Squark: It is often stated that in quantum gravity only charges coupled to gauge fields can be conserved. This is because of the no hair theorem. If a charge is coupled to a gauge field then when it falls into a black hole the black hole acquires a corresponding field. However if it is not then the black hole doesn't "remember" it. Apparently it implies we can't have exact global symmetries, only gauge symmetries.
How is this expectation realized in string theory? Is it true string vacuum sectors cannot posses global symmetry? Can we prove it?
We can prove it in perturbative string theory but it's probably valid beyond it.
In perturbative string theory, any (continuous) global symmetry has to be associated with a conserved charge which, because of the locality of the physics on the world sheet, implies the existence of a world sheet current \(j\) or \(\bar j\) or both (left movers vs right movers) whose left/right dimension is \((1,0)\) or \((0,1)\) or both (because its integral has to be a conformally invariant charge). Typically, such a symmetry might be something like the isometry of the target (spacetime) manifold or something on equal footing with it.
It follows that one may also construct operators
\[j\exp(ik\cdot X) \bar\partial X^\mu \quad{\rm or}\quad \bar j \exp(ik\cdot X) \partial X^\mu\] or both with a null vector \(k\) which have the dimension \((1,1)\), transform as spacetime vectors, and therefore belong to the spectrum of vertex operators of physical states which moreover transform as spacetime vectors, i.e. they're the gauge bosons (the \(X^\mu\) only go over the large spacetime coordinates). One would need to prove that the multiplication of the operators doesn't spoil their tensor character but it usually holds.
Consequently, any wouldbe global symmetry may automatically be shown to be a gauge symmetry as well.
The argument above only holds for the gauge symmetries that transform things nontrivially in the bulk of the world sheet. But even symmetries acting on the boundary degrees of freedom, i.e. the ChanPaton factors, obey the same requirement because one may also construct (open string) vertex operators for the corresponding group that transform properly.
We don't have a universal nonperturbative definition of string theory but it's likely that the conclusion holds nonperturbatively, too.
In some moral sense, it holds for discrete symmetries as well even though discrete symmetries don't allow gauge bosons.
Ask Ethan: Does Dark Energy Mean We’re Losing Information About The Universe? (Synopsis)

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snail feedback (1) :
the relativistics transformation of spacetime given by lorentz's transformations are intrically associated by the stronger violations of cpto weak interactions or deeply by pt symmetry breaking;thence generating the speed of light as constant and the invariance of lorentz's transformations,or the conatancy of speed of light is vinculated by violation of pt,and the variation of space and time to relativistic velocities
are symmetries to the observers.
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