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Physlib.Electromagnetism.Kinematics.FieldStrength

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definition

Field strength tensor FμνF^{\mu\nu} from electromagnetic potential AA

#toFieldStrength

Given an electromagnetic potential AA defined on a (1+d)(1+d)-dimensional spacetime, the function `toFieldStrength` assigns to AA its corresponding field strength tensor field FF (also known as the Faraday tensor). At each point xx in spacetime, the value F(x)F(x) is an element of the tensor product of two Lorentz vector spaces VVV \otimes V. The tensor is defined as the antisymmetrized derivative of the potential: \[ F^{\mu\nu} = \partial^\mu A^\nu - \partial^\nu A^\mu \] This is constructed by taking the difference between the derivative of the potential contracted with the Minkowski metric η\eta and its transpose (the same term with indices swapped).

theorem

The field strength tensor FμνF^{\mu\nu} equals ημρρAνηνρρAμ\eta^{\mu\rho} \partial_\rho A^\nu - \eta^{\nu\rho} \partial_\rho A^\mu

#toFieldStrength_eq_add

For an electromagnetic potential AA in (1+d)(1+d)-dimensional spacetime, the value of the field strength tensor FF at a point xx is given by the difference: Fμν(x)=ημρρAν(x)ηνρρAμ(x)F^{\mu\nu}(x) = \eta^{\mu\rho} \partial_\rho A^\nu(x) - \eta^{\nu\rho} \partial_\rho A^\mu(x) where η\eta is the Minkowski metric and ρAν\partial_\rho A^\nu denotes the derivative of the potential. In the formal expression, this is represented by taking the tensor Tμν=ημρρAνT^{\mu\nu} = \eta^{\mu\rho} \partial_\rho A^\nu and subtracting its index-permuted version TνμT^{\nu\mu}.

theorem

The Tensor Representation of FμνF^{\mu\nu} equals ημρρAνηνρρAμ\eta^{\mu\rho} \partial_\rho A^\nu - \eta^{\nu\rho} \partial_\rho A^\mu

#toTensor_toFieldStrength

For an electromagnetic potential AA on a (1+d)(1+d)-dimensional spacetime and a point xx in spacetime, the representation of the field strength tensor F(x)F(x) (the Faraday tensor) in the formal tensor index notation is equal to the antisymmetrized raised-index derivative of the potential: toTensor(F(x))=TμνTνμ\text{toTensor}(F(x)) = T^{\mu\nu} - T^{\nu\mu} where Tμν=ημρρAν(x)T^{\mu\nu} = \eta^{\mu\rho} \partial_\rho A^\nu(x) is the tensor constructed by contracting the Minkowski metric η\eta with the derivative of the potential A\partial A at xx. Here, toTensor\text{toTensor} denotes the linear equivalence between the physical tensor product space and the formal index-based tensor species representation.

theorem

Component Formula for the Field Strength Tensor Fμν=ημκκAνηνκκAμF^{\mu\nu} = \eta^{\mu\kappa} \partial_\kappa A^\nu - \eta^{\nu\kappa} \partial_\kappa A^\mu

#toTensor_toFieldStrength_basis_repr

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and FF be the associated field strength tensor (Faraday tensor). For any point xx in spacetime and any multi-index b=(b0,b1)b = (b_0, b_1) corresponding to the contravariant components of a rank-2 tensor, the component of the field strength tensor FF at index bb in the canonical basis is given by the formula: [F(x)]μν=κ(ημκκAν(x)ηνκκAμ(x)) [F(x)]^{\mu\nu} = \sum_{\kappa} \left( \eta^{\mu\kappa} \partial_\kappa A^\nu(x) - \eta^{\nu\kappa} \partial_\kappa A^\mu(x) \right) where μ\mu and ν\nu are the spacetime indices in Fin 1Fin d\text{Fin } 1 \oplus \text{Fin } d corresponding to the multi-index components b0b_0 and b1b_1, η\eta denotes the Minkowski metric, and κAν(x)\partial_\kappa A^\nu(x) is the partial derivative of the ν\nu-th component of the potential with respect to the κ\kappa-th coordinate at point xx.

theorem

Basis Representation Equivalence for the Field Strength Tensor FμνF^{\mu\nu}

#toFieldStrength_tensor_basis_eq_basis

Let dd be the number of spatial dimensions. For an electromagnetic potential AA and a point xx in spacetime, let F(x)F(x) be the field strength tensor (Faraday tensor) at xx, which is an element of the tensor product space VVV \otimes V of Lorentz vectors. Let Btensor\mathcal{B}_{\text{tensor}} be the canonical basis for the formal (2,0)(2,0)-tensor space of the species `realLorentzTensor d`, and let BVBV\mathcal{B}_V \otimes \mathcal{B}_V be the basis for VVV \otimes V induced by the standard Lorentz vector basis BV\mathcal{B}_V. For any multi-index b=(b0,b1)b = (b_0, b_1) in the component index set of the formal tensor, the coordinate of F(x)F(x) in the formal tensor basis at bb is equal to the coordinate of F(x)F(x) in the tensor product basis at the index pair (μ,ν)(\mu, \nu), where μ\mu and ν\nu are the Lorentz indices in Fin 1Fin d\text{Fin } 1 \oplus \text{Fin } d corresponding to b0b_0 and b1b_1 via the standard index equivalence.

theorem

Component Formula for Fμν=κ(ημκκAνηνκκAμ)F^{\mu\nu} = \sum_{\kappa} (\eta^{\mu\kappa} \partial_\kappa A^\nu - \eta^{\nu\kappa} \partial_\kappa A^\mu)

#toFieldStrength_basis_repr_apply

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and F(x)F(x) be the field strength tensor (Faraday tensor) at a point xx. For any pair of spacetime indices (μ,ν)(\mu, \nu), the component of the field strength tensor in the standard tensor product basis is given by: [F(x)]μν=κ(ημκκAν(x)ηνκκAμ(x)) [F(x)]^{\mu\nu} = \sum_{\kappa} \left( \eta^{\mu\kappa} \partial_\kappa A^\nu(x) - \eta^{\nu\kappa} \partial_\kappa A^\mu(x) \right) where η\eta is the Minkowski metric and κAν(x)\partial_\kappa A^\nu(x) denotes the partial derivative of the ν\nu-th component of the potential with respect to the κ\kappa-th coordinate at point xx.

theorem

Fμν=ημμμAνηνννAμF^{\mu\nu} = \eta^{\mu\mu} \partial_\mu A^\nu - \eta^{\nu\nu} \partial_\nu A^\mu

#toFieldStrength_basis_repr_apply_eq_single

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and xx be a point in spacetime. For any pair of spacetime indices (μ,ν)(\mu, \nu), the (μ,ν)(\mu, \nu)-th component of the field strength tensor FF at xx is given by: Fμν(x)=ημμμAν(x)ηνννAμ(x) F^{\mu\nu}(x) = \eta^{\mu\mu} \partial_\mu A^\nu(x) - \eta^{\nu\nu} \partial_\nu A^\mu(x) where η\eta is the Minkowski metric, and μAν(x)\partial_\mu A^\nu(x) denotes the partial derivative of the ν\nu-th component of the potential with respect to the μ\mu-th coordinate evaluated at point xx.

abbrev

Matrix representation of the field strength tensor FF at xx

#fieldStrengthMatrix

Given an electromagnetic potential AA defined on a (1+d)(1+d)-dimensional spacetime and a point xx in that spacetime, the function `fieldStrengthMatrix` returns the components of the field strength tensor FF (the Faraday tensor) at xx as a matrix. This matrix is obtained by taking the coordinate representation of the tensor F(x)=μAννAμF(x) = \partial^\mu A^\nu - \partial^\nu A^\mu with respect to the standard basis of the spacetime, where indices μ,ν\mu, \nu range over the temporal and spatial components {0,1,,d}\{0, 1, \dots, d\}.

theorem

Field Strength Matrix equals Tensor Representation

#fieldStrengthMatrix_eq

For an electromagnetic potential AA in a spacetime with dd spatial dimensions, the field strength matrix at a point xx is equal to the coordinate representation of the field strength tensor F(x)F(x) with respect to the basis formed by the tensor product of the standard Lorentz covector basis and the standard Lorentz vector basis.

theorem

The (μ,ν)(\mu, \nu)-th entry of the field strength matrix equals the (μ,ν)(\mu, \nu)-th component in the canonical tensor basis

#fieldStrengthMatrix_eq_tensor_basis_repr

For a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and xx be a point in spacetime. Let F(x)F(x) be the field strength tensor (Faraday tensor) at xx, and let F(x)\mathbf{F}(x) be its matrix representation (the field strength matrix). For any indices μ,ν{0,1,,d}\mu, \nu \in \{0, 1, \dots, d\}, the (μ,ν)(\mu, \nu)-th entry of the field strength matrix is equal to the coordinate of the formal tensor representation of F(x)F(x) with respect to the canonical tensor basis at the corresponding multi-index (μ,ν)(\mu, \nu).

theorem

F=μ,νFμν(eμeν)F = \sum_{\mu, \nu} F_{\mu\nu} (e_\mu \otimes e_\nu)

#toFieldStrength_eq_fieldStrengthMatrix

For an electromagnetic potential AA defined on a (1+d)(1+d)-dimensional spacetime, the field strength tensor FF (the Faraday tensor) at a point xx can be expressed as a linear combination of the standard basis tensors eμeνe_\mu \otimes e_\nu. Specifically, the tensor is given by: \[ F(x) = \sum_{\mu} \sum_{\nu} F_{\mu\nu}(x) (e_\mu \otimes e_\nu) \] where Fμν(x)F_{\mu\nu}(x) are the components of the field strength matrix at xx, and {eμ}\{e_\mu\} is the standard basis for the space of Lorentz vectors Vectord\text{Vector}_d.

theorem

If Electromagnetic Potential AA is C2C^2, then its Field Strength Matrix is Differentiable

#fieldStrengthMatrix_differentiable

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential. If AA is twice continuously differentiable (C2C^2), then for any pair of spacetime indices μ\mu and ν\nu, the (μ,ν)(\mu, \nu)-th component of the field strength matrix FμνF_{\mu\nu} (representing the Faraday tensor) is a differentiable function with respect to the spacetime coordinates.

theorem

If Electromagnetic Potential AA is C2C^2, then its Field Strength Tensor is Differentiable

#toFieldStrength_differentiable

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential. If AA is twice continuously differentiable (C2C^2), then the associated field strength tensor field FF (the Faraday tensor) is differentiable with respect to the spacetime coordinates.

theorem

Spatial Differentiability of the Field Strength Matrix for C2C^2 Electromagnetic Potentials

#fieldStrengthMatrix_differentiable_space

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential. If AA is twice continuously differentiable (of class C2C^2), then for any fixed time tt, speed of light cc, and indices μ,ν{0,1,,d}\mu, \nu \in \{0, 1, \dots, d\}, the (μ,ν)(\mu, \nu)-th component of the field strength matrix FμνF_{\mu\nu} is differentiable with respect to the spatial coordinates xx. Specifically, the function xFμν(t,x)x \mapsto F_{\mu\nu}(t, x), where (t,x)(t, x) is the spacetime point reconstructed from time tt and spatial position xx, is differentiable.

theorem

The Field Strength Matrix of a C2C^2 Potential is Differentiable in Time

#fieldStrengthMatrix_differentiable_time

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential that is twice continuously differentiable (of class C2C^2). For any fixed spatial point xx, speed of light cc, and any pair of spacetime indices μ\mu and ν\nu, the (μ,ν)(\mu, \nu)-th component of the field strength matrix FμνF_{\mu\nu} evaluated at the spacetime point corresponding to time tt and position xx is a differentiable function with respect to time tt.

theorem

Continuous Differentiability of the Field Strength Matrix

#fieldStrengthMatrix_contDiff

For a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential. If AA is n+1n+1 times continuously differentiable (of class Cn+1C^{n+1}), then for any spacetime indices μ\mu and ν\nu, the (μ,ν)(\mu, \nu)-th component of the field strength matrix of AA, mapping a spacetime point xx to Fμν(x)F^{\mu\nu}(x), is nn times continuously differentiable (of class CnC^n).

theorem

CC^\infty smoothness of the Field Strength Matrix components

#fieldStrengthMatrix_smooth

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential. If AA is infinitely differentiable (of class CC^\infty), then for any spacetime indices μ\mu and ν\nu, the (μ,ν)(\mu, \nu)-th component of the field strength matrix, mapping a spacetime point xx to the value Fμν(x)F_{\mu\nu}(x), is also infinitely differentiable (of class CC^\infty).

theorem

Antisymmetry of the Field Strength Tensor FμνF^{\mu\nu}

#toFieldStrength_antisymmetric

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and FF be the associated field strength tensor (Faraday tensor) defined by Fμν=μAννAμF^{\mu\nu} = \partial^\mu A^\nu - \partial^\nu A^\mu. For any point xx in spacetime and any indices μ\mu and ν\nu representing spacetime coordinates, the field strength tensor is antisymmetric: Fμν(x)=Fνμ(x) F^{\mu\nu}(x) = -F^{\nu\mu}(x)

theorem

Antisymmetry of the Field Strength Matrix FμνF_{\mu\nu}

#fieldStrengthMatrix_antisymm

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and F(x)F(x) be the field strength matrix (Faraday matrix) at a point xx. For any spacetime indices μ\mu and ν\nu, the components of the field strength matrix satisfy: F(x)μν=F(x)νμ F(x)_{\mu\nu} = -F(x)_{\nu\mu}

theorem

Diagonal components of the field strength matrix are zero (Fμμ=0F_{\mu\mu} = 0)

#fieldStrengthMatrix_diag_eq_zero

In a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and F(x)F(x) be its corresponding field strength matrix (Faraday matrix) at a point xx. For any spacetime index μ{0,1,,d}\mu \in \{0, 1, \dots, d\}, the diagonal entry of the field strength matrix is zero: Fμμ(x)=0 F_{\mu\mu}(x) = 0

theorem

Lorentz Equivariance of the Field Strength Tensor FμνF^{\mu\nu}

#toFieldStrength_equivariant

Let dd be a natural number representing spatial dimensions. Let AA be a differentiable electromagnetic potential field on a (1+d)(1+d)-dimensional spacetime. For any Lorentz transformation ΛL\Lambda \in \mathcal{L} and any spacetime point xx, let AA' be the Lorentz-transformed potential defined by A(x)=ΛA(Λ1x)A'(x) = \Lambda A(\Lambda^{-1} x). Then the field strength tensor FF (the Faraday tensor) satisfies the following equivariance property: \[ F[A'](x) = \Lambda \cdot F[A](\Lambda^{-1} x) \] where F[A]F[A] denotes the field strength tensor associated with potential AA, and the operation \cdot represents the action of the Lorentz group on a rank-2 tensor.

theorem

Lorentz transformation rule for the components of the field strength matrix FμνF^{\mu\nu}

#fieldStrengthMatrix_equivariant

Let dd be a natural number representing spatial dimensions. Let AA be a differentiable electromagnetic potential field on a (1+d)(1+d)-dimensional spacetime. For any Lorentz transformation Λ\Lambda in the Lorentz group L\mathcal{L} and any spacetime point xx, let AA' be the Lorentz-transformed potential defined by A(x)=ΛA(Λ1x)A'(x) = \Lambda A(\Lambda^{-1} x). Then, for any spacetime indices μ,ν{0,1,,d}\mu, \nu \in \{0, 1, \dots, d\}, the (μ,ν)(\mu, \nu)-th component of the field strength matrix FF associated with AA' at point xx satisfies: \[ F^{\mu\nu}[A'](x) = \sum_{\kappa} \sum_{\rho} \Lambda_{\mu\kappa} \Lambda_{\nu\rho} F^{\kappa\rho}[A](\Lambda^{-1} x) \] where Fκρ[A](Λ1x)F^{\kappa\rho}[A](\Lambda^{-1} x) is the (κ,ρ)(\kappa, \rho)-th component of the field strength matrix associated with AA evaluated at the point Λ1x\Lambda^{-1} x, and Λij\Lambda_{ij} denotes the (i,j)(i, j)-th entry of the matrix representation of the Lorentz transformation.

theorem

F[A1+A2]=F[A1]+F[A2]F[A_1 + A_2] = F[A_1] + F[A_2] (Additivity of the Field Strength Tensor)

#toFieldStrength_add

In a (1+d)(1+d)-dimensional spacetime, let A1A_1 and A2A_2 be two differentiable electromagnetic potentials. For any point xx in spacetime, the field strength tensor FF (also known as the Faraday tensor) satisfies the additivity property: \[ F[A_1 + A_2](x) = F[A_1](x) + F[A_2](x) \] where F[A]F[A] denotes the field strength tensor derived from the potential AA.

theorem

[F(A1+A2)]=[F(A1)]+[F(A2)][F(A_1 + A_2)] = [F(A_1)] + [F(A_2)] for the Field Strength Matrix

#fieldStrengthMatrix_add

In a (1+d)(1+d)-dimensional spacetime, let A1A_1 and A2A_2 be two differentiable electromagnetic potentials. For any point xx in spacetime, the field strength matrix (the coordinate representation of the Faraday tensor Fμν=μAννAμF^{\mu\nu} = \partial^\mu A^\nu - \partial^\nu A^\mu) of the sum A1+A2A_1 + A_2 is equal to the sum of the field strength matrices of A1A_1 and A2A_2. That is: \[ [F(A_1 + A_2)](x) = [F(A_1)](x) + [F(A_2)](x) \] where [F(A)](x)[F(A)](x) denotes the matrix representation of the field strength tensor derived from the potential AA at point xx.

theorem

F[cA]=cF[A]F[c \cdot A] = c \cdot F[A]

#toFieldStrength_smul

For any (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and cRc \in \mathbb{R} be a real scalar. If AA is differentiable, then at any point xx in spacetime, the field strength tensor FF (also known as the Faraday tensor) of the scaled potential cAc \cdot A is equal to cc times the field strength tensor of AA. That is: \[ F[c \cdot A](x) = c \cdot F[A](x) \] where F[A]F[A] denotes the field strength tensor derived from the potential AA via the relation Fμν=μAννAμF^{\mu\nu} = \partial^\mu A^\nu - \partial^\nu A^\mu.

theorem

[F(cA)]=c[F(A)][F(c \cdot A)] = c \cdot [F(A)] for the Field Strength Matrix

#fieldStrengthMatrix_smul

For a (1+d)(1+d)-dimensional spacetime, let AA be an electromagnetic potential and cRc \in \mathbb{R} be a real scalar. If AA is differentiable, then at any point xx in spacetime, the field strength matrix (the coordinate representation of the Faraday tensor Fμν=μAννAμF^{\mu\nu} = \partial^\mu A^\nu - \partial^\nu A^\mu) of the scaled potential cAc \cdot A is equal to cc times the field strength matrix of AA. That is: \[ [F(c \cdot A)](x) = c \cdot [F(A)](x) \] where [F(A)](x)[F(A)](x) denotes the matrix representation of the field strength tensor derived from the potential AA at point xx.

definition

Auxiliary field strength tensor FμνF^{\mu\nu} for a distributional potential AA

#fieldStrengthAux

Given a distributional electromagnetic potential AA in (1+d)(1+d)-dimensional spacetime and a Schwartz test function ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}), this function defines an auxiliary rank-2 contravariant tensor (an element of VectordRVectord\text{Vector}_d \otimes \mathbb{R} \text{Vector}_d). The tensor is constructed by antisymmetrizing the distributional derivative of the potential. Specifically, let ημν\eta^{\mu\nu} be the Minkowski metric and νAμ\partial_\nu A_{\mu'} represent the derivative of the distribution AμA_{\mu'} acting on the test function ϵ\epsilon. The auxiliary field strength tensor FμνF^{\mu\nu} is given by the expression: Fμν=ημμνAμηννμAνF^{\mu\nu} = \eta^{\mu\mu'} \partial_\nu A_{\mu'} - \eta^{\nu\nu'} \partial_\mu A_{\nu'} This represents νAμμAν\partial_\nu A^\mu - \partial_\mu A^\nu in distributional form. As an auxiliary definition, it provides the raw tensor value without additional linearity or continuity structures.

theorem

The Auxiliary Field Strength Tensor FμνF^{\mu\nu} Equals the Difference of Permuted Derivative Tensors

#fieldStrengthAux_eq_add

Let dd be the number of spatial dimensions. For a distributional electromagnetic potential AA and a Schwartz test function ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}), the auxiliary field strength tensor FμνF^{\mu\nu} (denoted by `fieldStrengthAux A ε`) is equal to the difference between the tensor Tμν=ημρνAρT^{\mu\nu} = \eta^{\mu\rho} \partial_\nu A_\rho and its index-permuted version TνμT^{\nu\mu}. Specifically, Fμν=TμνTνμF^{\mu\nu} = T^{\mu\nu} - T^{\nu\mu} where TμνT^{\mu\nu} is the tensor resulting from the contraction of the Minkowski metric η\eta with the distributional derivative of the potential AA evaluated at ϵ\epsilon.

theorem

The Formal Tensor of FμνF^{\mu\nu} Equals the Difference of Permuted Derivative Tensors

#toTensor_fieldStrengthAux

Let dd be the number of spatial dimensions. For a distributional electromagnetic potential AA and a Schwartz test function ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}), the formal tensor representation (given by the map `Tensorial.toTensor`) of the auxiliary field strength FμνF^{\mu\nu} is equal to the difference between the tensor Tμν=ημρνAρT^{\mu\nu} = \eta^{\mu\rho} \partial_\nu A_\rho and its index-permuted version TνμT^{\nu\mu}. Specifically, toTensor(Fμν)=TμνTνμ\text{toTensor}(F^{\mu\nu}) = T^{\mu\nu} - T^{\nu\mu} where TμνT^{\mu\nu} is the tensor resulting from the contraction of the Minkowski metric η\eta with the distributional derivative of the potential AA evaluated at ϵ\epsilon.

theorem

Basis Components of the Auxiliary Field Strength Tensor FμνF^{\mu\nu} as a Sum of Potential Derivatives

#toTensor_fieldStrengthAux_basis_repr

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. Let FμνF^{\mu\nu} be the auxiliary rank-2 contravariant field strength tensor (defined as `fieldStrengthAux A ε`). For a multi-index b=(b0,b1)b = (b_0, b_1) corresponding to spacetime indices μ\mu and ν\nu via the canonical isomorphism ϕ:ComponentIdxFin(1+d)\phi: \text{ComponentIdx} \cong \text{Fin}(1+d), the component of the tensor in the canonical basis is given by: [Fμν]b=κ=0d(ημκκAν[ϵ]ηνκκAμ[ϵ]) [F^{\mu\nu}]_b = \sum_{\kappa=0}^{d} \left( \eta^{\mu\kappa} \partial_\kappa A_\nu[\epsilon] - \eta^{\nu\kappa} \partial_\kappa A_\mu[\epsilon] \right) where ημκ\eta^{\mu\kappa} denotes the components of the Minkowski metric, and κAν[ϵ]\partial_\kappa A_\nu[\epsilon] is the κ\kappa-th distributional derivative of the ν\nu-th component of the potential AA evaluated at the test function ϵ\epsilon.

theorem

Component Equality of Auxiliary Field Strength Tensor in Tensor and Product Bases

#fieldStrengthAux_tensor_basis_eq_basis

Let AA be a distributional electromagnetic potential in a spacetime with dd spatial dimensions and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. Let F=fieldStrengthAux(A,ϵ)F = \text{fieldStrengthAux}(A, \epsilon) be the auxiliary rank-2 contravariant field strength tensor. For any tensor multi-index bb consisting of two contravariant indices, the component of FF (viewed as a formal tensor) in the canonical tensor basis at index bb is equal to the component of FF (viewed as an element of the tensor product of Lorentz vector spaces) in the product basis at indices μ=ϕ(b0)\mu = \phi(b_0) and ν=ϕ(b1)\nu = \phi(b_1), where ϕ\phi is the canonical equivalence between tensor component indices and the spacetime index set Fin 1Fin d\text{Fin } 1 \oplus \text{Fin } d.

theorem

Component Formula for the Auxiliary Field Strength Tensor FμνF^{\mu\nu} as a Sum of Potential Derivatives

#fieldStrengthAux_basis_repr_apply

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. Let FμνF^{\mu\nu} be the auxiliary rank-2 contravariant field strength tensor (defined as `fieldStrengthAux A ε`). For a pair of spacetime indices (μ,ν)(\mu, \nu), the component of this tensor in the product basis of Lorentz vectors is given by: [Fμν]μ,ν=κ=0d(ημκκAν[ϵ]ηνκκAμ[ϵ]) [F^{\mu\nu}]_{\mu, \nu} = \sum_{\kappa=0}^{d} \left( \eta^{\mu\kappa} \partial_\kappa A_\nu[\epsilon] - \eta^{\nu\kappa} \partial_\kappa A_\mu[\epsilon] \right) where η\eta denotes the Minkowski metric and κAν[ϵ]\partial_\kappa A_\nu[\epsilon] represents the κ\kappa-th distributional derivative of the ν\nu-th component of the potential AA evaluated at the test function ϵ\epsilon.

theorem

Component Formula for FμνF^{\mu\nu} using the Diagonal Minkowski Metric

#fieldStrengthAux_basis_repr_apply_eq_single

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. The component of the auxiliary rank-2 contravariant field strength tensor FμνF^{\mu\nu} (defined by `fieldStrengthAux A ε`) at the indices (μ,ν)(\mu, \nu) is given by: [Fμν]μ,ν=ημμμAν[ϵ]ηνννAμ[ϵ] [F^{\mu\nu}]_{\mu,\nu} = \eta^{\mu\mu} \partial_\mu A_\nu[\epsilon] - \eta^{\nu\nu} \partial_\nu A_\mu[\epsilon] where η\eta denotes the Minkowski metric and κAλ[ϵ]\partial_\kappa A_\lambda[\epsilon] represents the κ\kappa-th distributional derivative of the λ\lambda-th component of the potential AA evaluated at the test function ϵ\epsilon. This formula simplifies the general sum over indices by leveraging the fact that the Minkowski metric is diagonal.

theorem

Basis Expansion of the Auxiliary Field Strength Tensor FμνF^{\mu\nu}

#fieldStrengthAux_eq_basis

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. The auxiliary rank-2 contravariant field strength tensor Faux(A,ϵ)F_{\text{aux}}(A, \epsilon) can be expressed as a sum over the tensor products of the standard basis vectors {eμ}\{e_\mu\} and {eν}\{e_\nu\} as follows: Faux(A,ϵ)=μ,ν(ημμμAν[ϵ]ηνννAμ[ϵ])eμeν F_{\text{aux}}(A, \epsilon) = \sum_{\mu, \nu} \left( \eta^{\mu\mu} \partial_\mu A_\nu[\epsilon] - \eta^{\nu\nu} \partial_\nu A_\mu[\epsilon] \right) e_\mu \otimes e_\nu where ημμ\eta^{\mu\mu} are the diagonal components of the Minkowski metric and κAλ[ϵ]\partial_\kappa A_\lambda[\epsilon] denotes the κ\kappa-th distributional derivative of the λ\lambda-th component of the potential AA evaluated at the test function ϵ\epsilon.

definition

Field strength tensor FμνF^{\mu\nu} of a distributional potential AA

#fieldStrength

For a given spatial dimension dd, the field strength FF is a linear map that associates a distributional electromagnetic potential AA with a rank-2 contravariant tensor distribution. Specifically, for a distributional potential AA, the field strength F(A)F(A) is a distribution that maps a Schwartz test function ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) to a rank-2 tensor in VectordVectord\text{Vector}_d \otimes \text{Vector}_d. The components of this tensor are given by: (F(A)(ϵ))μν=ημμ(νAμ)(ϵ)ηνν(μAν)(ϵ) (F(A)(\epsilon))^{\mu\nu} = \eta^{\mu\mu} (\partial_\nu A_\mu)(\epsilon) - \eta^{\nu\nu} (\partial_\mu A_\nu)(\epsilon) where η\eta denotes the Minkowski metric and (νAμ)(ϵ)(\partial_\nu A_\mu)(\epsilon) represents the distributional derivative of the μ\mu-th component of the potential acting on the test function ϵ\epsilon. The map AF(A)A \mapsto F(A) is linear, and for each AA, the map ϵF(A)(ϵ)\epsilon \mapsto F(A)(\epsilon) is linear and continuous.

theorem

Equality of distributional field strength and auxiliary field strength tensor

#fieldStrength_eq_fieldStrengthAux

For a distributional electromagnetic potential AA in (1+d)(1+d)-dimensional spacetime and a Schwartz test function ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}), the evaluation of the field strength tensor distribution F(A)F(A) at ϵ\epsilon is equal to the auxiliary field strength tensor Faux(A,ϵ)F_{\text{aux}}(A, \epsilon) defined by the antisymmetrized distributional derivatives of the potential.

theorem

Basis Expansion of the Distributional Field Strength Tensor FμνF^{\mu\nu}

#fieldStrength_eq_basis

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. The field strength tensor distribution F(A)F(A) evaluated at ϵ\epsilon is given by the basis expansion: F(A)(ϵ)=μ,ν(ημμ(μAν)(ϵ)ηνν(νAμ)(ϵ))eμeν F(A)(\epsilon) = \sum_{\mu, \nu} \left( \eta^{\mu\mu} (\partial_\mu A_\nu)(\epsilon) - \eta^{\nu\nu} (\partial_\nu A_\mu)(\epsilon) \right) e_\mu \otimes e_\nu where {eμ}\{e_\mu\} is the standard basis for the space of Lorentz vectors, ημμ\eta^{\mu\mu} are the diagonal components of the Minkowski metric, and (κAλ)(ϵ)(\partial_\kappa A_\lambda)(\epsilon) denotes the κ\kappa-th distributional derivative of the λ\lambda-th component of the potential AA acting on the test function ϵ\epsilon.

theorem

Component formula (F(A)(ϵ))μν=ημμ(μAν)(ϵ)ηνν(νAμ)(ϵ)(F(A)(\epsilon))^{\mu\nu} = \eta^{\mu\mu} (\partial_\mu A_\nu)(\epsilon) - \eta^{\nu\nu} (\partial_\nu A_\mu)(\epsilon)

#fieldStrength_basis_repr_eq_single

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. The (μ,ν)(\mu, \nu) component of the rank-2 field strength tensor distribution F(A)F(A) evaluated at ϵ\epsilon is given by: (F(A)(ϵ))μν=ημμ(μAν)(ϵ)ηνν(νAμ)(ϵ) (F(A)(\epsilon))^{\mu\nu} = \eta^{\mu\mu} (\partial_\mu A_\nu)(\epsilon) - \eta^{\nu\nu} (\partial_\nu A_\mu)(\epsilon) where η\eta denotes the diagonal Minkowski metric and (κAλ)(ϵ)(\partial_\kappa A_\lambda)(\epsilon) represents the κ\kappa-th distributional derivative of the λ\lambda-th component of the potential AA acting on the test function ϵ\epsilon.

theorem

Diagonal components of the field strength tensor (F(A)(ϵ))μμ=0(F(A)(\epsilon))^{\mu\mu} = 0

#fieldStrength_diag_zero

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. The diagonal components of the rank-2 field strength tensor distribution F(A)F(A) evaluated at ϵ\epsilon are zero. That is, for any spacetime index μ\mu: (F(A)(ϵ))μμ=0 (F(A)(\epsilon))^{\mu\mu} = 0 where (F(A)(ϵ))μμ(F(A)(\epsilon))^{\mu\mu} denotes the (μ,μ)(\mu, \mu) component of the tensor in the standard coordinate basis.

theorem

Diagonal Components of the Derivative of the Distributional Field Strength Tensor are Zero: (νF)μμ=0(\partial_\nu F)^{\mu\mu} = 0

#distDeriv_fieldStrength_diag_zero

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. For any spacetime indices μ\mu and ν\nu in the standard coordinate basis, the diagonal components of the ν\nu-th distributional derivative of the field strength tensor F(A)F(A) evaluated at ϵ\epsilon are zero. That is: (νF(A)(ϵ))μμ=0 (\partial_\nu F(A)(\epsilon))^{\mu\mu} = 0 where (νF(A)(ϵ))μμ(\partial_\nu F(A)(\epsilon))^{\mu\mu} denotes the (μ,μ)(\mu, \mu) component of the tensor-valued distribution derivative in the standard basis.

theorem

Antisymmetry of the Distributional Field Strength Tensor: Fμν=FνμF^{\mu\nu} = -F^{\nu\mu}

#fieldStrength_antisymmetric_basis

In (1+d)(1+d)-dimensional spacetime, let AA be a distributional electromagnetic potential and ϵS(R1+d,R)\epsilon \in \mathcal{S}(\mathbb{R}^{1+d}, \mathbb{R}) be a Schwartz test function. Let F(A)(ϵ)F(A)(\epsilon) be the rank-2 field strength tensor evaluated at ϵ\epsilon. For any spacetime indices μ\mu and ν\nu in the standard basis, the components of the tensor satisfy the antisymmetry property: (F(A)(ϵ))μν=(F(A)(ϵ))νμ (F(A)(\epsilon))^{\mu\nu} = -(F(A)(\epsilon))^{\nu\mu}

theorem

Lorentz Equivariance of the Field Strength Tensor: F(ΛA)=ΛF(A)F(\Lambda \cdot A) = \Lambda \cdot F(A)

#fieldStrength_equivariant

For a given number of spatial dimensions dd, let AA be a distributional electromagnetic potential and Λ\Lambda be an element of the Lorentz group L\mathcal{L}. The operation of taking the field strength tensor FF is equivariant under the Lorentz group action, meaning: F(ΛA)=ΛF(A) F(\Lambda \cdot A) = \Lambda \cdot F(A) In other words, the field strength of a Lorentz-transformed potential is equal to the Lorentz transformation of the field strength of the original potential.