Editing Tractor beam (section)

===1990s – Podkletnov experiment===
In 1992, Professor [[Yevgeny Podkletnov]] and R. Nieminen, of the [[Tampere University of Technology]], claimed to have discovered weight fluctuations in objects above an electromagnetically levitated, massive, composite superconducting disk.<ref>{{cite journal | doi = 10.1016/0921-4534(92)90055-H | last1 = Podkletnov | first1 = E. | last2 = Nieminen | first2 = R. | date = 1992 | title = A possibility of gravitational force shielding by bulk YBa2Cu3O7-x superconductor | journal = Physica C | volume = 203 | issue = 3–4| pages = 441–444 |bibcode = 1992PhyC..203..441P }}</ref>  Three years later, Podkletnov reported the results of additional experiments with a toroidal disk superconductor.<ref>Podkletnov, E. E. (1995, January).  Weak gravitational shielding properties of composite bulk YBa2Cu3O7-x superconductor below 70 K under e.m. field [Report MSU-chem 95]. Moscow, Russia: Moscow Chemical Scientific Research-Center.  Also, LANL Physics Preprint Server, arXiv: cond-mat/9701074v3.</ref>  They reported the weight of the samples would fluctuate between −2.5% and +5.4% as the angular speed of the superconductor increased.  Certain combinations of disk angular speeds and electromagnetic frequencies caused the fluctuations to stabilize at a 0.3% reduction.  The experiments with the toroidal disk yielded reductions that reached a maximum of 1.9–2.1%.  Reports about both sets of experiments stated the weight loss region was cylindrical, extending vertically for at least three meters above the disk.  Qualitative observations of an expulsive force at the border of the shielded zone were reported in the Fall of 1995.<ref name="Congress">Modanese, G. (1997).  Updating the theoretical analysis of the weak gravitational shielding experiments.  ''Proceedings of the 1997 IAF Congress'', nr. IAA-97-4.107.</ref>{{dubious|date=July 2024|Should there be some balance with the withdrawn paper and Podkletnov being expelled from his lab and fired as a professor?}} Several groups around the world tried to replicate Podkletnov's gravity shielding observations.<ref name="Allen">{{cite journal | doi = 10.1016/S0376-0421(02)00049-0 | last1 = Allen | first1 = J. E. | date = 2003 | title = Quest for novel force: a possible revolution in aerospace | journal = Progress in Aerospace Sciences | volume = 39 | issue = 1 | pages = 1–60 | bibcode=2003PrAeS..39....1A}}</ref>{{explain|date=July 2024|successfully or not?}}

Italian physicist Giovanni Modanese, while a [[Alexander von Humboldt Foundation|Von Humboldt Fellow]] at the [[Max Planck Institute for Physics]], made the first attempt to provide a theoretical explanation of Podkletnov's alleged observations.<ref name="Congress"/><ref>{{cite journal | last1 = Modanese | first1 = G | year = 1996 | title = Theoretical analysis of a reported weak-gravitational-shielding effect | journal = Europhysics Letters | volume = 35 | issue = 6| pages = 413–418 |arxiv=hep-th/9505094 | doi=10.1209/epl/i1996-00129-8| bibcode = 1996EL.....35..413M| s2cid = 10365722 }}</ref>  He argued that the shielding effect and slight expulsive force at the border of the shielded zone could be explained in terms of induced changes in the local [[cosmological constant]].  Modanese described several effects regarding responses to modifications to the local cosmological constant within the superconductor.<ref>{{cite journal | last1 = Modanese | first1 = G | year = 1996 | title = Role of a "local" cosmological constant in Euclidean quantum gravity | journal = Physical Review D | volume = 54 | issue = 8| pages = 5002–5009 |arxiv=hep-th/9601160 | doi=10.1103/PhysRevD.54.5002| pmid = 10021190 | bibcode = 1996PhRvD..54.5002M| s2cid = 17444342 }}</ref>  Ning Wu of the [[Institute of High Energy Physics]] (Beijing), used the quantum gauge theory of gravity he had developed in 2001 to explain Podkletnov's observations.<ref>{{cite journal | last1 = Wu | first1 = N | year = 2004 | title = Gravitational shielding effects in gauge theory of gravity | journal = Communications in Theoretical Physics | volume = 41 | issue = 4| pages = 567–572 |arxiv=hep-th/0307225 | doi=10.1088/0253-6102/41/4/567| bibcode = 2004CoTPh..41..567W| s2cid = 119407101 }}</ref>  Wu's theory approximated the relative gravity loss as 0.03% (an order of magnitude smaller than the reported range of 0.3–0.5%).{{citation needed|date=July 2024}}

[[C.S._Unnikrishnan|C. S. Unnikrishan]], [[TIFR|Tata Institute of Fundamental Research]], Mumbai, showed that if the effect had been caused by gravitational shielding, the shape of the shielded region would be similar to a shadow from the gravitational shield.  For example, the shape of the shielded region above a disk would be conical.  The height of the cone's apex above the disk would vary directly with the height of the shielding disk above the earth.<ref>{{cite journal | doi = 10.1016/0921-4534(96)00340-1 | last1 = Unnikrishan | first1 = C. S. | date = 1996 | title = Does a superconductor shield gravity? | journal = Physica C | volume = 266 | issue = 1 | pages = 133–137 |bibcode = 1996PhyC..266..133U }}</ref>  Podkletnov and Nieminen described the shape of the weight loss region as a cylinder that extended through the ceiling above the [[cryostat]].{{citation needed|date=July 2024<!--need further explanation as well.-->}}