Hughes–Drever experiment

Hughes–Drever experiments (also clock comparison-, clock anisotropy-, mass isotropy-, or energy isotropy experiments) are testing the isotropy of mass and space. As in Michelson–Morley experiments, the existence of a preferred frame of reference, or deviations from Lorentz invariance can be tested, which also affects the validity of the equivalence principle. Thus these experiments concern fundamental aspects of both special and general relativity. Contrary to Michelson–Morley, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of protons, neutrons, and electrons. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity (see also Tests of special relativity).^{[A 1] [A 2] [A 3] [A 4] [A 5] [A 6]}

Early experiments

Giuseppe Cocconi and Edwin Ernest Salpeter (1958) theorized, that inertia depends on the surrounding masses according to Mach's principle. Anisotropic distribution of matter thus would lead to anisotropy of inertia in different directions. This might be observed by investigating the Zeeman effect in atomic nuclei.^[1]

Hughes et al. (1960) and Ronald Drever (1961), independently conducted experiments of that kind. They observed the nucleus of lithium-7, whose ground state possesses a spin of ³⁄₂. By that, four magnetic energy levels exist when measured in a magnetic field in accordance with its allowed magnetic quantum number. If mass isotropy is satisfied, no frequency shift in the energy levels arises and only one resonance line should exist, if not, a triplet or broadened resonance line should exist. In fact, no frequency shift of the energy levels was observed, and due to the experiment's high precision, the maximum anisotropy only amounted 0,04 Hz = 10^-25 GeV.^{[2] [3]}

Modern interpretation

While this experiment was initially only related to Mach's principle, it is also considered as an important test of Lorentz invariance and thus special relativity. This is because anisotropy effects also occur in the presence of a preferred and Lorentz-violating frame of reference – usually identified with the CMBR-rest frame as some sort of luminiferous aether. Therefore, the negative results of the Hughes-Drever experiments (like the Michelson–Morley experiments) rule out the existence of such a frame. In addition, one fundamental statement of the equivalence principle of general relativity says that Lorentz invariance locally holds in freely moving reference frames = local Lorentz invariance (LLI). This means that the results of this experiment concern both special and general relativity.

Or, as it was put by Clifford Will, it's about the question of whether the limiting velocity of matter is identical with the speed of light, as predicted by relativity. It they are not the same, the properties and frequencies of matter interactions will change and that's exactly what can be measured by Hughes-Drever experiments. Due to the fact that different frequencies are compared, those experiments are also denoted as clock-comparison experiments.^{[A 1][A 2][A 3][A 4]}

Modern experiments

Further information: Modern searches for Lorentz violation

Besides Lorentz violations due to a preferred frame or influences based on Mach's principle, also spontaneous violations of Lorentz invariance and CPT symmetry are considered, especially in the course of the continuing development of quantum gravity models. Therefore, a series of modern variants of Hughes-Drever experiments have been conducted. Those measurements are related to neutrons and protons, and by application of spin-polarized systems and co-magnetometers (to suppress magnetic influences), it was possible to considerably increase the accuracy. In addition, by using spin-polarized torsion balances the electron sector was also tested.^{[A 5][A 6]}

All of those experiments gave negative results, so there is still no sign of the existence of a preferred frame or any other form of Lorentz violation. The values of the following table are related to the coefficients given by the Standard-Model Extension (see also Test theories of special relativity). From that, any deviation of Lorentz invariance can be connected with specific coefficients. Since a series of coefficients are tested in those experiments, only the value of maximal sensitivity is given (for precise data, see the individual articles):^{[A 3][A 7][A 4]}

Author	Year	max. anisotropy in GeV
		Proton	Neutron	Electron
Prestage et al.[4]	1985		10−27
Phillips[5]	1987			10−27
Lamoreaux et al.[6]	1989		10−29
Chupp et al.[7]	1989		10−27
Wineland et al.[8]	1991			10−25
Wang et al.[9]	1993			10−27
Berglund et al.[10]	1995	10−27	10−30	10−27
Bear et al.[11]	2000		10−31
Walsworth et al.[12]	2000	10−27	10−31
Phillips et al.[13]	2000	10−27
Humphrey et al.[14]	2003	10−27		10−27
Hou et al.[15]	2003			10−29
Canè et al.[16]	2004		10−32
Wolf et al.[17]	2006	10−25
Heckel et al.[18]	2006			10−30
Heckel et al.[19]	2008			10−31
Altarev et al.[20]	2009		10−20
Brown et al.[21]	2010	10−32	10−33
Gemmel et al.[22]	2010		10−32

See also

Secondary sources

1. ^ ^{a b} Will, C. M. (2006). "The Confrontation between General Relativity and Experiment". Living Reviews in Relativity 9 (3). http://www.livingreviews.org/lrr-2006-3. Retrieved June 23, 2011. 
2. ^ ^{a b} Will, C. M. (1995). "Stable clocks and general relativity". Proceedings of the 30th Rencontres de Moriond: 417. arXiv:gr-qc/9504017. 
3. ^ ^{a b c} Kostelecký, V. Alan; Lane, Charles D. (1999). "Constraints on Lorentz violation from clock-comparison experiments". Physical Review D 60 (11): 116010. arXiv:hep-ph/9908504. doi:10.1103/PhysRevD.60.116010. 
4. ^ ^{a b c} Mattingly, David (2005). "Modern Tests of Lorentz Invariance". Living Rev. Relativity 8 (5). http://www.livingreviews.org/lrr-2005-5. 
5. ^ ^{a b} Pospelov, Maxim; Romalis, Michael (2004). "Lorentz Invariance on Trial". Physics Today 57 (7): 40–46. doi:10.1063/1.1784301. http://physics.princeton.edu/romalis/articles/Pospelov%20and%20Romalis%20-%20Lorentz%20Invariance%20on%20Trial.pdf. 
6. ^ ^{a b} Walsworth, R.L. (2006). "Tests of Lorentz Symmetry in the Spin-Coupling Sector". Lecture Notes in Physics 702: 493–505. doi:10.1007/3-540-34523-X_18. http://www.cfa.harvard.edu/Walsworth/pdf/LNP_ch_19.pdf. 
7. ^ Hou, Li-Shing; Ni, Wei-Tou; Li, Yu-Chu M. (2003). "Test of Cosmic Spatial Isotropy for Polarized Electrons Using a Rotatable Torsion Balance". Physical Review Letters 90 (20): 201101. arXiv:physics/0009012. doi:10.1103/PhysRevLett.90.201101. 

Primary sources

1. ^ Cocconi, G.; Salpeter E. (1958). "A search for anisotropy of inertia". Il Nuovo Cimento 10 (4): 646–651. doi:10.1007/BF02859800. 
2. ^ Hughes, V. W.; Robinson, H. G.; Beltran-Lopez, V. (1960). "Upper Limit for the Anisotropy of Inertial Mass from Nuclear Resonance Experiments". Physical Review Letters 4 (7): 342–344. doi:10.1103/PhysRevLett.4.342. 
3. ^ Drever, R. W. P. (1961). "A search for anisotropy of inertial mass using a free precession technique". Philosophical Magazine 6 (65): 683–687. doi:10.1080/14786436108244418. 
4. ^ Prestage, J. D.; Bollinger, J. J.; Itano, W. M.; Wineland, D. J. (1985). "Limits for spatial anisotropy by use of nuclear-spin-polarized Be-9(+) ions". Physical Review Letters 54: 2387–2390. doi:10.1103/PhysRevLett.54.2387. 
5. ^ Phillips, P. R. (1987). "Test of spatial isotropy using a cryogenic torsion pendulum". Physical Review Letter 59 (5): 1784–1787. doi:10.1103/PhysRevLett.59.1784. 
6. ^ Lamoreaux, S. K.; Jacobs, J. P.; Heckel, B. R.; Raab, F. J.; Fortson, E. N. (1989). "Optical pumping technique for measuring small nuclear quadrupole shifts in 1S(0) atoms and testing spatial isotropy". Physical Review A 39: 1082–1111. doi:10.1103/PhysRevA.39.1082. 
7. ^ Chupp, T. E.; Hoare, R. J.; Loveman, R. A.; Oteiza, E. R.; Richardson, J. M.; Wagshul, M. E.; Thompson, A. K. (1989). "Results of a new test of local Lorentz invariance: A search for mass anisotropy in 21Ne". Physical Review Letters 63 (15): 1541–1545. doi:10.1103/PhysRevLett.63.1541. 
8. ^ Wineland, D. J.; Bollinger, J. J.; Heinzen, D. J.; Itano, W. M.; Raizen, M. G. (1991). "Search for anomalous spin-dependent forces using stored-ion spectroscopy". Physical Review Letters 67 (13): 1735–1738. doi:10.1103/PhysRevLett.67.1735. 
9. ^ Wang, Shih-Liang; Ni, Wei-Tou; Pan, Sheau-Shi (1993). "New Experimental Limit on the Spatial Anisotropy for Polarized Electrons". Modern Physics Letters A 8 (39): 3715–3725. doi:10.1142/S0217732393003445. 
10. ^ Berglund, C. J.; Hunter, L. R.; Krause, D., Jr.; Prigge, E. O.; Ronfeldt, M. S.; Lamoreaux, S. K. (1995). "New Limits on Local Lorentz Invariance from Hg and Cs Magnetometers". Physical Review Letters 75 (10): 1879–1882. doi:10.1103/PhysRevLett.75.1879. 
11. ^ Bear, D.; Stoner, R. E.; Walsworth, R. L.; Kostelecký, V. Alan; Lane, Charles D. (2000). "Limit on Lorentz and CPT Violation of the Neutron Using a Two-Species Noble-Gas Maser". Physical Review Letters 85 (24): 5038–5041. arXiv:physics/0007049. doi:10.1103/PhysRevLett.85.5038. 
12. ^ Walsworth, R. L.; Bear, D.; Humphrey, M.; Mattison, E. M.; Phillips, D. F.; Stoner, R. E.; Vessot, R. F. C. (2000). "New clock comparison searches for Lorentz and CPT violation". AIP Conference Proceedings 539: 119–129. arXiv:physics/0007063. doi:10.1063/1.1330910. 
13. ^ Phillips, D. F.; Humphrey, M. A.; Mattison, E. M.; Stoner, R. E.; Vessot, R. F.; Walsworth, R. L. (2000). "Limit on Lorentz and CPT violation of the proton using a hydrogen maser". Physical Review D 63 (11): 111101. arXiv:physics/0008230. doi:10.1103/PhysRevD.63.111101. 
14. ^ Humphrey, M. A.; Phillips, D. F.; Mattison, E. M.; Vessot, R. F.; Stoner, R. E.; Walsworth, R. L. (2003). "Testing CPT and Lorentz symmetry with hydrogen masers". Physical Review A 68 (6): 063807. arXiv:physics/0103068. doi:10.1103/PhysRevA.68.063807. 
15. ^ Hou, Li-Shing; Ni, Wei-Tou; Li, Yu-Chu M. (2003). "Test of Cosmic Spatial Isotropy for Polarized Electrons Using a Rotatable Torsion Balance". Physical Review Letters 90 (20): 201101. arXiv:physics/0009012. doi:10.1103/PhysRevLett.90.201101. 
16. ^ Canè, F.; Bear, D.; Phillips, D. F.; Rosen, M. S.; Smallwood, C. L.; Stoner, R. E.; Walsworth, R. L.; Kostelecký, V. Alan (2004). "Bound on Lorentz and CPT Violating Boost Effects for the Neutron". Physical Review Letters 93 (23): 230801. arXiv:physics/0309070. doi:10.1103/PhysRevLett.93.230801. 
17. ^ Wolf, P.; Chapelet, F.; Bize, S.; Clairon, A. (2006). "Cold Atom Clock Test of Lorentz Invariance in the Matter Sector". Physical Review Letters 96 (6): 060801. arXiv:hep-ph/0601024. doi:10.1103/PhysRevLett.96.060801. 
18. ^ Heckel, B. R.; Cramer, C. E.; Cook, T. S.; Adelberger, E. G.; Schlamminger, S.; Schmidt, U. (2006). "New CP-Violation and Preferred-Frame Tests with Polarized Electrons". Physical Review Letters 97 (2): 021603. arXiv:hep-ph/0606218. doi:10.1103/PhysRevLett.97.021603. 
19. ^ Heckel, B. R.; Adelberger, E. G.; Cramer, C. E.; Cook, T. S.; Schlamminger, S.; Schmidt, U. (2008). "Preferred-frame and CP-violation tests with polarized electrons". Physical Review D 78 (9): 092006. arXiv:0808.2673. doi:10.1103/PhysRevD.78.092006. 
20. ^ Altarev, I. et al. (2009). "Test of Lorentz Invariance with Spin Precession of Ultracold Neutrons". Physical Review Letters 103 (8): 081602. arXiv:0905.3221. doi:10.1103/PhysRevLett.103.081602. 
21. ^ Brown, J. M.; Smullin, S. J.; Kornack, T. W.; Romalis, M. V. (2010). "New Limit on Lorentz- and CPT-Violating Neutron Spin Interactions". Physical Review Letters 105 (15): 151604. arXiv:1006.5425. doi:10.1103/PhysRevLett.105.151604. 
22. ^ Gemmel, C.; Heil, W.; Karpuk, S.; Lenz, K.; Sobolev, Yu.; Tullney, K.; Burghoff, M.; Kilian, W.; Knappe-Grüneberg, S.; Müller, W.; Schnabel, A.; Seifert, F.; Trahms, L.; Schmidt, U. (2010). "Limit on Lorentz and CPT violation of the bound neutron using a free precession He3/Xe129 comagnetometer". Physical Review D 82 (11): 111901. arXiv:1011.2143. doi:10.1103/PhysRevD.82.111901. 

External links

• T. Roberts (2007) & S. Schleif, Relativity FAQ, What is the experimental basis of Special Relativity?