 Doubly special relativity

Doubly special relativity (DSR)— also called deformed special relativity or, by some, extraspecial relativity — is a modified theory of special relativity in which there is not only an observerindependent maximum velocity (the speed of light), but an observerindependent maximum energy scale and minimum length scale (the Planck energy and Planck length).^{[1]}
Contents
History
Such a modification of special relativity was first proposed in a paper by Giovanni AmelinoCamelia in 2000^{[2]}, though it is at least implicit in a paper by Paul Merriam. AmelinoCamelia proposed a specific realization of preserving invariance of the Planck length. A different model, inspired by that of AmelinoCamelia, was proposed in 2001 by João Magueijo and Lee Smolin, who focused on the invariance of Planck energy.^{[3]} It was quickly realized that there are indeed infinitely many deformations of special relativity that allow to achieve an invariance of the Planck energy, either as a maximum energy, as a maximal momentum, or both. It has been shown^{by whom?} that theories are related to loop quantum gravity in 2+1 dimensions (two space, one time), and it has been conjectured^{by whom?} that a relation also exists in 3+1 dimensions.
The motivation to these proposals is mainly theoretical, based on the following observation: The Planck energy is expected to play a fundamental role in a theory of quantum gravity, setting the scale at which quantum gravity effects cannot be neglected and new phenomena might become important. If special relativity is to hold up exactly to this scale, different observers would observe quantum gravity effects at different scales, due to the LorentzFitzGerald contraction, in contradiction to the principle that all inertial observers should be able to describe phenomena by the same physical laws. This motivation has been criticized on the grounds that the result of a Lorentztransformation does not itself constitute an observable.^{[4]}
The model suffers from several inconsistencies in formulation that have yet to be resolved.^{[5]}^{[6]} Most notably it is difficult to recover the standard transformation behavior for macroscopic bodies, known as the soccerballproblem. The other conceptual difficulty is that DSR is a priori formulated in momentum space. There is as yet no consistent formulation of the model in position space.
Main
In principle, it seems difficult to incorporate an invariant length magnitude in a theory which preserves Lorentz invariance due to LorentzFitzGerald contraction, but in the same way that special relativity incorporates an invariant velocity by modifying the highvelocity behavior of Galilean transformations, DSR modifies Lorentz transformations at small distances (large energies) in such a way to admit a length invariant scale without destroying the principle of relativity. The postulates on which DSR theories are constructed are:
 The principle of relativity holds, i.e. equivalence of all inertial observers.
 There are two observerindependent scales: the speed of light, c, and a length (energy) scale λ (η = 1 / λ) in such a way that when λ → 0 (η → ∞), special relativity is recovered.
As noted by Jerzy KowalskiGlikman, an immediate consequence of these postulates is that the symmetry group of DSR theories must be ten dimensional, corresponding to boosts, rotations and translations in 4 dimensions. Translations, however, cannot be the usual Poincaré generators as it would be in contradiction with postulate 2). As translation operators are expected to be modified, the usual dispersion relation
 E^{2} − p^{2} = m^{2}
is expected to be modified and, indeed, the presence of an energy scale, namely η, allows introducing ηsuppressed terms of higher order in the dispersion relation. These higher momenta powers in the dispersion relation can be traced back as having their origin in higher dimensional (i.e. nonrenormalizable) terms in the Lagrangian.
It was soon realized that by deforming the Poincaré (i.e. translation) sector of the Poincaré algebra, consistent DSR theories can be constructed. In accordance with postulate 1), the Lorentz sector of the algebra is not modified, but just nonlinearly realized in their action on momenta coordinates. More precisely, the Lorentz Algebra
 [M_{i},p_{0}] = 0
remains unmodified, while the most general modification on its action on momenta is
 [N_{i},p_{0}] = Dp_{i}
where A, B, C and D are arbitrary functions of p_{i},p_{0},η and M,N are the rotation generators and boost generators, respectively. It can be shown that C must be zero and in order to satisfy the Jacobi identity, A, B and D must satisfy a nonlinear first order differential equation. It was also shown by KowalskiGlikman that these constraints are automatically satisfied by requiring that the boost and rotation generators N and M, act as usual on some coordinates η_{A} (A=0,...,4) that satisfy
i.e. that belong to de Sitter space. The physical momenta p_{μ} are identified as coordinates in this space, i.e.
 p_{μ} = p_{μ}(η_{A},η)
and the dispersion relation that these momenta satisfy is given by the invariant
 .
This way, different choices for the "physical momenta coordinates" in this space give rise to different modified dispersion relations, a corresponding modified Poincare algebra in the Poincaré sector and a preserved underlying Lorentz invariance.
One of the most common examples is the socalled MagueijoSmolin basis (Also known as the DSR2 model), in which:
which implies, for instance,
 ,
showing explicitly the existence of the invariant energy scale P_{0} = η as [N_{i},P_{0} = η] = 0.
The theory is highly speculative as of first publishing in 2002, as it relies on no experimental evidence so far. It would be fair to say that DSR is not considered a promising approach by a majority of members of the highenergy physics community, as it lacks experimental evidence and there's so far no guiding principle in the choice for the particular DSR model (i.e. basis in momenta de Sitter space) that should be realized in nature, if any.
DSR is based upon a generalization of symmetry to quantum groups. The Poincaré symmetry of ordinary special relativity is deformed into some noncommutative symmetry and Minkowski space is deformed into some noncommutative space. As explained before, this theory is not a violation of Poincaré symmetry as much as a deformation of it and there is an exact de Sitter symmetry. This deformation is scale dependent in the sense that the deformation is huge at the Planck scale but negligible at much larger length scales. It's been argued that models which are significantly Lorentz violating at the Planck scale are also significantly Lorentz violating in the infrared limit because of radiative corrections, unless a highly unnatural finetuning mechanism is implemented. Without any exact Lorentz symmetry to protect them, such Lorentz violating terms will be generated with abandon by quantum corrections. However, DSR models do not succumb to this difficulty since the deformed symmetry is exact and will protect the theory from unwanted radiative corrections — assuming the absence of quantum anomalies. Furthermore, models where a privileged rest frame exists can escape this difficulty due to other mechanisms.
Jafari and Shariati have constructed canonical transformations that relate both the doubly special relativity theories of AmelinoCamelia and of Magueijo and Smolin to ordinary special relativity. They claim that doubly special relativity is therefore only a complicated set of coordinates for an old and simple theory. However, all theories are related to free theories by canonical transformations. Therefore supporters of doubly special relativity may claim that while it is equivalent to ordinary relativity, the momentum and energy coordinates of doubly special relativity are those that appear in the usual form of the standard model interactions.
Predictions
Experiments to date have not observed contradictions to special relativity (see Modern searches for Lorentz violation).
It was initially speculated that ordinary special relativity and doubly special relativity would make distinct physical predictions in high energy processes, and in particular the derivation of the GreisenZatsepinKuzmin limit would not be valid. However, it is now established that standard doubly special relativity does not predict any suppression of the GZK cutoff, contrary to the models where an absolute local rest frame exists, such as effective field theories like the StandardModel Extension.
Since DSR generically (though not necessarily) implies an energydependence of the speed of light, it has further been predicted that, if there are modifications to first order in energy over the Planck mass, this energydependence would be observable in high energetic photons reaching Earth from distant gamma ray bursts. Depending on whether the now energydependent speed of light increases or decreases with energy (a modeldependent feature) highly energetic photons would be faster or slower than the lower energetic ones .^{[7]} However, the FermiLAT experiment in 2009 measured a 31GeV photon, which nearly simultaneously arrived with other photons from the same burst, which excluded such dispersion effects even above the Planck energy.^{[8]} It has moreover been argued, that DSR with an energydependent speed of light is inconsistent and first order effects are ruled out already because they would lead to nonlocal particle interactions that would long have been observed in particle physics experiments.^{[9]}
de Sitter relativity
Main article: de Sitter relativitySince the de Sitter group naturally incorporates an invariant length–parameter, de Sitter relativity can be interpreted as an example of doubly special relativity. There is a fundamental difference, though: whereas in all doubly special relativity models the Lorentz symmetry is violated, in de Sitter relativity it remains as a physical symmetry. A drawback of the usual doubly special relativity models is that they are valid only at the energy scales where ordinary special relativity is supposed to break down, giving rise to a patchwork relativity. On the other hand, de Sitter relativity is found to be invariant under a simultaneous rescaling of mass, energy and momentum, and is consequently valid at all energy scales.
Inline notes and references
 ^ AmelinoCamelia, G. (2010). "DoublySpecial Relativity: Facts, Myths and Some Key Open Issues". Symmetry 2: 230–271. arXiv:1003.3942.
 ^ AmelinoCamelia, G. (2000). "Relativity in spacetimes with shortdistance structure governed by an observerindependent (Planckian) length scale". Int. J. Mod. Phys. D11: 35–60. arXiv:grqc/0012051. doi:10.1142/S0218271802001330.
 ^ Magueijo, J.; Smolin, L (2001). "Lorentz invariance with an invariant energy scale". Physical Review Letters 88 (19): 190403. arXiv:hepth/0112090. doi:10.1103/PhysRevLett.88.190403.
 ^ Hossenfelder, S. (2006). "Interpretation of Quantum Field Theories with a Minimal Length Scale". Physical Review D 73: 105013. arXiv:hepth/0603032. Bibcode 2006PhRvD..73j5013H. doi:10.1103/PhysRevD.73.105013.
 ^ Aloisio, R.; Galante, A.; Grillo, A.F.; Luzio, E.; Mendez, F. (2004). "Approaching Space Time Through Velocity in Doubly Special Relativity". Physical Review D 70: 125012. arXiv:grqc/0410020. Bibcode 2004PhRvD..70l5012A. doi:10.1103/PhysRevD.70.125012.
 ^ Aloisio, R.; Galante, A.; Grillo, A.F.; Luzio, E.; Mendez, F. (2005). "A note on DSRlike approach to spacetime". Physics Letters B 610: 101–106. arXiv:grqc/0501079. Bibcode 2005PhLB..610..101A. doi:10.1016/j.physletb.2005.01.090.
 ^ AmelinoCamelia, G.; Smolin, L. (2009). "Prospects for constraining quantum gravity dispersion with near term observations". Physical Review D 80: 084017. arXiv:0906.3731. Bibcode 2009PhRvD..80h4017A. doi:10.1103/PhysRevD.80.084017.
 ^ Fermi LAT Collaboration (2009). "A limit on the variation of the speed of light arising from quantum gravity effects". Nature 462 (7271): 331–334. arXiv:0908.1832. doi:10.1038/nature08574.
 ^ Hossenfelder, S. (2009). "The BoxProblem in Deformed Special Relativity". arXiv:0912.0090. Bibcode 2009arXiv0912.0090H.
See also
 Scale relativity
 Planck scale
 Planck units
 Planck epoch
 FockLorentz symmetry
Further reading
 AmelinoCamelia, G. (2002). "DoublySpecial Relativity: First Results and Key Open Problems". International Journal of Modern Physics D 11 (10): 1643–1669. arXiv:grqc/0210063. Bibcode 2002IJMPD..11.1643A. doi:10.1142/S021827180200302X.
 AmelinoCamelia, G. (2002). "Relativity: Special treatment". Nature 418 (6893): 34–35. arXiv:grqc/0207049. Bibcode 2002Natur.418...34A. doi:10.1038/418034a. PMID 12097897.
 Cardone, F.; Mignani, R. (2004). Energy and Geometry: An Introduction to Deformed Special Relativity. World Scientific. ISBN 9812387285.
 Jafari, N.; Shariati, A. (2006). "Doubly Special Relativity: A New Relativity or Not?". AIP Conference Proceedings. 841. pp. 462–465. arXiv:grqc/0602075. doi:10.1063/1.2218214.
 KowalskiGlikman, J. (2005). "Introduction to Doubly Special Relativity". Planck Scale Effects in Astrophysics and Cosmology. Lecture Notes in Physics. 669. Springer. pp. 131–159. arXiv:hepth/0405273. doi:10.1007/b105189. ISBN 9783540252634.
 Smolin, Lee. (2006). "Chapter 14. Building on Einstein". The trouble with physics : the rise of string theory, the fall of a science, and what comes next. Boston, MA: Houghton Mifflin. ISBN 9780618551057. OCLC 64453453. Smolin writes for the layman a brief history of the development of DSR and how it ties in with string theory and cosmology.
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