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There is no loss of generality in using light-front relativistic quantum theories. For systems of a finite number of degrees of freedom there are explicit <math>S</math>-matrix-preserving unitary transformations that transform theories with light-front kinematic subgroups to equivalent theories with instant-form or point-form kinematic subgroups. One expects that this is true in quantum field theory, although establishing the equivalence requires a nonperturbative definition of the theories in different forms of dynamics.
=== Light-front Commutation Relations ===
[[Canonical commutation relations]] at equal time are the central element of the [[Canonical quantization]] method to quantized fields. In the standard quantization method (called Instant Form in Dirac's classification of dynamical forms), the relations are, for example here for a spin-0 field <math> \phi </math> and its canonical conjugate <math> \pi </math>:
<math display="block">{\rm Instant~Form:}~~[\phi(t, \vec x),\phi(t, \vec y)] = 0, \ \ [\pi( \vec x), \pi( \vec y)] = 0, \ \ [\phi(t, \vec x),\pi(t, \vec y)] = i\hbar \delta^3( \vec x- \vec y).</math>
where the relation are taken at equal time <math> t </math>, and <math> \vec x </math> and <math> \vec y </math> are the space variables. The equal-time requirement imposes that <math> \vec x - \vec y </math> is a [[Spacetime#Spacetime_interval|spacelike]] quantity. The non-zero value of the commutator <math>[\phi(t, \vec x),\pi(t, \vec y)</math> expresses the fact that when <math> \phi </math> and <math> \pi </math> are separated by a spacelike distance cannot communicate with each other and thus commute, except when the separation <math> \vec x - \vec y \to 0</math>.<ref>{{cite book | last=Carroll | first=Sean | title=Spacetime and Geometry: An Introduction to General Relativity | publisher=Addison Wesley | year=2003 | isbn=0-8053-8732-3 | edition=Reprinted 2019 }}</ref>
This is not true however in the Light-Front form where the Light-Front time <math> x^+ \equiv t-z </math> is along the light cone and therefore, fields at equal time <math> x^+ </math> are causally linked. Indeed, the Light-Front canonical commutation relations are different, for instance:<ref>{{cite book | last=Harindranath | first=A. | title=An Introduction to Light Front Dynamics for Pedestrians; In Light-Front Quantization and Non-Perturbative QCD | editor-last1=Vary | editor-first1=J.P. | editor-last2=Wolz | editor-first2=F. | publisher=International Institute of Theoretical and Applied Physics | ___location=Ames, IA | year=2000 | isbn=1-891815-00-8 | arxiv=hep-ph/9612244 }}</ref>
<math display="block">{\rm Light-Front~form:}~~[\phi(x^+, \vec x),\phi(x^+, \vec y)] = \frac{i}{4}\epsilon(x^- -y^-)\delta^2( \vec{x_\bot} - \vec{y_\bot}).</math>
where <math>\epsilon(x)=\theta(x)-\theta(-x)</math> is the antisymmetric [[Heaviside step function]].
On the other hand, the commutation relations for the [[creation and annihilation operators]] are similar for both the Instant and Light-Front forms:
<math display="block">{\rm Instant~Form:}~~[a(t, \vec k),a(t, \vec l)] = 0, \ \ [a^\dagger(t, \vec k),a^\dagger(t, \vec l)] = 0, \ \ [a(t, \vec k),a^\dagger(t, \vec l)]= \hbar \delta^3( \vec k- \vec l).</math>
<math display="block">{\rm Light-Front~form:}~~[a(x^+, \vec k),a(x^+, \vec l)] = 0, \ \ [a^\dagger(x^+, \vec k),a^\dagger(x^+, \vec l)] = 0, \ \ [a(x^+, \vec k),a^\dagger(x^+, \vec l)]= \hbar \delta(k^+-l^+) \delta^2( \vec{k_\bot}- \vec{l_\bot}).</math>
where <math> \vec k</math> and <math> \vec l</math> are the [[Wave vector|wavevectors]] of the fields, <math> k^+ = k_0 + k_3 </math> and <math> l^+ = l_0 + l_3 </math>.
=== Light-front boosts ===
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