The case for Anti-gravity

antigravity

A hypothetical force that acts in a direction opposite to that of normal gravity. More generally, any means of modifying the effects of gravity, through some form of shielding or otherwise.

Antigravity, gravity modification, and gravitational anomalies are subjects that have been considered both within and beyond the confines of conventional science. Most physicists remain skeptical of these ideas. However, that has not stopped aerospace companies carrying out research in them, scientific papers being published on them, and numerous plans being drawn up by inventors for antigravity devices, one of which was awarded a US patent.


Antigravity and conventional physics

In both Newton's law of gravitation and the general theory of relativity (Einstein's theory of gravitation), a requirement for antigravity to be possible is the existence of negative mass.1, 2 Most scientists regard negative mass as a purely hypothetical concept with no basis in reality. However, there is nothing in physics to say that it is actually impossible. A related idea that has been discussed by theoretical physicists involves gravitational shielding.


Majorana shielding

The notion of gravitational shielding was investigated in the 1920s by the Italian physicist Quirino Majorana. So-called Majorana shielding is hypothetical effect by which large masses (such as the moon) can partially block the gravitational force from more distant objects (such as the sun). This might explain the unusual and highly controversial efforts observed by some researchers in the behavior of pendulums during solar eclipses.

The origins of the idea of gravitational shielding go back to Nicolas Fatio de Duillier, a Swiss mathematician and one-time close friend of Isaac Newton. When Newton admitted he didn’t know how gravity really worked, de Duillier suggested, in 1690, that it arose as a shadowing effect associated with the absorption by material bodies of minute particles. This “push” theory of gravity was then developed further, in the eighteenth century, by another Swiss mathematician, George-Louis LeSage, also remembered for building and patenting the first electric telegraph. LeSage believed there was some kind of pressure in space. Masses, he thought, shielded one another from this space pressure and are thus pushed together by the unshielded pressure on their opposite sides. Although LeSage’s theory never won much support in the wider scientific community, it did strongly influence John Herapath, an English amateur scientist, in developing an early version of the kinetic theory of gases. It also came back into play when attempts were made to explain some anomalies in the motion of the moon that had been detected in the first decade of the twentieth century by Simon Newcomb.

In 1912 the German astronomer Kurt Bottlinger calculated the effects that would occur if the gravitational force between the sun and the moon decreased during lunar eclipses. What he found was a fluctuation in the moon’s longitude that agreed with Newcomb’s observations. Subsequently, Bottlinger’s results were criticized by the Dutch astronomer Willem de Sitter, and Einstein tried to supply an alternative explanation in terms of changes of the Earth’s rotation due to tidal effects. However, Einstein’s analysis was soon proved to be wrong, and for many years the moon‘s anomalous movements went unexplained. In the 1930s the mystery disappeared from view when astronomers began to use so-called ephemeris time, which was defined in a way that assumed the motion of the moon to be regular. Even before this, the widespread acceptance of general relativity undermined belief that an effect involving gravitational absorption could exist, pulling the rug from any further experimental and astronomical studies of this hypothesis. But it didn’t stop Majorona. In the 1920s, the decade in which general relativity came of age, he did a series of lab experiments, involving lead and mercury shields, in which he reported a small gravitational absorption effect. There is a need for Majorona’s research to be repeated to check if the shielding he found is real.


Podkletnov phenomenon



.1-Solenoids create magnetic field
.2-Spinning, super-conducting ceramic ring
.3-Liquid Nitrogen acts as coolant
.4-Podkletnov claims weight can be reduced by 2% (1kg=980g)

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In the 1992, the Russian emigré scientist Yevgeny Podkletnov reported an antigravity or gravitational shielding effect involving a spinning superconductor.3 Podkletnov claimed he saw tobacco smoke rise over the spinning superconductor, so he measured the gravitational acceleration above the device and made the discovery. Podkletnov now claims to have created a force beam that is 200 times stronger than his first experiments.

In the wake of Podkletnov's announcement, Boeing, BAE Systems, and NASA all funded research to try to replicate the Podkletnov effect but with no reported success. An American scientist, Ning Li, independently predicted a gravity shielding effect with superconductors. In 1999, Popular Mechanics reported that Li and her team had built a working prototype to generate what Li described as AC Gravity. However, there has been no subsequent verification of the work.


Antigravity in science fiction

The theme of antigravity appeared early in science fiction, a typical 19th century example being "apergy" – an antigravity principle used to propel a spacecraft from Earth to Mars in Percy Greg's Across the Zodiac (1880) and borrowed for the same purpose by John Jacob Astor in A Journey in Other Worlds (1894). More famously, in The First Men in the Moon (1901), H. G. Wells used moveable shutters made of "Cavorite," a metal that shields against gravity, to navigate a spacecraft to the Moon.4


References
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Bondi. H. "Negative mass in general relativity." Reviews of Modern Physics 29 (1957): 423.
Price, R. "Negative mass can be positively amusing." American Jurnal of Physics, March 1993, 216-17.
Podkletnov, E., and Nieminen. "A Possibility of Gravitational Force Shielding by Bulk YBa2Cu307-x Superconductor," Physica C, 203, 441-44 (1992).
Wells, H. G. The First Men in the Moon. London: Newnes (1901).
Burridge, Gaston. "Another Step Toward Anti-Gravity." American Monthly, 86, 77-82 (1958).

Related categories

• GRAVITATIONAL PHYSICS
• SPACE AND TIME
• ADVANCED PROPULSION CONCEPTS


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Could antigravity be real? (Aug 7, 2002)



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Alcubierre warp drive



An idea for achieving faster-than-light travel suggested by the Mexican theoretical physicist Miguel Alcubierre in 1994.1 It starts from the notion, implicit in Einstein's general theory of relativity, that matter causes the surface of spacetime around it to curve. Alcubierre was interested in the possibility of whether Star Trek's fictional "warp drive" could ever be realized. This led him to search for a valid mathematical description of the gravitational field that would allow a kind of spacetime warp to serve as a means of superluminal propulsion. Alcubierre concluded that a warp drive would be feasible if matter could be arranged so as to expand the spacetime behind a starship (thus pushing the departure point many light-years back) and contract the spacetime in front (bringing the destination closer), while leaving the starship itself in a locally flat region of spacetime bounded by a "warp bubble" that lay between the two distortions. The ship would then surf along in its bubble at an arbitrarily high velocity, pushed forward by the expansion of space at its rear and the contraction of space in front. It could travel faster than light without breaking any physical law because, with respect to the spacetime in its warp bubble, it would be at rest. Also, being locally stationary, the starship and its crew would be immune from any devastatingly high accelerations and decelerations (obviating the need for "inertial dampers"), and from relativistic effects such as time dilation (since the passage of time inside the warp bubble would be the same as that outside).

Could such a warp drive be built? It would require, as Alcubierre pointed out, the manipulation of matter with a negative energy density. Such matter, known as exotic matter, is the same kind of peculiar stuff apparently needed to maintain stable wormholes – another proposed means of circumventing the light barrier. Quantum mechanics allows the existence of regions of negative energy density under special circumstances, such as in the Casimir effect.

Further analysis of Alubierre's warp drive concept by Chris Van Den Broeck of the Catholic University in Leuven, Belgium,2 has perhaps brought the construction of the starship Enterprise a little closer. Van Den Broeck's calculations put the amount of energy required much lower than that quoted in Alcubierre's paper. But this is not to say we are on the verge of warp capability. As Van Den Broeck concludes: "The first warp drive is still a long way off but maybe it has now become slightly less improbable."


References
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Alcubierre, M. "The Warp Drive: Hyper-fast Travel within General Relativity," Classical and Quantum Gravity, 11(5), L73-77 (1994).
Abstract: It is shown how, within the framework of general relativity and without the introduction of wormholes, it is possible to modify a spacetime in a way that allows a spaceship to travel with an arbitrarily large speed. By a purely local expansion of spacetime behind the spaceship and an opposite contraction in front of it, motion faster than the speed of light as seen by observers outside the disturbed region is possible. The resulting distortion is reminicent of the 'warp drive' of science fiction. However, just as happens with wormholes, exotic matter will be needed in order to generate a distortion of spacetime like the one discussed here.
Broeck, C. van den. "A 'warp drive' with more reasonable energy requirements," Classical and Quantum Gravity, 16, 3973-79 (1999).
Pfenning, M. J., and L. H. Ford. "The unphysical nature of 'warp drive'." Classical and Quantum Gravity 14 (1997): 1743-51.
Puthoff, H. E. "SETI, the velocity-of-light limitation, and the Alcubierre warp drive: An integrating overview." Physics Essays 9 (1): 156-58 (1996).

Related entry

• interstellar propulsion


Related categories

• ADVANCED PROPULSION CONCEPTS
• SCIENCE FICTION
• SCIENCE OF STAR TREK

antimatter propulsion



Devotees of Star Trek will need no reminding that the starships Enterprise and Voyager are powered by engines that utilize antimatter. Far from being fictional, the idea of propelling spacecraft by the annihilation of matter and antimatter is being actively investigated at NASA's Marshall Space Flight Center, Pennsylvania State University, and elsewhere. The principle is simple: an equal mixture of matter and antimatter provides the highest energy density of any known propellant. Whereas the most efficient chemical reactions produce about 1 × 107 joules(J)/kg, nuclear fission 8 × 1013 J/kg, and nuclear fusion 3 × 1014 J/kg, the complete annihilation of matter and antimatter, according to Einstein's mass-energy relationship (E = mc2), yields 9 × 1016 J/kg. In other words, kilogram for kilogram, matter-antimatter annihilation releases about ten billion times more energy than the hydrogen/oxygen mixture that powers the Space Shuttle Main Engines and 300 times more than the fusion reactions at the Sun's core.

However, there are several (major!) technical hurdles to be overcome before an antimatter rocket can be built. The first is that antimatter does not exist in significant amounts in nature – at least, not anywhere near the solar system. It has to be manufactured. Currently the only way to do this is by energetic collisions in giant particle accelerators, such as those at FermiLab, near Chicago, and at CERN, in Switzerland. The process typically involves accelerating protons to almost the speed of light and then slamming them into a target made of a metal such as tungsten. The fast-moving protons are slowed or stopped by collisions with the nuclei of the target atoms, and the protons' kinetic energy converted into matter in the form of various subatomic particles, some of which are antiprotons – the simplest form of antimatter. So efficient is matter-antimatter annihilation that 71 milligrams of antimatter would produce as much energy as that stored by all the fuel in the Space Shuttle External Tank. Unfortunately, the annual amount of antimatter (in the form of antiprotons) presently produced at Fermilab and CERN is only 1-10 nanograms (a nanogram is a million times smaller than a milligram).1 On top of this production shortfall, there is the problem of storage. Antimatter cannot be kept in a normal container because it will annihilate instantly on coming into contact with the container's walls. One solution is the Penning Trap – a supercold, evacuated electromagnetic bottle in which charged particles of antimatter can be suspended. Antielectrons, or positrons, are difficult to store in this way, so antiprotons are stored instead. Penn State and NASA scientists have already built such a device capable of holding 10 million antiprotons for a week. Now they are developing a Penning Trap with a capacity 100 times greater.2 At the same time, FermiLab is installing new equipment that will boost its production of antimatter by a factor of 10-100.


A spacecraft propulsion system that works by expelling the products of direct one-to-one annihilation of protons and antiprotons – a
so-called beamed core engine – would need 1-1,000 grams of antimatter for an interplanetary or interstellar journey.3 Even with the improved antiproton production and storage capacities expected soon, this amount of antimatter is beyond our reach. However, the antimatter group at Penn State has proposed a highly efficient space propulsion system that would need only a tiny fraction of the antimatter consumed by a beamed core engine. It would work by a process called antiproton-catalyzed microfission (ACMF).4 Whereas conventional nuclear fission can only transfer heat energy from a uranium core to surrounding chemical propellant, ACMF permits all energy from fission reactions to be used for propulsion. The result is a more efficient engine that could be used for interplanetary manned missions. The ICAN-II (ion compressed antimatter nuclear II) spacecraft designed at Penn State would use the ACMF engine and only 140 nanograms of antimatter for a manned 30-day crossing to Mars.

A follow-up to ACMF and ICAN is a spacecraft propelled by AIM (antiproton initiated microfission/fusion) in which a small concentration of antimatter and fissionable material would be used to spark a microfusion reaction with nearby material. Using 30-130 micrograms of antimatter, an unmanned AIM-powered probe – AIMStar – would be able travel to the Oort Cloud in 50 years, while a greater supply of antiprotons might bring Alpha Centauri within reach.

Combining antimatter technology with the concept of the space sail has led to the idea of the antimatter-driven sail.


References
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Schmidt, G., Gerrish, H., Martin, J. J. "Antimatter Production for Near-term Propulsion Applications," 1999 Joint Propulsion Conference.
Smith, G. A., et al. "Antiproton Production and Trapping for Space Propulsion Applications," unpublished paper. Read on-line here.
Forward, R. L. "Antiproton Annihilation Propulsion," Journal of Propulsion, 1 (5), 370-74 (1985).
Smith, G. A., Gaidos, G., Lewis, R. A., Meyer, K., and Schmid, T. "Aimstar: Antimatter Initiated Microfusion for Precursor Interstellar Missions," Acta Astronautica, 44 183-86 (1999).

Related entry

• nuclear propulsion


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• ADVANCED PROPULSION CONCEPTS
• ROCKET ENGINE TYPES


External site
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Antimatter and space propulsion (NASA/JPL)



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