter.
For much of the later quarter of the 20th century, the physics community has been involved in an attempt to produce a unified field theory, a single physical theory that explains the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. Scientists have made progress in unifying the three quantum forces, but gravity has remained "the problem" in every attempt. This has not stopped any number of such attempts being made, however.
Generally these attempts tried to "quantize gravity" by positing a new particle, the graviton, that carried gravity in the same way that photons (light) carry electromagnetism. Simple attempts along this direction all failed, however, leading to more complex examples that attempted to account for these problems. Two of these, supergravity and supersymmetry, both required the existence of an extremely weak "fifth force" that coupled together several "loose ends" in quantum theory. As a side-effect, both theories also posited that antimatter would be affected by this fifth force in a way similar to anti-gravity, being repelled from mass. Several experiments were carried out in the 1990s to measure this effect, but all failed.
There are solutions of the field equations of general relativity which describe "warp drives" (such as the famous Alcubierre metric) and stable, traversable wormholes. This by itself is not significant, since any spacetime geometry is a solution of the field equations for some configuration of the stress-energy tensor field (see exact solutions in general relativity). General relativity does not constrain the geometry of spacetime unless outside constraints are placed on the stress-energy tensor. Warp-drive and traversable-wormhole geometries are well-behaved in most areas, but require regions of exotic matter; thus they are excluded as solutions if the stress-energy tensor is limited to known forms of matter (including dark matter and dark energy).
During the close of the twentieth century NASA provided funding for the Breakthrough Propulsion Physics Program from 1996 through 2002. This program studied a number of "far out" designs for space propulsion that were not receiving funding through normal university or commercial channels. Anti-gravity-like concepts were investigated under the name "diametric drive".
Anti-gravity devices are a common invention in the "alt" field, often requiring a completely new physics framework in order to work. Most of these devices rather obviously do not work, and are often parts of grander conspiracy theories. 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, and have often become the subject of conspiracy theories as a result.
Perhaps the best known example is a series of patents issued to Henry William Wallace, an engineer at GE Aerospace in Valley Forge PA, 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. The work appears to have been largely ignored.
Hayasaka and Takeuchi had reported weight decreases along the axis of a right spinning gyroscope. Tests of their claims by Nitschke and Wilmath yielded null results. A few years later, recommendations were made to conduct further tests.
During the 1920s Thomas Townsend Brown, a high-voltage experimenter, produced a device he called the "gravitator" which he claimed used an unknown force to produce anti-gravity effects by applying high voltages to materials with high dielectric constants. Although it was claimed that the device operated outside of working mass, Brown abandoned this work and moved on to produce a series of successful high-voltage devices in the following years.
The Biefeld-Brown effect nevertheless lives on. A 1956 analysis by the Gravity Research Group and by a technical writer, under the pen name of Intel (1956), claimed the Biefeld-Brown effect was the primary theory tested by the aerospace firms in the 1950s. It has remained a constant theme in the UFO field, and has recently been a topic of some discussion in this field under the name "lifters". There appears to be a general understanding that the lifters require a working mass, air specifically (ion wind), and that they do not demonstrate new physics.
Experimental evidence.The Russian researcher Eugene Podkletnov claims to have discovered experimenting with superconductors in 1995, that a fast rotating superconductor reduces the gravitational effect. All attempts of different physicists were in vain to reproduce Podkletnov's results.
In 1989, Ning Li, of the University of Alabama in Huntsville theoretically demonstrated how a time dependent magnetic field could cause the spins of the lattice ions in a superconductor to generate detectable gravitomagnetic and gravitoelectric fields. In 1999, Li and her team appeared in Popular Mechanics, claiming to have constructed a working prototype to generate what she described as "AC Gravity." No further evidence of this prototype has been offered.
The Institute for Gravity Research of the Göde Scientific Foundation has tried to reproduce different experiments which allegedly show an antigravity effect. All attempts to observe an antigravity effect have been unsuccessful. The foundation has offered a reward of one million Euros for a reproducible antigravity experiment.
A paper by Martin Tajmar et al in 2006 claims detection of an artificial gravitational field around a rotating superconductor, proportional to the angular acceleration of the superconductor. A subsequent paper claims to explain the phenomenon in terms of the nonzero cosmological constant. Neither the experimental results nor the theoretical explanation are widely accepted.
In July 2007, Graham et al of the Canterbury Ring Laser Group, New Zealand, reported results from an attempt to test the same effect with a larger rotating superconductor. They report no indication of any effect within the measurement accuracy of the experiment. Given the conditions of the experiment, the Canterbury group conclude that if any such 'Tajmar' effect exists, it is at least 22 times smaller than predicted by Tajmar in 2006.