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Canonical Phase Space Constraints

Physical scope

Classical canonical phase-space form and first-class metric constraints of the flat tensile Polyakov string after conformal gauge fixing; no quantization claim.

Statement

For the adopted flat Polyakov model in conformal gauge hab=diag⁡(−1,+1), the Lagrangian density is L=T2(X˙2−X′2), the canonical momentum is Πμ=TX˙μ, and the Hamiltonian density is H=12TΠ2+T2X′2. The metric equation is equivalent to the constraints D(σ)=Π⋅X′=0 and G(σ)=Π2+T2X′2=0. On J=R/(2πZ) for a closed string or J=(0,π) for the open-string interior, use the canonical phase-space bracket specified in the Facts & Assumptions below. For smooth smearings f,g that are periodic on the circle or compactly supported in the open-string interior, the brackets of D[f]=∫JfD dσ and G[g]=∫JgG dσ are {D[f],D[g]}=D[fg′−gf′], {D[f],G[g]}=G[fg′−gf′], and {G[f],G[g]}=4T2D[fg′−gf′]. Their closure as linear combinations of the constraints is the meaning of first-class here.

Facts & Assumptions

Given: A smooth equal-time string configuration X(σ), momentum density Πμ(σ), the flat-target metric η, the adopted Polyakov action, and conformal gauge hab=diag⁡(−1,+1).

[F1]

Flat Bosonic String Model: X takes values in R1,D−1 with η=diag⁡(−1,+1,…,+1); τ,σ are dimensionless.

[F2]

String Tension Alpha Prime: T=(2πα′)−1>0.

[A1]

Bosonic Polyakov Model: SP=−T2∫−h hab∂aX⋅∂bX d2σ.

[F3]

Conformal Gauge and Residual Symmetry: a local classical gauge choice permits hab=e2ϕdiag⁡(−1,+1), and Weyl gauge sets ϕ=0.

[F4]

Polyakov Equations and Virasoro Constraints: the metric equation gives T00=12(X˙2+X′2)=0 and T01=X˙⋅X′=0.

[F5]

Canonical Phase Space for the Flat String: the canonical bracket is {F,G}=∫J(δF/δXμ δG/δΠμ−δF/δΠμ δG/δXμ) dσ, with periodic smearings on the circle and compactly supported smearings on the open-string interior.

Proof

1.1givenF1A1F3algebra

For the flat target and its mostly-plus metric [F1], substituting hab=diag⁡(−1,+1) into [A1] gives L=−T2(−X˙2+X′2)=T2(X˙2−X′2).

1.2F5algebra

For smooth smearings f,g of the boundary type in [F5], set D[f]=∫Jf Π⋅X′ dσ and G[g]=∫Jg(Π2+T2X′2) dσ. Their variational derivatives are δD[f]/δX=−(fΠ)′, δD[f]/δΠ=fX′, δG[g]/δX=−2T2(gX′)′, and δG[g]/δΠ=2gΠ, by differentiating the integrands and integrating by parts with no boundary term.

2.1F2step 1.1algebra

Differentiating this density with respect to X˙μ gives Πμ=∂L/∂X˙μ=TX˙μ. Since T>0, X˙μ=Πμ/T.

3.1step 1.1step 2.1algebra

The Legendre transform is H=ΠμX˙μ−L=TX˙2−T2(X˙2−X′2)=T2(X˙2+X′2). Substituting step 2.1 yields H=12TΠ2+T2X′2.

3.2F2F4step 2.1algebra

Multiplying the two metric constraints in [F4] by T and T2, respectively, and using step 2.1 gives D=Π⋅X′=TX˙⋅X′=0 and G=Π2+T2X′2=T2(X˙2+X′2)=0. Conversely, because T>0, these canonical constraints imply both metric constraints.

4.1F5step 1.2algebra∎

Substituting the derivatives from step 1.2 into the bracket, the first constraint bracket is {D[f],D[g]}=∫J(−(fΠ)′⋅gX′+fX′⋅(gΠ)′)dσ=∫J(fg′−gf′)Π⋅X′ dσ, because the Π′⋅X′ terms cancel. For the mixed bracket, expand the derivatives: −2gΠ⋅(fΠ)′=−2gf′Π2−fg(Π2)′,2T2fX′⋅(gX′)′=2T2fg′X′2+T2fg(X′2)′. Integration by parts on the total-derivative terms, with no endpoint term by [F5], makes their sum (fg′−gf′)(Π2+T2X′2), hence {D[f],G[g]}=G[fg′−gf′]. Finally, {G[f],G[g]}=4T2∫J(−gΠ⋅(fX′)′+fΠ⋅(gX′)′)dσ=4T2D[fg′−gf′], since the terms proportional to fg Π⋅X′′ cancel. Thus all three brackets are smearings of the constraints, which is first-class closure by definition here.

Depends on

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