Editing Lorentz covariance

{{Use American English|date = March 2019}}
{{Short description|Concept in relativistic physics}}
In [[relativistic mechanics|relativistic physics]], '''Lorentz symmetry''' or '''Lorentz invariance''', named after the Dutch physicist [[Hendrik Lorentz]], is an equivalence of observation or observational symmetry due to [[special relativity]] implying that the laws of physics stay the same for all observers that are moving with respect to one another within an [[inertial frame]]. It has also been described as "the feature of nature that says experimental results are independent of the orientation or the boost velocity of the laboratory through space".<ref>{{cite web|first=Neil| last=Russell |url= https://cerncourier.com/a/framing-lorentz-symmetry/ |title=Framing Lorentz symmetry |publisher=CERN Courier |date=2004-11-24 |access-date=2019-11-08}}</ref>

'''Lorentz covariance''', a related concept, is a property of the underlying [[spacetime]] manifold.  Lorentz covariance has two distinct, but closely related meanings:

# A [[physical quantity]] is said to be Lorentz covariant if it transforms under a given [[group representation|representation]] of the [[Lorentz group]]. According to the [[representation theory of the Lorentz group]], these quantities are built out of [[scalar (physics)|scalar]]s, [[four-vector]]s, [[four-tensor]]s, and [[spinor]]s. In particular, a [[Lorentz scalar|Lorentz covariant scalar]] (e.g., the [[space-time interval]]) remains the same under [[Lorentz transformation]]s and is said to be a ''Lorentz invariant'' (i.e., they transform under the [[trivial representation]]).
# An [[equation]] is said to be Lorentz covariant if it can be written in terms of Lorentz covariant quantities (confusingly, some use the term ''invariant'' here). The key property of such equations is that if they hold in one inertial frame, then they hold in any inertial frame; this follows from the result that if all the components of a tensor vanish in one frame, they vanish in every frame. This condition is a requirement according to the [[principle of relativity]]; i.e., all non-[[gravitation]]al laws must make the same predictions for identical experiments taking place at the same spacetime event in two different [[inertial frames of reference]].

On [[manifold]]s, the words [[covariance and contravariance of vectors|''covariant'' and ''contravariant'']] refer to how objects transform under general coordinate transformations. Both covariant and contravariant four-vectors can be Lorentz covariant quantities.

'''Local Lorentz covariance''', which follows from [[general relativity]], refers to Lorentz covariance applying only [[local symmetry|''locally'']] in an infinitesimal region of spacetime at every point. There is a generalization of this concept to cover [[Poincare covariance|Poincaré covariance]] and Poincaré invariance.

==Examples==

In general, the (transformational) nature of a Lorentz tensor{{clarify|this terminology should be introduced before use|date=March 2017}} can be identified by its [[tensor order]], which is the number of free indices it has. No indices implies it is a scalar, one implies that it is a vector, etc.  Some tensors with a physical interpretation are listed below.

The [[sign convention]] of the [[Minkowski metric]] {{nowrap|1=''η'' = [[diagonal matrix|diag]] (1, −1, −1, −1)}} is used throughout the article.

===Scalars===

;[[Spacetime interval]]:<math>\Delta s^2=\Delta x^a \Delta x^b \eta_{ab}=c^2 \Delta t^2 - \Delta x^2 - \Delta y^2 - \Delta z^2</math>
;[[Proper time]] (for [[timelike]] intervals):<math>\Delta \tau = \sqrt{\frac{\Delta s^2}{c^2}},\, \Delta s^2 > 0</math>
;[[Proper distance]] (for [[spacelike]] intervals):<math>L = \sqrt{-\Delta s^2},\, \Delta s^2 < 0</math>
;[[Mass]]:<math>m_0^2 c^2 = P^a P^b \eta_{ab}= \frac{E^2}{c^2} - p_x^2 - p_y^2 - p_z^2</math>
;Electromagnetism invariants:<math>\begin{align}
  F_{ab} F^{ab} &= \ 2 \left( B^2 - \frac{E^2}{c^2} \right) \\
  G_{cd} F^{cd} &= \frac{1}{2}\epsilon_{abcd}F^{ab} F^{cd} = - \frac{4}{c} \left( \vec{B} \cdot \vec{E} \right)
\end{align}</math>
;[[D'Alembertian]]/wave operator:<math>\Box = \eta^{\mu\nu}\partial_\mu \partial_\nu = \frac{1}{c^2}\frac{\partial^2}{\partial t^2} - \frac{\partial^2}{\partial x^2} - \frac{\partial^2}{\partial y^2} - \frac{\partial^2}{\partial z^2}</math>

===Four-vectors===
;[[Displacement (vector)|4-displacement]]: <math>\Delta X^a = \left(c\Delta t, \Delta\vec{x}\right) = (c\Delta t, \Delta x, \Delta y, \Delta z)</math>
;[[Four-position|4-position]]: <math>X^a = \left(ct, \vec{x}\right) = (ct, x, y, z)</math>
;[[Four-gradient|4-gradient]]: which is the 4D [[partial derivative]]:{{paragraph}} <math>\partial^a = \left(\frac{\partial_t}{c}, -\vec{\nabla}\right) = \left(\frac{1}{c}\frac{\partial}{\partial t}, -\frac{\partial}{\partial x}, -\frac{\partial}{\partial y}, -\frac{\partial}{\partial z} \right)</math>
;[[Four-velocity|4-velocity]]: <math>U^a = \gamma\left(c, \vec{u}\right) = \gamma \left(c, \frac{dx}{dt}, \frac{dy}{dt}, \frac{dz}{dt}\right)</math>{{paragraph}} where <math>U^a = \frac{dX^a}{d\tau}</math>
;[[Four-momentum|4-momentum]]: <math>P^a = \left(\gamma mc, \gamma m\vec{v}\right) = \left(\frac{E}{c}, \vec{p}\right) = \left(\frac{E}{c}, p_x, p_y, p_z\right)</math>{{paragraph}} where <math>P^a = m U^a</math> and <math>m</math> is the [[Mass_in_special_relativity|rest mass]].
;[[Four-current|4-current]]: <math>J^a = \left(c\rho, \vec{j}\right) = \left(c\rho, j_x, j_y, j_z\right)</math>{{paragraph}} where <math>J^a = \rho_o U^a</math>
;[[Electromagnetic four-potential|4-potential]]: <math>A^a = \left(\frac{\phi}{c}, \vec{A}\right)= \left(\frac{\phi}{c}, A_x, A_y, A_z\right)</math>

===Four-tensors===

;[[Kronecker delta]]:<math>\delta^a_b = \begin{cases} 1 & \mbox{if } a = b, \\ 0 & \mbox{if } a \ne b. \end{cases}</math>
;[[Minkowski metric]] (the metric of flat space according to [[general relativity]]):<math>\eta_{ab} = \eta^{ab} = \begin{cases} 1 & \mbox{if } a = b = 0, \\ -1 & \mbox{if }a = b = 1, 2, 3, \\ 0 & \mbox{if } a \ne b. \end{cases}</math>
;[[Electromagnetic field tensor]] (using a [[sign convention#Metric signature|metric signature]] of + − − −):<math>F_{ab} = \begin{bmatrix}
   0              &  \frac{1}{c}E_x &  \frac{1}{c}E_y &  \frac{1}{c}E_z \\
  -\frac{1}{c}E_x &  0              & -B_z            &  B_y \\
  -\frac{1}{c}E_y &  B_z            &  0              & -B_x \\
  -\frac{1}{c}E_z & -B_y            &  B_x            &  0
\end{bmatrix}</math>
;[[Hodge dual|Dual]] electromagnetic field tensor:<math>G_{cd} = \frac{1}{2}\epsilon_{abcd}F^{ab} = \begin{bmatrix}
   0   &  B_x            &  B_y            &  B_z \\
  -B_x &  0              &  \frac{1}{c}E_z & -\frac{1}{c}E_y \\
  -B_y & -\frac{1}{c}E_z &  0              &  \frac{1}{c}E_x \\
  -B_z &  \frac{1}{c}E_y & -\frac{1}{c}E_x &  0
\end{bmatrix}</math>

==Lorentz violating models==
{{See also|Modern searches for Lorentz violation}}

In standard field theory, there are very strict and severe constraints on [[Renormalization group#Relevant and irrelevant operators and universality classes|marginal and relevant]] Lorentz violating operators within both [[Quantum electrodynamics|QED]] and the [[Standard Model]]. Irrelevant Lorentz violating operators may be suppressed by a high [[cutoff (physics)|cutoff]] scale, but they typically induce marginal and relevant Lorentz violating operators via radiative corrections. So, we also have very strict and severe constraints on irrelevant Lorentz violating operators.

Since some approaches to [[quantum gravity]] lead to violations of Lorentz invariance,<ref name="Mattingly">{{Cite journal|doi=10.12942/lrr-2005-5|pmid=28163649|pmc=5253993|title=Modern Tests of Lorentz Invariance|year=2005|last1=Mattingly|first1=David|journal=Living Reviews in Relativity|volume=8|issue=1|pages=5|doi-access=free |arxiv = gr-qc/0502097 |bibcode = 2005LRR.....8....5M }}</ref> these studies are part of [[phenomenological quantum gravity]]. Lorentz violations are allowed in [[string theory]], [[supersymmetry]] and [[Hořava–Lifshitz gravity]].<ref>{{Cite journal |arxiv = 1709.03434|doi = 10.1038/s41567-018-0172-2|title = Neutrino interferometry for high-precision tests of Lorentz symmetry with Ice ''Cube''|journal = Nature Physics|volume = 14|issue = 9|pages = 961–966|year = 2018|last1 = Collaboration|first1 = IceCube|last2 = Aartsen|first2 = M. G.|last3 = Ackermann|first3 = M.|last4 = Adams|first4 = J.|last5 = Aguilar|first5 = J. A.|last6 = Ahlers|first6 = M.|last7 = Ahrens|first7 = M.|last8 = Al Samarai|first8 = I.|last9 = Altmann|first9 = D.|last10 = Andeen|first10 = K.|last11 = Anderson|first11 = T.|last12 = Ansseau|first12 = I.|last13 = Anton|first13 = G.|last14 = Argüelles|first14 = C.|last15 = Auffenberg|first15 = J.|last16 = Axani|first16 = S.|last17 = Bagherpour|first17 = H.|last18 = Bai|first18 = X.|last19 = Barron|first19 = J. P.|last20 = Barwick|first20 = S. W.|last21 = Baum|first21 = V.|last22 = Bay|first22 = R.|last23 = Beatty|first23 = J. J.|last24 = Becker Tjus|first24 = J.|last25 = Becker|first25 = K. -H.|last26 = BenZvi|first26 = S.|last27 = Berley|first27 = D.|last28 = Bernardini|first28 = E.|last29 = Besson|first29 = D. Z.|last30 = Binder|first30 = G.|bibcode = 2018NatPh..14..961I|s2cid = 59497861|display-authors = 29}}</ref>

Lorentz violating models typically fall into four classes:{{Citation needed|date=October 2011}}

* The laws of physics are exactly [[Lorentz covariant]] but this symmetry is [[spontaneously broken]]. In [[special relativity|special relativistic]] theories, this leads to [[phonon]]s, which are the [[Goldstone boson]]s. The phonons travel at ''less'' than the [[speed of light]].
* Similar to the approximate Lorentz symmetry of phonons in a lattice (where the speed of sound plays the role of the critical speed), the Lorentz symmetry of special relativity (with the speed of light as the critical speed in vacuum) is only a low-energy limit of the laws of physics, which involve new phenomena at some fundamental scale. Bare conventional "elementary" particles are not point-like field-theoretical objects at very small distance scales, and a nonzero fundamental length must be taken into account. Lorentz symmetry violation is governed by an energy-dependent parameter which tends to zero as momentum decreases.<ref name="Gonzalez-MestresMoriond1995">{{Cite journal|title=Properties of a possible class of particles able to travel faster than light |url=https://archive.org/details/arxiv-astro-ph9505117 |journal=Dark Matter in Cosmology |pages=645 |author=Luis Gonzalez-Mestres |date=1995-05-25 |arxiv=astro-ph/9505117 |bibcode=1995dmcc.conf..645G }}</ref> Such patterns require the existence of a [[preferred frame|privileged local inertial frame]] (the "vacuum rest frame"). They can be tested, at least partially, by ultra-high energy cosmic ray experiments like the [[Pierre Auger Observatory]].<ref name="Gonzalez-MestresICRC97">{{Cite journal|title=Absence of Greisen-Zatsepin-Kuzmin Cutoff and Stability of Unstable Particles at Very High Energy, as a Consequence of Lorentz Symmetry Violation |journal=Proceedings of the 25th International Cosmic Ray Conference (Held 30 July - 6 August) |author=Luis Gonzalez-Mestres |volume = 6|pages=113 |date=1997-05-26 |bibcode = 1997ICRC....6..113G|arxiv=physics/9705031}}</ref>
* The laws of physics are symmetric under a [[deformation theory|deformation]] of the Lorentz or more generally, the [[Poincaré group]], and this deformed symmetry is exact and unbroken. This deformed symmetry is also typically a [[quantum group]] symmetry, which is a generalization of a group symmetry. [[Deformed special relativity]] is an example of this class of models. The deformation is scale dependent, meaning that at length scales much larger than the Planck scale, the symmetry looks pretty much like the Poincaré group. Ultra-high energy cosmic ray experiments cannot test such models.
* [[Very special relativity]] forms a class of its own; if [[CP symmetry|charge-parity]] (CP) is an exact symmetry, a subgroup of the Lorentz group is sufficient to give us all the standard predictions. This is, however, not the case.

Models belonging to the first two classes can be consistent with experiment if Lorentz breaking happens at Planck scale or beyond it, or even before it in suitable [[preon]]ic models,<ref name="Gonzalez-MestresCrete2013">{{Cite journal|doi=10.1051/epjconf/20147100062|title=Ultra-high energy physics and standard basic principles. Do Planck units really make sense?|journal=EPJ Web of Conferences|volume=71|pages=00062|year=2014|author=Luis Gonzalez-Mestres|url=http://www.epj-conferences.org/articles/epjconf/pdf/2014/08/epjconf_icnfp2013_00062.pdf|bibcode=2014EPJWC..7100062G|doi-access=free}}</ref> and if Lorentz symmetry violation is governed by a suitable energy-dependent parameter. One then has a class of models which deviate from Poincaré symmetry near the Planck scale but still flows towards an exact Poincaré group at very large length scales.  This is also true for the third class, which is furthermore protected from radiative corrections as one still has an exact (quantum) symmetry.

Even though there is no evidence of the violation of Lorentz invariance, several experimental searches for such violations have been performed during recent years. A detailed summary of the results of these searches is given in the Data Tables for Lorentz and CPT Violation.<ref name="DataTables">
{{cite arXiv
 |first1=V.A. |last1=Kostelecky |first2=N. |last2=Russell
 |title=Data Tables for Lorentz and CPT Violation
 |year=2010
 |eprint=0801.0287v3
 |class=hep-ph
}}</ref>

Lorentz invariance is also violated in QFT assuming non-zero temperature.<ref>{{Cite book|last1=Laine|first1=Mikko|last2=Vuorinen|first2=Aleksi|date=2016|title=Basics of Thermal Field Theory|series=Lecture Notes in Physics|volume=925|language=en-gb|doi=10.1007/978-3-319-31933-9|issn=0075-8450|arxiv=1701.01554|bibcode=2016LNP...925.....L|isbn=978-3-319-31932-2|s2cid=119067016}}</ref><ref>{{Cite journal|last=Ojima|first=Izumi|date=January 1986|title=Lorentz invariance vs. temperature in QFT|journal=Letters in Mathematical Physics|language=en|volume=11|issue=1|pages=73–80|doi=10.1007/bf00417467|issn=0377-9017|bibcode=1986LMaPh..11...73O|s2cid=122316546}}</ref><ref>{{Cite web|url=https://physics.stackexchange.com/q/131197|title=Proof of Loss of Lorentz Invariance in Finite Temperature Quantum Field Theory|website=Physics Stack Exchange|access-date=2018-06-18}}</ref>

There is also growing evidence of Lorentz violation in [[Weyl semimetal]]s and [[Dirac semimetal]]s.<ref>{{Cite journal |doi = 10.1126/sciadv.1603266|title = Discovery of Lorentz-violating type II Weyl fermions in LaAl ''Ge''|journal = Science Advances|volume = 3|issue = 6|pages = e1603266|year = 2017|last1 = Xu|first1 = Su-Yang|last2 = Alidoust|first2 = Nasser|last3 = Chang|first3 = Guoqing|last4 = Lu|first4 = Hong|last5 = Singh|first5 = Bahadur|last6 = Belopolski|first6 = Ilya|last7 = Sanchez|first7 = Daniel S.|last8 = Zhang|first8 = Xiao|last9 = Bian|first9 = Guang|last10 = Zheng|first10 = Hao|last11 = Husanu|first11 = Marious-Adrian|last12 = Bian|first12 = Yi|last13 = Huang|first13 = Shin-Ming|last14 = Hsu|first14 = Chuang-Han|last15 = Chang|first15 = Tay-Rong|last16 = Jeng|first16 = Horng-Tay|last17 = Bansil|first17 = Arun|last18 = Neupert|first18 = Titus|last19 = Strocov|first19 = Vladimir N.|last20 = Lin|first20 = Hsin|last21 = Jia|first21 = Shuang|last22 = Hasan|first22 = M. Zahid|pmid = 28630919|pmc = 5457030|bibcode = 2017SciA....3E3266X|doi-access = free}}</ref><ref>{{Cite journal | doi=10.1038/s41467-017-00280-6| pmid=28811465| pmc=5557853| title=Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe2| journal=Nature Communications| volume=8| issue=1| pages=257| year=2017| last1=Yan| first1=Mingzhe| last2=Huang| first2=Huaqing| last3=Zhang| first3=Kenan| last4=Wang| first4=Eryin| last5=Yao| first5=Wei| last6=Deng| first6=Ke| last7=Wan| first7=Guoliang| last8=Zhang| first8=Hongyun| last9=Arita| first9=Masashi| last10=Yang| first10=Haitao| last11=Sun| first11=Zhe| last12=Yao| first12=Hong| last13=Wu| first13=Yang| last14=Fan| first14=Shoushan| last15=Duan| first15=Wenhui| last16=Zhou| first16=Shuyun| author-link16= Shuyun Zhou|bibcode=2017NatCo...8..257Y| arxiv=1607.03643}}</ref><ref>{{cite journal | arxiv=1603.08508| doi=10.1038/nphys3871| title=Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2| journal=Nature Physics| volume=12| issue=12| pages=1105–1110| year=2016| last1=Deng| first1=Ke| last2=Wan| first2=Guoliang| last3=Deng| first3=Peng| last4=Zhang| first4=Kenan| last5=Ding| first5=Shijie| last6=Wang| first6=Eryin| last7=Yan| first7=Mingzhe| last8=Huang| first8=Huaqing| last9=Zhang| first9=Hongyun| last10=Xu| first10=Zhilin| last11=Denlinger| first11=Jonathan| last12=Fedorov| first12=Alexei| last13=Yang| first13=Haitao| last14=Duan| first14=Wenhui| last15=Yao| first15=Hong| last16=Wu| first16=Yang| last17=Fan| first17=Shoushan| last18=Zhang| first18=Haijun| last19=Chen| first19=Xi| last20=Zhou| first20=Shuyun| bibcode=2016NatPh..12.1105D| s2cid=118474909}}</ref><ref>{{Cite journal | doi=10.1038/nmat4685 |pmid = 27400386|title = Spectroscopic evidence for a type II Weyl semimetallic state in MoTe2|journal = Nature Materials|volume = 15|issue = 11|pages = 1155–1160|year = 2016|last1 = Huang|first1 = Lunan|last2 = McCormick|first2 = Timothy M.|last3 = Ochi|first3 = Masayuki|last4 = Zhao|first4 = Zhiying|last5 = Suzuki|first5 = Michi-To|last6 = Arita|first6 = Ryotaro|last7 = Wu|first7 = Yun|last8 = Mou|first8 = Daixiang|last9 = Cao|first9 = Huibo|last10 = Yan|first10 = Jiaqiang|last11 = Trivedi|first11 = Nandini|last12 = Kaminski|first12 = Adam|bibcode = 2016NatMa..15.1155H|arxiv = 1603.06482|s2cid = 2762780}}</ref><ref>{{Cite journal |doi = 10.1038/ncomms13643|pmid = 27917858|pmc = 5150217|title = Discovery of a new type of topological Weyl fermion semimetal state in MoxW1−xTe2|journal = Nature Communications|volume = 7|pages = 13643|year = 2016|last1 = Belopolski|first1 = Ilya|last2 = Sanchez|first2 = Daniel S.|last3 = Ishida|first3 = Yukiaki|last4 = Pan|first4 = Xingchen|last5 = Yu|first5 = Peng|last6 = Xu|first6 = Su-Yang|last7 = Chang|first7 = Guoqing|last8 = Chang|first8 = Tay-Rong|last9 = Zheng|first9 = Hao|last10 = Alidoust|first10 = Nasser|last11 = Bian|first11 = Guang|last12 = Neupane|first12 = Madhab|last13 = Huang|first13 = Shin-Ming|last14 = Lee|first14 = Chi-Cheng|last15 = Song|first15 = You|last16 = Bu|first16 = Haijun|last17 = Wang|first17 = Guanghou|last18 = Li|first18 = Shisheng|last19 = Eda|first19 = Goki|last20 = Jeng|first20 = Horng-Tay|last21 = Kondo|first21 = Takeshi|last22 = Lin|first22 = Hsin|last23 = Liu|first23 = Zheng|last24 = Song|first24 = Fengqi|last25 = Shin|first25 = Shik|last26 = Hasan|first26 = M. Zahid|bibcode = 2016NatCo...713643B|arxiv = 1612.05990}}</ref>

==See also==
{{cols|colwidth=21em}}
* [[Four vector|4-vector]]
* [[Antimatter tests of Lorentz violation]]
* [[Fock–Lorentz symmetry]]
* [[General covariance]]
* [[Lorentz invariance in loop quantum gravity]]
* [[Lorentz-violating electrodynamics]]
* [[Lorentz-violating neutrino oscillations]]
* [[Planck length]]
* [[Symmetry in physics]]
{{colend}}

==Notes==
{{reflist|2}}

==References==
* Background information on Lorentz and CPT violation: http://www.physics.indiana.edu/~kostelec/faq.html
* {{Cite journal|doi=10.12942/lrr-2005-5|pmid=28163649|pmc=5253993|title=Modern Tests of Lorentz Invariance|year=2005|last1=Mattingly|first1=David|journal=Living Reviews in Relativity|volume=8|issue=1|pages=5|doi-access=free |arxiv = gr-qc/0502097 |bibcode = 2005LRR.....8....5M }}
* {{cite journal|vauthors=Amelino-Camelia G, Ellis J, Mavromatos NE, Nanopoulos DV, Sarkar S | title=Tests of quantum gravity from observations of bold gamma-ray bursts | journal=Nature | volume=393|issue=6687 | pages=763–765 |date=June 1998 | doi=10.1038/31647 |  url=http://www.nature.com/nature/journal/v393/n6687/full/393763a0_fs.html | access-date=2007-12-22|arxiv = astro-ph/9712103 |bibcode = 1998Natur.393..763A | s2cid=4373934 }}
* {{cite journal|vauthors=Jacobson T, Liberati S, Mattingly D | title=A strong astrophysical constraint on the violation of special relativity by quantum gravity | journal=Nature | volume=424 | pages=1019–1021 |date=August 2003 | doi=10.1038/nature01882 |pmid=12944959|issue=6952|arxiv = astro-ph/0212190 |bibcode = 2003Natur.424.1019J | citeseerx=10.1.1.256.1937 | s2cid=17027443 }}
* {{cite journal|author=Carroll S | title=Quantum gravity: An astrophysical constraint | journal=Nature | volume=424 | pages=1007–1008 |date=August 2003 | doi=10.1038/4241007a |pmid=12944951|issue=6952|bibcode = 2003Natur.424.1007C | s2cid=4322563 | doi-access=free }}
* {{cite journal|doi=10.1103/PhysRevD.67.124011|title=Threshold effects and Planck scale Lorentz violation: Combined constraints from high energy astrophysics|year=2003|last1=Jacobson|first1=T.|last2=Liberati|first2=S.|last3=Mattingly|first3=D.|journal=Physical Review D|volume=67|issue=12|pages=124011|arxiv = hep-ph/0209264 |bibcode = 2003PhRvD..67l4011J |s2cid=119452240}}

[[Category:Special relativity]]
[[Category:Symmetry]]
[[Category:Hendrik Lorentz]]