Metric theories of gravitation (e.g., General Relativity) postulate the Equivalence Principle (EP): all bodies in vacuum fall identically regardless of composition and internal structure; inertial and gravitational masses are fundamentally indistinguishable. Spacetime curvature immediately follows.
Affine theories of gravitation ignore the EP, instead modeling gravitation as spacetime torsion. The two entirely different approaches give wholly identical predictions, with one class of exceptions: affine theories predict at least three classes of Equivalence Principle violation based upon test mass physical spin[1], test mass polarized electron spin[2], or test mass atomic lattice opposite geometric parity[3]. The first two are theoretically too small to observe.
Testing for reproducible Equivalence Principle violation is therefore of major interest. No violation beyond experimental error was ever observed. Test mass opposite parity EP experiments have never been performed. The proper test of spacetime geometry may be test mass geometry. Somebody should look.
Historic Equivalence Principle Tests[6]
Year
Investigator
Accuracy*
Method
500?
Philoponus[7]
"small"
Drop Tower
1590?
Galileo[4]
2·10-2
Pendulum, Drop Tower
1686
Newton[5]
10-3
Pendulum
1832
Bessel[8]
2·10-5
Pendulum
1910
Southerns[9]
5·10-6
Pendulum
1918
Zeeman[10]
3·10-8
Torsion Balance
1922
Eötvös[11]
5·10-9
Torsion Balance
1923
Potter[12]
3·10-6
Pendulum
1935
Renner[13]
2·10-9
Torsion Balance
1964
Dicke,Roll,Krotkov[14]
3·10-11
Torsion Balance
1972
Braginsky,Panov[15]
10-12
Torsion Balance
1976
Shapiro, et al.[16]
10-12
Lunar Laser Ranging
1981
Keiser,Faller[17]
4·10-11
Fluid Support
1987
Niebauer, et al.[18]
10-10
Drop Tower
1989
Heckel, et al.[19]
10-11
Torsion Balance
1990
Adelberger, et al.[20]
10-12
Torsion Balance
1999
Baeßler, et al.[21]
5·10-13
Torsion Balance
2005?
MiniSTEP[22]
10-17
Earth Orbit
"Accuracy" is the measurable difference/average of the weight vectors of the contrasted (sets of) test masses.
[1] Phys. Rev. D 66 022002 (2002)
[2] Phys. Rev. D 42 977 (1990) http://arXiv.org/abs/gr-qc/0102020
[3] http://www.mazepath.com/uncleal/qz.pdf
[4] Galilei, Galileo. Discorsi e Dimostrazioni Matematiche Intorno a Due Nuove Scienze (Appresso gli Elsevirii, Leida: 1638) (Discourses and Mathematical Demonstrations Concerning Two New Sciences, Elsevier Press, Leiden: Netherlands, 1638)
[5] Jahrbuch der Radioaktivität Elect.4 411 (1907)
[6]Ciufolini & Wheeler Gravitation and Inertia (Princeton University Press: Princeton, 1995) pp. 117-119 http://einstein.stanford.edu/STEP/information/data/gravityhist2.html
[7]Philoponus, J, Corollaries on Place and Void David Furley trans.
(Ithaca, NY: Cornell University Press, 1987)
[8]Ann. Physik und Chemie (Poggendorff) 25 401 (1832)
[9]Proc. Roy. Soc. Lond.84 325 (1910)
[10]Proc. K. Akad. Amsterdam20(4) 542 (1918)
[11]Math. Naturw. Ber. aus. Ungarn8 65 (1889) Ann. Physik (Leipzig) 68 11 (1922) Phys. Rev. D61(2) 022001 (1999)
[12]Proc. Roy. Soc. Lond.104 588 (1923)
[13]és Természettudományi Értesitö53 569 (1935)
[14]Ann. Phys. (NY)26 442 (1964)
[15]Zh. Eksp. Teor. Fiz.61 873 (1971) Sov. Phys. JETP34(3) 463 (1972)
[16]Phys. Rev. Lett.36 555 (1976)
[17]Bull. Am. Phys. Soc.24 579 (1979)
[18]Phys. Rev. Lett.59 609 (1987)
[19]Phys. Rev. Lett.62 609 (1989)
[20]Phys. Rev. D42 3267 (1990)
[21]Class. Quantum. Grav.18(13) 2393 (2001) Phys.Rev. Lett83(18) 3585 (1999)
[22]http://einstein.stanford.edu/STEP/
