Editing Anti-gravity (section)

===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>