View source for Physics:Hughes–Drever experiment

[[File:Lithium-7-NMR spectrum of LiCl (1M) in D2O.gif|thumb|250px|<sup>7</sup>Li NMR spectrum of LiCl (1M) in D<sub>2</sub>O. The sharp, unsplit NMR line of this isotope of lithium is evidence for the isotropy of mass and space.]]
'''Hughes–Drever experiments''' (also '''clock comparison-''', '''clock anisotropy-''', '''mass isotropy-''', or '''energy isotropy experiments''') are [[Physics:Spectroscopy|spectroscopic]] tests of the [[Isotropy|isotropy]] of [[Physics:Mass|mass]] and [[Space|space]]. Although originally conceived of as a test of [[Physics:Mach's principle|Mach's principle]], they are now understood to be an important test of [[Lorentz covariance|Lorentz invariance]]. As in [[Physics:Michelson–Morley experiment|Michelson–Morley experiment]]s, the existence of a [[Physics:Preferred frame|preferred frame]] of reference or other deviations from Lorentz invariance can be tested, which also affects the validity of the [[Astronomy:Equivalence principle|equivalence principle]]. Thus these experiments concern fundamental aspects of both [[Physics:Special relativity|special]] and [[Physics:General relativity|general relativity]]. Unlike Michelson–Morley type experiments, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of [[Physics:Proton|proton]]s, [[Physics:Neutron|neutron]]s, and [[Physics:Electron|electron]]s. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity (see also [[Physics:Tests of special relativity|Tests of special relativity]]).<ref group=A name=willb>{{cite journal|author=Will, C. M.|title=The Confrontation between General Relativity and Experiment
|year=2006|volume=9|issue=3|journal=Living Reviews in Relativity|page=3
|url=http://www.livingreviews.org/lrr-2006-3
|accessdate = June 23, 2011|doi=10.12942/lrr-2006-3|arxiv=gr-qc/0510072|bibcode=2006LRR.....9....3W|pmid=28179873|pmc=5256066}}</ref>
<ref group=A name=will>{{cite journal|author=Will, C. M.|year=1995|title=Stable clocks and general relativity|journal=Proceedings of the 30th Rencontres de Moriond|pages=417|arxiv=gr-qc/9504017|bibcode = 1995dmcc.conf..417W }}</ref><ref name=koste group=A>{{cite journal|author1=Kostelecký, V. Alan |author2=Lane, Charles D. |year=1999|title=Constraints on Lorentz violation from clock-comparison experiments|journal=Physical Review D|volume=60|issue=11|pages=116010|doi=10.1103/PhysRevD.60.116010|arxiv=hep-ph/9908504|bibcode = 1999PhRvD..60k6010K |s2cid=119039071 }}</ref><ref name=matt group=A>{{Cite journal| author=Mattingly, David| title=Modern Tests of Lorentz Invariance|journal=Living Rev. Relativ.| volume=8| year=2005| issue=5| url=http://www.livingreviews.org/lrr-2005-5| bibcode=2005LRR.....8....5M| pages=5|arxiv = gr-qc/0502097 |doi = 10.12942/lrr-2005-5| pmid=28163649| pmc=5253993 }}</ref><ref group=A name=posp>{{cite journal|author1=Pospelov, Maxim|author2=Romalis, Michael|year=2004|title=Lorentz Invariance on Trial|journal=Physics Today|volume=57|issue=7|pages=40–46|doi=10.1063/1.1784301|url=http://physics.princeton.edu/romalis/articles/Pospelov%20and%20Romalis%20-%20Lorentz%20Invariance%20on%20Trial.pdf|bibcode=2004PhT....57g..40P|access-date=2011-05-26|archive-date=2022-01-02|archive-url=https://web.archive.org/web/20220102164626/https://physics.princeton.edu//romalis/articles/Pospelov%20and%20Romalis%20-%20Lorentz%20Invariance%20on%20Trial.pdf|url-status=dead}}</ref><ref group=A name=wals>{{cite book|author=Walsworth, R. L.|title=Special Relativity |year=2006|chapter=Tests of Lorentz Symmetry in the Spin-Coupling Sector|volume=702|pages=493–505|doi=10.1007/3-540-34523-X_18|chapter-url=http://www.cfa.harvard.edu/Walsworth/pdf/LNP_ch_19.pdf|series=Lecture Notes in Physics|isbn=978-3-540-34522-0}}</ref>

==Experiments by Hughes and Drever==
[[Biography:Giuseppe Cocconi|Giuseppe Cocconi]] and Edwin Ernest Salpeter (1958) theorized that [[Physics:Inertia|inertia]] depends on the surrounding masses according to [[Physics:Mach's principle|Mach's principle]]. Nonuniform distribution of matter thus would lead to [[Anisotropy|anisotropy]] of inertia in different directions. Heuristic arguments led them to believe that any inertial anisotropy, if one existed, would be dominated by mass contributions from the center of our Milky Way galaxy. They argued that this anisotropy might be observed in two ways: measuring the [[Physics:Zeeman effect|Zeeman splitting]] in an atom<ref>{{cite journal|author1=Cocconi, G. |author2=Salpeter E. |year=1958|title=A search for anisotropy of inertia |journal=Il Nuovo Cimento|volume=10|issue=4|pages=646–651|doi=10.1007/BF02859800|bibcode=1958NCim...10..646C|s2cid=123226805 }}</ref> or measuring the Zeeman splitting in the [[Physics:Excited state|excited nuclear state]] of {{SimpleNuclide|Iron|57|link=yes}} using the [[Physics:Mössbauer effect|Mössbauer effect]].<ref>{{cite journal|author1=Cocconi, G. |author2=Salpeter E. |year=1960|title=Upper Limit for the Anisotropy of Inertia from the Mössbauer Effect|journal=Physical Review Letters|volume=4|issue=4|pages=176–177|doi=10.1103/PhysRevLett.4.176|bibcode = 1960PhRvL...4..176C }}</ref>

[[Biography:Vernon W. Hughes|Vernon W. Hughes]] ''et al.'' (1960)<ref>{{cite journal|author1=Hughes, V. W. |author2=Robinson, H. G. |author3=Beltran-Lopez, V. |year=1960|title=Upper Limit for the Anisotropy of Inertial Mass from Nuclear Resonance Experiments|journal=Physical Review Letters|volume=4|issue=7|pages=342–344|doi=10.1103/PhysRevLett.4.342|bibcode = 1960PhRvL...4..342H  }}</ref> and [[Biography:Ronald Drever|Ronald Drever]] (1961)<ref>{{cite journal|author=Drever, R. W. P.|year=1961|title=A search for anisotropy of inertial mass using a free precession technique|journal=Philosophical Magazine|volume=6|issue=65|pages=683–687|doi=10.1080/14786436108244418|bibcode = 1961PMag....6..683D }}</ref> independently conducted similar [[Physics:Spectroscopy|spectroscopic]] experiments to test Mach's principle. However, they didn't use the Mössbauer effect but made [[Physics:Nuclear magnetic resonance spectroscopy|magnetic resonance]] measurements of the [[Physics:Atomic nucleus|nucleus]] of [[Chemistry:Lithium|lithium]]-7, whose [[Physics:Ground state|ground state]] possesses a [[Physics:Spin|spin]] of {{frac|3|2}}. The ground state is split into four equally spaced magnetic [[Physics:Energy level|energy level]]s when measured in a magnetic field in accordance with its allowed [[Physics:Magnetic quantum number|magnetic quantum number]]. The nuclear wave functions for the different energy levels have different spatial distributions relative to the magnetic field, and thus have different directional properties. If mass isotropy is satisfied, each transition between a pair of adjacent levels should emit a photon of equal frequency, resulting in a single, sharp spectral line. On the other hand, if inertia has a directional dependence, a triplet or broadened resonance line should be observed. During the 24-hour course of Drever's version of the experiment, the Earth turned, and the magnetic field axis swept different sections of the sky. Drever paid particular attention to the behavior of the spectral line as the magnetic field crossed the center of the galaxy.<ref group=A name=Bartusiak>{{cite book|last=Bartusiak |first=Marcia |title=Einstein's Unfinished Symphony: Listening to the Sounds of Space-Time|year=2003|publisher=Joseph Henry Press |isbn=0425186202 |pages=96–97 |url=https://books.google.com/books?id=pSYbbgSAZR8C&pg=PT96 |accessdate=15 Jul 2012 |quote='I watched that line over a 24-hour period as the Earth rotated. As the axis of the field swung past the center of the galaxy and other directions, I looked for a change,' recalls Drever.}}</ref> Neither Hughes nor Drever observed any frequency shift of the energy levels, and due to their experiments' high precision, the maximal anisotropy could be limited to 0.04&nbsp;[[Hertz|Hz]] = 10<sup>−25</sup>&nbsp;GeV.

Regarding the consequences of the null result for Mach's principle, it was shown by [[Biography:Robert H. Dicke|Robert H. Dicke]] (1961) that it is in agreement with this principle, as long as the spatial anisotropy is the same for all particles. Thus the null result is rather showing that inertial anisotropy effects are, if they exist, universal for all particles and locally unobservable.<ref>{{cite journal|author=Dicke, R. H.|year=1961|title=Experimental Tests of Mach's Principle|journal=Physical Review Letters|volume=7|issue=9|pages=359–360|doi=10.1103/PhysRevLett.7.359|bibcode = 1961PhRvL...7..359D }}</ref><ref>{{cite book|author=Dicke, R. H.|year=1964|title=The Theoretical Significance of Experimental Relativity|url=https://archive.org/details/theoreticalsigni0000dick|url-access=registration|publisher=Gordon and Breach}}</ref>

==Modern interpretation==
While the motivation for this experiment was to test Mach's principle, it has since become recognized as an important test of [[Lorentz covariance|Lorentz invariance]] and thus [[Physics:Special relativity|special relativity]]. This is because anisotropy effects also occur in the presence of a [[Physics:Preferred frame|preferred]] and Lorentz-violating frame of reference – usually identified with the CMBR rest frame as some sort of [[Physics:Luminiferous aether|luminiferous aether]] (relative velocity about 368&nbsp;km/s). Therefore, the negative results of the Hughes–Drever experiments (as well as the [[Physics:Michelson–Morley experiment|Michelson–Morley experiment]]s) rule out the existence of such a frame. In particular, Hughes–Drever tests of Lorentz violations are often described by a test theory of special relativity put forward by Clifford Will. According to this model, Lorentz violations in the presence of preferred frames can lead to differences between the maximal attainable velocity of massive particles and the speed of light. If they were different, the properties and frequencies of matter interactions would change as well. In addition, it is a fundamental consequence of the [[Astronomy:Equivalence principle|equivalence principle]] of [[Physics:General relativity|general relativity]] 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.<ref group=A name=willb /><ref group=A name=will />

Due to the fact that different frequencies ("clocks") are compared, these experiments are also denoted as clock-comparison experiments.<ref group=A name=koste /><ref group=A name=matt />

==Recent experiments==
{{Further|Physics:Modern searches for Lorentz violation}}
Besides Lorentz violations due to a preferred frame or influences based on Mach's principle, spontaneous violations of Lorentz invariance and [[Physics:CPT symmetry|CPT symmetry]] are also being searched for, motivated by the predictions of various [[Physics:Quantum gravity|quantum gravity]] models that suggest their existence. Modern updates of the Hughes–Drever experiments have been conducted studying possible Lorentz and CPT violation in [[Physics:Neutron|neutron]]s and [[Physics:Proton|proton]]s. Using [[Physics:Spin polarization|spin-polarized]] systems and co-magnetometers (to suppress magnetic influences), the accuracy and sensitivity of these experiments have been greatly increased. In addition, by using spin-polarized torsion balances, the [[Physics:Electron|electron]] sector has also been tested.<ref group=A name=posp /><ref group=A name=wals />

All of these experiments have thus far given 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 [[Physics:Standard-Model Extension|Standard-Model Extension]] (SME), an often used [[Physics:Effective field theory|effective field theory]] to assess possible Lorentz violations (see also other [[Physics:Test theories of special relativity|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):<ref group=A name=koste /><ref group=A>{{cite journal|author1=Hou, Li-Shing |author2=Ni, Wei-Tou |author3=Li, Yu-Chu M. |year=2003|title=Test of Cosmic Spatial Isotropy for Polarized Electrons Using a Rotatable Torsion Balance|journal=Physical Review Letters|volume=90|issue=20|pages=201101|doi=10.1103/PhysRevLett.90.201101|arxiv=physics/0009012|bibcode = 2003PhRvL..90t1101H|pmid=12785879 |s2cid=28211115 }}</ref><ref group=A name=matt />

{|class=wikitable
! rowspan=2 | Author
! rowspan=2 | Year
! colspan=3 | SME constraints
! rowspan=2 | Description
|-
!Proton!!Neutron!!Electron
|-
|-
|Phillips<ref>{{cite journal|author=Phillips, P. R.|year=1987|title=Test of spatial isotropy using a cryogenic spin-torsion pendulum|journal=Physical Review Letters|volume=59|issue=5|pages=1784–1787|doi=10.1103/PhysRevLett.59.1784|pmid=10035328|bibcode = 1987PhRvL..59.1784P }}</ref>||1987|| || ||10<sup>−27</sup>||[[Physics:Sine wave|Sinusoidal oscillations]] were investigated using a [[Physics:Cryogenics|cryogenic]] spin-torsion pendulum carrying a transversely polarized magnet.
|-
|-
|-
|-
|Wang ''et al.''<ref>{{cite journal|author1=Wang, Shih-Liang |author2=Ni, Wei-Tou |author3=Pan, Sheau-Shi |year=1993|title=New Experimental Limit on the Spatial Anisotropy for Polarized Electrons|journal=Modern Physics Letters A|volume=8|issue=39|pages=3715–3725|doi=10.1142/S0217732393003445|bibcode = 1993MPLA....8.3715W }}</ref>||1993|| || ||10<sup>−27</sup> || A spin-torsion pendulum carrying a spin-polarized [[Chemistry:Dysprosium|Dy]]–[[Chemistry:Iron|Fe]] mass is investigated for sidereal variations.
|-
|Berglund ''et al.''<ref>{{cite journal|author1=Berglund, C. J. |author2=Hunter, L. R. |author3=Krause, D. Jr. |author4=Prigge, E. O. |author5=Ronfeldt, M. S. |author6=Lamoreaux, S. K. |year=1995|title=New Limits on Local Lorentz Invariance from Hg and Cs Magnetometers|journal=Physical Review Letters|volume=75|issue=10|pages=1879–1882|doi=10.1103/PhysRevLett.75.1879|bibcode = 1995PhRvL..75.1879B|pmid=10059152 }}</ref>||1995||10<sup>−27</sup>||10<sup>−30</sup>||10<sup>−27</sup>|| The frequencies of [[Physics:Isotopes of mercury|<sup>199</sup>Hg]] and [[Chemistry:Caesium|<sup>133</sup>Cs]] are compared by applying a magnetic field.
|-
|-
|Phillips ''et al.''<ref>{{cite journal|author1=Phillips, D. F. |author2=Humphrey, M. A. |author3=Mattison, E. M. |author4=Stoner, R. E. |author5=Vessot, R. F. |author6=Walsworth, R. L. |year=2001|title=Limit on Lorentz and CPT violation of the proton using a hydrogen maser|journal=Physical Review D|volume=63|issue=11|pages=111101|doi=10.1103/PhysRevD.63.111101|arxiv=physics/0008230|bibcode = 2001PhRvD..63k1101P |s2cid=10665017 }}</ref>||2000||10<sup>−27</sup>|| || || The Zeeman frequency is measured using [[Physics:Hydrogen maser|hydrogen maser]]s.
|-
|Humphrey ''et al.''<ref>{{cite journal|author1=Humphrey, M. A. |author2=Phillips, D. F. |author3=Mattison, E. M. |author4=Vessot, R. F. |author5=Stoner, R. E. |author6=Walsworth, R. L. |year=2003|title=Testing CPT and Lorentz symmetry with hydrogen masers|journal=Physical Review A|volume=68|issue=6|pages=063807|doi=10.1103/PhysRevA.68.063807|arxiv=physics/0103068|bibcode = 2003PhRvA..68f3807H |s2cid=13659676 }}</ref>||2003||10<sup>−27</sup>|| ||10<sup>−27</sup> || Similar to Phillips ''et al.'' (2000).
|-
|Hou ''et al.''<ref>{{cite journal|author1=Hou, Li-Shing |author2=Ni, Wei-Tou |author3=Li, Yu-Chu M. |year=2003|title=Test of Cosmic Spatial Isotropy for Polarized Electrons Using a Rotatable Torsion Balance|journal=Physical Review Letters|volume=90|issue=20|pages=201101|doi=10.1103/PhysRevLett.90.201101|arxiv=physics/0009012|bibcode = 2003PhRvL..90t1101H|pmid=12785879 |s2cid=28211115 }}</ref>||2003|| || ||10<sup>−29</sup> || Similar to Wang ''et al.'' (1993).
|-
|Canè ''et al.''<ref>{{cite journal|author1=Canè, F. |author2=Bear, D. |author3=Phillips, D. F. |author4=Rosen, M. S. |author5=Smallwood, C. L. |author6=Stoner, R. E. |author7=Walsworth, R. L. |author8=Kostelecký, V. Alan |year=2004|title=Bound on Lorentz and CPT Violating Boost Effects for the Neutron|journal=Physical Review Letters|volume=93|issue=23|pages=230801|doi=10.1103/PhysRevLett.93.230801|arxiv=physics/0309070|bibcode = 2004PhRvL..93w0801C|pmid=15601138 |s2cid=20974775 }}</ref>||2004|| ||10<sup>−32</sup>|| || Similar to Bear ''et al.'' (2000).
|-
|-
|Heckel ''et al.''<ref>{{cite journal|author1=Heckel, B. R. |author2=Cramer, C. E. |author3=Cook, T. S. |author4=Adelberger, E. G. |author5=Schlamminger, S. |author6=Schmidt, U. |year=2006|title=New CP-Violation and Preferred-Frame Tests with Polarized Electrons|journal=Physical Review Letters|volume=97|issue=2|pages=021603|doi=10.1103/PhysRevLett.97.021603|arxiv=hep-ph/0606218|bibcode = 2006PhRvL..97b1603H|pmid=16907432 |s2cid=27027816 }}</ref>||2006|| || ||10<sup>−30</sup>|| They used a spin-torsion pendulum with four sections of [[Physics:Alnico|Alnico]] and four sections of [[Chemistry:Samarium|Sm]][[Chemistry:Cobalt|<sub>5</sub>Co]].
|-
|Heckel ''et al.''<ref>{{cite journal|author1=Heckel, B. R. |author2=Adelberger, E. G. |author3=Cramer, C. E. |author4=Cook, T. S. |author5=Schlamminger, S. |author6=Schmidt, U. |year=2008|title=Preferred-frame and CP-violation tests with polarized electrons|journal=Physical Review D|volume=78|issue=9|pages=092006|doi=10.1103/PhysRevD.78.092006|arxiv=0808.2673|bibcode = 2008PhRvD..78i2006H |s2cid=119259958 }}</ref>||2008|| || ||10<sup>−31</sup>|| Similar to Heckel et al. (2006).
|-
|-
|-
|-
|-
|Peck ''et al.''<ref>{{cite journal|last1=Peck|first1=S.K.|year=2012|title= New Limits on Local Lorentz Invariance in Mercury and Cesium|journal=Physical Review A|volume=86|issue=1|pages=012109|doi=10.1103/PhysRevA.86.012109|arxiv=1205.5022|bibcode = 2012PhRvA..86a2109P|last2=Kim|first2=D. K.|last3=Stein|first3=D.|last4=Orbaker|first4=D.|last5=Foss|first5=A.|last6=Hummon|first6=M. T.|last7=Hunter|first7=L. R.|s2cid=118619087|display-authors=1}}</ref>||2012||10<sup>−30</sup>||10<sup>−31</sup>|| || Similar to Berglund ''et al.'' (1995).
|-
|-
|Allmendinger ''et al.''<ref>{{cite journal |title=New limit on Lorentz and CPT violating neutron spin interactions using a free precession 3He-129Xe co-magnetometer|journal=Physical Review Letters|volume=112|issue=11|year=2014|pages=110801|arxiv=1312.3225|doi=10.1103/PhysRevLett.112.110801|pmid=24702343|bibcode = 2014PhRvL.112k0801A |last1=Allmendinger|first1=F.|last2=Heil|first2=W.|display-authors=1|last3=Karpuk|first3=S.|last4=Kilian|first4=W.|last5=Scharth|first5=A.|last6=Schmidt|first6=U.|last7=Schnabel|first7=A.|last8=Sobolev|first8=Yu.|last9=Tullney|first9=K.|s2cid=8122573}}</ref>||2013||||10<sup>−34</sup>|||| Similar to Gemmel ''et al.'' (2010).
|}

==Secondary sources==
<references group=A />

==Primary sources==
<references />

==External links==
* T. Roberts (2007) & S. Schleif, Relativity FAQ, [http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html What is the experimental basis of Special Relativity?]

{{Tests of special relativity}}

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[[Category:Tests of general relativity]]

{{Sourceattribution|Hughes–Drever experiment}}