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Worked current boost law and coordinate versus rest density

Example

Worked computation. In an inertial frame with coordinates (x0,x1,x2,x3) consider the constant vector field U with components (U0,U1,U2,U3)=(5/4,3/4,0,0) on an open set Ω (C^k tensor fields on affine Minkowski space and their inertial components), so that η0(U,U)=−25/16+9/16=−1 with U0>∣Uvec∣=3/4; U is a future timelike unit vector field. Let ρ0 be a constant scalar field and put J:=ρ0U. Choose the primed inertial chart x′=Bxx, where Bx is the matrix of B(35e1); it exists by Inertial charts are related by proper orthochronous affine Lorentz maps, and every such map is a chart transition. Thus the primed frame basis has e′a=B(−35e1)ea and the primed component columns are [U]x′=Bx[U]x, [J]x′=Bx[J]x; these are changes of components of the same fields. Then:

(i) J has components (J0,J1)=(5ρ0/4,3ρ0/4), and ∂aJa=0 because the components are constant; the continuity equation of Transformation of tensor, covector, derivative and current components under affine Lorentz maps reads ∂0J0+∂1J1=0 identically.

(ii) Under the passive boost B=B(35e1) one computes BU=e0: the first component of BU is (5/4)(5/4−(3/5)(3/4))=(5/4)(16/20)=1, the second is (5/4)(3/4−(3/5)(5/4))=(5/4)⋅0=0, and the transverse components vanish. Correspondingly [J]x′=(ρ0,0,0,0): in the primed chart the field J has those components. Hence the coordinate density J0 is 5ρ0/4 in the original chart and ρ0 in the chart in which U=e′0, while the scalar multiplier ρ0 is the same in both; this is the worked instance of clause (vii) of Transformation of tensor, covector, derivative and current components under affine Lorentz maps.

(iii) The general boost law of clause (vi) of Transformation of tensor, covector, derivative and current components under affine Lorentz maps reproduces (ii): J′0=γ(J0−βJ1)=(5/4)(5ρ0/4−(3/5)(3ρ0/4))=(5/4)(16ρ0/20)=ρ0 and J′1=γ(J1−βJ0)=(5/4)(3ρ0/4−(3/5)(5ρ0/4))=0.

The example carries no physical charge assignment: it illustrates the transformation of a specified vector field, and the identification of J with a physical current is a model premise of the electromagnetic framework.

Facts & Assumptions

Given: An inertial frame with coordinates (x0,x1,x2,x3) and η0=diag⁡(−1,1,1,1), an open set Ω, the constant future unit timelike vector field U with components (5/4,3/4,0,0), a constant scalar field ρ0, and J:=ρ0U.

[L1]

A Ck vector field on Ω is a map assigning to each point a vector, with components in the inertial chart; for a constant field all partial derivatives of the components vanish, and for a C1 vector field J in an inertial chart the continuity equation ∂aJa=0 is equivalent to ∂0J0+∑i∂iJi=0; under the passive boost with parameter β, Λ=B(β), the components transform as J′0=γ(J0−β⋅Jcov) and Jcov′=Jcov+γ−1∣β∣2(β⋅Jcov)β−γβJ0 (Transformation of tensor, covector, derivative and current components under affine Lorentz maps).

[L2]

For a C1 future timelike vector field U and a C1 scalar field ρ0, J=ρ0U is a C1 vector field with first component J0=ρ0U0; the normalized chart direction u:=U/U0 satisfies u0=1 and ρ:=J0=ρ0U0 is the coordinate density, while ρ0 is the scalar multiplier; ρ=ρ0 holds exactly when ρ0(U0−1)=0, hence at points with ρ0≠0 exactly when U0=1 (Transformation of tensor, covector, derivative and current components under affine Lorentz maps).

[L3]

For β=βe1 with ∣β∣<1 the boost acts by x′0=γ(x0−βx1), x′1=γ(x1−βx0) on the (e0,e1) plane with γ=(1−β2)−1/2, and B(β) is a proper orthochronous Lorentz map (The passive Lorentz boost with parameter beta, The passive boost is a proper orthochronous Lorentz map with inverse B(-beta)).

[L4]

In the frame, e0∈C+ and the coordinates identify the timelike sets: T+={v:v0>∣vvec∣}, T−={v:v0<−∣vvec∣}, with C+=T+; a nonzero vector with η(v,v)<0 is timelike, and η(v,v)=−(v0)2+(v1)2+(v2)2+(v3)2 (Inertial frames and inertial charts of the affine Minkowski model, The timelike set has exactly two convex components, described in orthonormal coordinates, Time orientation, future and past timelike/null/causal vectors, and the closed cones, Timelike, null, spacelike and causal vectors of a Lorentz vector space).

Proof

technique · direct
1.1L1L2L4algebra

By [L4], η0(U,U)=−(5/4)2+(3/4)2=(−25+9)/16=−1, so U is timelike, and U0=5/4>3/4=∣Uvec∣ places U in T+=C+: U is future timelike unit. Since ρ0 is constant, [L2] gives J=ρ0U with components (J0,J1,J2,J3)=ρ0(5/4,3/4,0,0)=(5ρ0/4,3ρ0/4,0,0), and all partial derivatives of the components vanish; the sum ∂0J0+∑i∂iJi is then a sum of vanishing terms, so the continuity equation ∂aJa=0 holds by [L1].

1.2L2L3algebra

By [L3] with β=(3/5,0,0), γ=5/4: the first component of BU is (5/4)(5/4−(3/5)(3/4))=(5/4)(5/4−9/20)=(5/4)(16/20)=1, the second is (5/4)((3/4)−(3/5)(5/4))=(5/4)(3/4−3/4)=0, and the transverse components of U vanish, so BU=e0. By linearity BJ=ρ0e0 as an active image in V, and by the primed-chart component law [L1], [J]x′=Bx[J]x=(ρ0,0,0,0). Likewise [U]x′=(1,0,0,0), so U=e′0 and J=ρ0e′0 intrinsically. This is clause (ii).

2.1L1step 1.1step 1.2algebra

For β=35e1 the general law of [L1] reduces, since J2=J3=0 and β⋅Jcov=35J1, to J′0=γ(J0−35J1) and J′1=γ(J1−35J0) (the correction term (γ−1)J1e1 combining with J1 to give γJ1). Substituting γ=5/4, J0=5ρ0/4 and J1=3ρ0/4: J′0=(5/4)(5ρ0/4−(3/5)(3ρ0/4))=(5/4)(25ρ0/20−9ρ0/20)=(5/4)(16ρ0/20)=ρ0 and J′1=(5/4)(3ρ0/4−(3/5)(5ρ0/4))=(5/4)(3ρ0/4−3ρ0/4)=0, reproducing the components of step 1.2; this is clause (iii).

3.1step 1.1step 1.2step 2.1∎

Clause (i) is step 1.1, clause (ii) is step 1.2, and clause (iii) is step 2.1. Every entry is an exact rational evaluation of the boost matrix on the displayed components, and no physical identification of J with a charge-current is made.

Depends on

Dependency tree · 2 levels

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