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| In [[physics]], '''Lorentz symmetry''', named for [[Hendrik Lorentz]], is "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|url=http://cerncourier.com/cws/article/cern/29224 |title=Framing Lorentz symmetry |publisher=CERN Courier |date=2004-11-24 |accessdate=2013-05-26}}</ref> '''Lorentz covariance''', a related concept, is a key property of [[spacetime]] following from the [[special theory of relativity]]. Lorentz covariance has two distinct, but closely related meanings:
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| # 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 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]]).
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| # 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]].
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| This usage of the term ''covariant'' should not be confused with the related concept of a ''[[covariant vector]]''. On [[manifold]]s, the words [[Covariance and contravariance of vectors|''covariant'' and ''contravariant'']] refer to how objects transform under general coordinate transformations. Confusingly, both covariant and contravariant four-vectors can be Lorentz covariant quantities.
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| '''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 [[Poincare invariance|Poincaré invariance]].
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| ==Examples==
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| In general, the nature of a Lorentz tensor can be identified by its [[tensor order]], which is the number of indices it has. No indices implies it is a scalar, one implies that it is a vector, etc. Furthermore, any number of new scalars, vectors etc. can be made by contracting any kinds of tensors together, but many of these may not have any real physical meaning. Some of those tensors that do have a physical interpretation are listed (by no means exhaustively) below.
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| Please note, the [[metric tensor|metric]] sign convention such that η = [[diagonal matrix|diag]] (1, −1, −1, −1) is used throughout the article.
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| ===Scalars===
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| [[Spacetime interval]]:
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| :<math>\Delta s^2=x^a x^b \eta_{ab}=c^2 \Delta t^2 - \Delta x^2 - \Delta y^2 - \Delta z^2</math>
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| [[Proper time]] (for [[timelike]] intervals):
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| :<math>\Delta \tau = \sqrt{\frac{\Delta s^2}{c^2}},\, \Delta s^2 > 0</math>
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| [[Rest mass]]:
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| :<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>
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| Electromagnetism invariants:
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| :<math>F_{ab} F^{ab} = \ 2 \left( B^2 - \frac{E^2}{c^2} \right)</math>
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| :<math>G_{cd}F^{cd}=\frac{1}{2}\epsilon_{abcd}F^{ab} F^{cd} = - \frac{4}{c} \left( \vec B \cdot \vec E \right)</math>
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| [[D'Alembertian]]/wave operator:
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| :<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>
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| ===Four-vectors===
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| 4-[[Displacement (vector)|displacement]]:
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| :<math>X^a = \left[ct, x, y, z\right]</math>
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| Partial derivative:
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| :<math>\partial_a = \left[ \frac{1}{c}\frac{\partial}{\partial t}, \frac{\partial}{\partial x}, \frac{\partial}{\partial y}, \frac{\partial}{\partial z} \right]</math>
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| [[4-velocity]]:
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| :<math>U^a = \frac{dX^a}{d\tau} = \gamma \left[c, \frac{dx}{dt}, \frac{dy}{dt}, \frac{dz}{dt}\right]</math>
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| [[4-momentum]]:
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| :<math>P^a = m_0 U^a = \left[\frac{E}{c}, p_x, p_y, p_z\right]</math>
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| [[4-current]]:
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| :<math>j^a = \left[c\rho, j_x, j_y, j_z\right]</math>
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| ===Four-tensors===
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| The [[Kronecker delta]]:
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| :<math>\delta^a_b = \begin{cases} 1 & \mbox{if } a = b, \\ 0 & \mbox{if } a \ne b. \end{cases}</math>
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| The [[Minkowski metric]] (the metric of flat space according to [[general relativity]]):
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| :<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>
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| The [[Levi-Civita symbol]]:
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| :<math>\epsilon_{abcd} = -\epsilon^{abcd} = \begin{cases} +1 & \mbox{if } \{abcd\} \mbox{ is an even permutation of } \{0123\}, \\ -1 & \mbox{if } \{abcd\} \mbox{ is an odd permutation of } \{0123\}, \\ 0 & \mbox{otherwise.} \end{cases}</math>
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| [[Electromagnetic field tensor]] (using a [[sign convention#Metric signature|metric signature]] of + − − − ):
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| :<math>F_{ab} = \begin{bmatrix} 0 & E_x/c & E_y/c & E_z/c \\ -E_x/c & 0 & -B_z & B_y \\ -E_y/c & B_z & 0 & -B_x \\ -E_z/c & -B_y & B_x & 0 \end{bmatrix}</math>
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| [[Hodge dual|Dual]] electromagnetic field tensor:
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| :<math>G_{cd} = \frac{1}{2}\epsilon_{abcd}F^{ab} = \begin{bmatrix} 0 & B_x & B_y & B_z \\ -B_x & 0 & E_z/c & -E_y/c \\ -B_y & -E_z/c & 0 & E_x/c \\ -B_z & E_y/c & -E_x/c & 0 \end{bmatrix}</math>
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| == Lorentz violating models ==
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| {{See also|Modern searches for Lorentz violation}}
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| In standard field theory, there are very strict and severe constraints on [[Renormalization group#Relevant and irrelevant operators, 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.
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| Since some approaches to [[quantum gravity]] lead to violations of Lorentz invariance,<ref name="Mattingly">{{Cite journal|doi=10.12942/lrr-2005-5|title=Modern Tests of Lorentz Invariance|year=2005|last1=Mattingly|first1=David|journal=Living Reviews in Relativity|volume=8|arxiv = gr-qc/0502097 |bibcode = 2005LRR.....8....5M }}</ref> these studies are part of [[Phenomenological Quantum Gravity]].
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| Lorentz violating models typically fall into four classes{{Citation needed|date=October 2011}}:
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| *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]].
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| *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. 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]].
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| *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. It is not accurate to call such models Lorentz-violating as much as Lorentz deformed any more than special relativity can be called a violation of Galilean symmetry rather than a deformation of it. 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.
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| *This is a class of its own; a subgroup of the Lorentz group is sufficient to give us all the standard predictions if [[CP symmetry|CP]] is an exact symmetry. However, CP isn't exact. This is called [[Very Special Relativity]].
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| Models belonging to the first two classes can be consistent with experiment if Lorentz breaking happens at Planck scale or beyond it, 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.
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| 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">
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| {{cite journal
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| |first=V.A. |last=Kostelecky |first2=N. |last2=Russell
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| |title=Data Tables for Lorentz and CPT Violation
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| |year=2010
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| |arxiv=0801.0287v3
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| }}</ref>
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| == See also ==
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| *[[Antimatter tests of Lorentz violation]]
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| *[[General covariance]]
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| *[[Lorentz invariance in loop quantum gravity]]
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| *[[Lorentz-violating neutrino oscillations]]
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| *[[Symmetry in physics]]
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| ==References==
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| {{reflist}}
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| *Background information on Lorentz and CPT violation: http://www.physics.indiana.edu/~kostelec/faq.html
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| *{{Cite journal|doi=10.12942/lrr-2005-5|title=Modern Tests of Lorentz Invariance|year=2005|last1=Mattingly|first1=David|journal=Living Reviews in Relativity|volume=8|arxiv = gr-qc/0502097 |bibcode = 2005LRR.....8....5M }}
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| *{{cite journal|author=Amelino-Camelia G, Ellis J, Mavromatos N E, Nanopoulos D V, and 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 | accessdate=2007-12-22|arxiv = astro-ph/9712103 |bibcode = 1998Natur.393..763A }}
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| *{{cite journal|author=Jacobson T, Liberati S, and 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 | url=http://www.nature.com/nature/journal/v424/n6952/full/nature01882.html | accessdate=2007-12-22|pmid=12944959|issue=6952|arxiv = astro-ph/0212190 |bibcode = 2003Natur.424.1019J }}
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| *{{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 | url=http://www.nature.com/nature/journal/v424/n6952/full/4241007a.html | accessdate=2007-12-22|pmid=12944951|issue=6952|bibcode = 2003Natur.424.1007C }}
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| *{{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|arxiv = hep-ph/0209264 |bibcode = 2003PhRvD..67l4011J }}
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| *{{Cite arxiv |eprint=0802.2536 |author1=Luis Gonzalez-Mestres |title=Lorentz symmetry violation and the results of the AUGER experiment |class=hep-ph |year=2008}}
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| *{{Cite arxiv|eprint =0908.1832|author1 =Fermi GBM/LAT Collaborations|title =Testing Einstein's special relativity with Fermi's short hard gamma-ray burst GRB090510|class =astro-ph.HE|year =2009}}
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| ==External links==
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| [[Category:Special relativity]]
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| [[Category:Symmetry]]
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