Editing Anti-gravity (section)

==Probable solutions==
In [[Newton's law of universal gravitation]], gravity was an external force transmitted by unknown means. In the 20th century, Newton's model was replaced by [[general relativity]] where gravity is not a force but the result of the geometry of spacetime. Under general relativity, anti-gravity is impossible except under contrived circumstances.<ref>Peskin, M and Schroeder, D.; ''An Introduction to Quantum Field Theory'' (Westview Press, 1995) {{ISBN|0-201-50397-2}}</ref><ref>{{cite book | author = Wald, Robert M.| author-link = Robert M. Wald| title = ''General Relativity'' | location = Chicago | publisher = University of Chicago Press | date = 1984 | isbn = 978-0-226-87033-5 | title-link = General Relativity (book)}}</ref><ref>[[Joseph Polchinski|Polchinski, Joseph]] (1998). ''String Theory'', Cambridge University Press. A modern textbook</ref>

===Gravity shields===
[[File:New boston babson monument.JPG|thumb|right|A monument at [[Babson College]] dedicated to [[Roger Babson]] for research into anti-gravity and partial gravity insulators]]
{{Main|Gravitational shielding}}

In 1948 businessman [[Roger Babson]] (founder of [[Babson College]]) formed the [[Gravity Research Foundation]] to study ways to reduce the effects of gravity.<ref>{{cite magazine | last1 = Mooallem | first1 = J. | date = October 2007 | title = A curious attraction | magazine = Harper's Magazine | volume = 315 | issue = 1889 | pages = 84–91 }}</ref> Their efforts were initially somewhat "[[crank (person)|crank]]ish", but they held occasional conferences that drew such people as [[Clarence Birdseye]], known for his frozen-food products, and helicopter pioneer [[Igor Sikorsky]].{{Citation needed|date=January 2022}} Over time the Foundation turned its attention away from trying to control gravity, to simply better understanding it. The Foundation nearly disappeared after Babson's death in 1967. However, it continues to run an essay award, offering prizes of up to $4,000. As of 2017, it is still administered out of [[Wellesley, Massachusetts]], by George Rideout Jr., son of the foundation's original director.<ref>[http://www.gravityresearchfoundation.org/winners_name.html#s List of winners] {{webarchive|url=https://web.archive.org/web/20121228174605/http://www.gravityresearchfoundation.org/winners_name.html |date=28 December 2012 }}</ref> Winners include California astrophysicist [[George F. Smoot]] (1993), who later won the 2006 [[Nobel Prize in Physics]], and [[Gerard 't Hooft]] (2015) who previously won the 1999 Nobel Prize in Physics.<ref>{{Cite web|url=https://www.nobelprize.org/prizes/physics/1999/summary/|title = The Nobel Prize in Physics 1999}}</ref>

===General relativity research in the 1950s===
{{Main|United States gravity control propulsion research}}

General relativity was introduced in the 1910s, but development of the theory was greatly slowed by a lack of suitable mathematical tools.{{clarify|reason=The mathematics of general relativity was little known at the time, but it is dubious that new tools would have to be developed to further explore the theory, once it had been formulated. A more important factor is likely that the empirical evidence for GR over Newtonian gravity for several decades consisted of only three astronomical phenomena — why worry about fine details, when you can just barely measure any effect at all?|date=July 2018}} It appeared that anti-gravity was outlawed under general relativity.

It is claimed the [[US Air Force]] also ran a study effort throughout the 1950s and into the 1960s.<ref>Goldberg, J. M. (1992). US air force support of general relativity: 1956–1972. In, J. Eisenstaedt & A. J. Kox (Ed.), ''Studies in the History of General Relativity, Volume 3'' Boston, Massachusetts: Center for Einstein Studies. {{ISBN|0-8176-3479-7}}</ref> 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 no 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 Advanced Study.<ref>Mallan, L. (1958). ''Space satellites'' (How to book 364). Greenwich, CT: Fawcett Publications, pp. 9–10, 137, 139. LCCN 58-001060</ref><ref>{{cite journal | last1 = Clarke | first1 = A. C. | date = 1957 | title = The conquest of gravity | journal = Holiday | volume = 22 | issue = 6| page = 62 }}</ref> Major newspapers announced the contract that had been made between theoretical physicist [[Burkhard Heim]] and the Glenn L. Martin Company. Another effort in the private sector to master 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 [[United States Department of Defense|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 no results.

Under general relativity, gravity is the result of following spatial geometry (change in the normal shape of space) caused by local mass-energy. This theory holds that it is the altered shape of space, deformed by massive objects, that causes gravity, which is actually a property of deformed space rather than being a true force. Although the equations cannot normally produce a "negative geometry", it is possible to do so by using "[[negative mass]]". 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 that negative gravitational mass, combined with negative inertial mass, would 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.<ref name="Bondi">{{cite journal | last1 = Bondi | first1 = H. | date = 1957 | title = Negative mass in general relativity | journal = Reviews of Modern Physics | volume = 29 | issue = 3| pages = 423–428 | doi=10.1103/revmodphys.29.423|bibcode = 1957RvMP...29..423B }}</ref> 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.<ref name= "Forward">{{cite journal | last1 = Forward | first1 = R. L. | year = 1990 | title =  Negative matter propulsion | journal = Journal of Propulsion and Power | volume = 6 | issue = 1| pages = 28–37 | doi=10.2514/3.23219}}; see also commentary {{cite journal | last1 = Landis | first1 = G.A. | year = 1991 | title = Comments on Negative Mass Propulsion | journal = Journal of Propulsion and Power | volume = 7 | issue = 2| page = 304 | doi=10.2514/3.23327}}</ref>

Bondi pointed out that a negative mass will fall toward (and not away from) "normal" matter, since although the gravitational force is repulsive, the negative mass (according to Newton's law, F=ma) responds by accelerating in the opposite of the direction of the force. Normal mass, on the other hand, will fall away from the negative matter. He noted that two identical masses, one positive and one negative, placed near each other will therefore self-accelerate in the direction of the line between them, with the negative mass chasing after the positive mass.<ref name="Bondi" /> Notice that because the negative mass acquires negative [[kinetic energy]], the total energy of the accelerating masses remains at zero. Forward pointed out that the self-acceleration effect is due to the negative inertial mass, and could be seen induced without the gravitational forces between the particles.<ref name="Forward" />

The [[Standard Model]] of particle physics, which describes all currently known forms of matter, does not include negative mass. Although cosmological [[dark matter]] may consist of particles outside the Standard Model whose nature is unknown, their mass is ostensibly known – since they were postulated from their gravitational effects on surrounding objects, which implies their mass is positive. The proposed cosmological [[dark energy]], on the other hand, is more complicated, since according to general relativity the effects of both its energy density and its negative pressure contribute to its gravitational effect.

===Unique force===
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 1960 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 (see [[gravitational interaction of antimatter]]).

For much of the last quarter of the 20th century, the physics community was involved in attempts 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 [[grand unification theory|unifying the three quantum forces]], but gravity has remained "the problem" in every attempt. This has not stopped any number of such attempts from being made, however.

Generally these attempts tried to "quantize gravity" by positing a particle, the [[graviton]], that carried gravity in the same way that [[photon]]s (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, [[supersymmetry]] and the relativity related [[supergravity]], both required the existence of an extremely weak "fifth force" carried by a [[graviphoton]], which coupled together several "loose ends" in quantum field theory, in an organized manner. As a side effect, both theories also all but required that antimatter be affected by this fifth force in a way similar to anti-gravity, dictating repulsion away from mass. Several experiments were carried out in the 1990s to measure this effect, but none yielded positive results.<ref name=SciAm79>''Supergravity and the Unification of the Laws of Physics,'' by [[Daniel Z. Freedman]] and Peter van Nieuwenhuizen, Scientific American, February 1978</ref>

In 2013 [[CERN]] looked for an antigravity effect in an experiment designed to study the energy levels within antihydrogen. The antigravity measurement was just an "interesting sideshow" and was inconclusive.<ref>[https://www.bbc.co.uk/news/science-environment-22355187 Jason Palmer, Antigravity gets first test at Cern's Alpha experiment, bbc.co.uk, 30 April 2013 ]</ref>

===Breakthrough Propulsion Physics Program===
During the close of the twentieth century [[NASA]] provided funding for the [[Breakthrough Propulsion Physics Program]] (BPP) 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". The work of the BPP program continues in the independent, non-NASA affiliated [[Tau Zero Foundation]].<ref>[http://www.tauzero.aero/ Tau Zero Foundation]</ref>