# History of Topics in Special Relativity/Four-current

## Overview

The w:Four-current is the four-dimensional analogue of the w:electric current density

${\displaystyle {\begin{matrix}J^{\alpha }&\underbrace {=\left(c\rho ,j^{1},j^{2},j^{3}\right)=\left(c\rho ,\mathbf {j} \right)} &=\rho _{0}U^{\alpha }\\&(a)&(b)\end{matrix}}\left(\rho _{0}=\rho {\sqrt {1-{\frac {v^{2}}{c^{2}}}}}\right)}$

where c is the w:speed of light, ${\displaystyle U^{\alpha }}$ the four-velocity, ρ is the w:charge density, ${\displaystyle \rho _{0}}$ the rest charge density , and j the conventional w:current density. Alternatively, it can be defined in terms of the inhomogeneous Maxwell equations as the negative product of the D'Alembert operator and the electromagnetic potential ${\displaystyle A^{\beta }}$, or the four-divergence of the electromagnetic tensor ${\displaystyle F^{\alpha \beta }}$:

${\displaystyle {\begin{matrix}\mu _{0}J^{\beta }&=-\square A^{\beta }&=\partial _{\alpha }F^{\alpha \beta }\\&(c)&(d)\end{matrix}}}$

and the generally covariant form

${\displaystyle (e)\ J^{\mu }=\partial _{\nu }{\mathcal {D}}^{\mu \nu },\ \left[{\mathcal {D}}^{\mu \nu }\,=\,{\frac {1}{\mu _{0}}}\,g^{\mu \alpha }\,F_{\alpha \beta }\,g^{\beta \nu }\,{\sqrt {-g}}\right]}$

The four-current was already used by #Herglotz (1904) before the advent of relativity, while the Lorentz transformation of its four components was given by #Poincaré (1905/6) and #Marcolongo (1906). It was explicitly formulated in modern form by #Minkowski (1907/15) and reformulated in different notations by #Born (1909), #Bateman (1909/10), #Ignatowski (1910), #Sommerfeld (1910), #Lewis (1910), Wilson/Lewis (1912), #Von Laue (1911), #Silberstein (1911). The generally covariant form was first given by #Kottler (1912) and #Einstein (1913).

## Historical notation

### Herglotz (1904)

w:Gustav Herglotz gave the following solution for the retarded potential in terms of charge density ${\displaystyle \varrho }$:[R 1]

${\displaystyle {\begin{matrix}\Phi ={\frac {c}{4\pi ^{2}i}}\int d\tau \int {\frac {\varrho \left(x',y',z',t-\tau \right)}{R^{2}-c^{2}\tau ^{2}}}dv\\\left[{\frac {\partial ^{2}\varphi }{\partial t^{2}}}-c^{2}\left({\frac {\partial ^{2}\varphi }{\partial t^{2}}}+{\frac {\partial ^{2}\varphi }{\partial t^{2}}}+{\frac {\partial ^{2}\varphi }{\partial t^{2}}}\right)=0,\ R^{2}=(x-x')^{2}+(y-y')^{2}+(z-z')^{2}\right]\end{matrix}}}$

in which ${\displaystyle \varrho \left(x',y',z',t-\tau \right)}$ is the four-current equivalent to (a) while ${\displaystyle \Phi }$ represents the four-potential, which is notable because this solution was given by Herglotz already in 1904 before the spacetime representations of Poincaré and Minkowski. It was also emphasized by Minkowski himself in a conversation reported by Sommerfeld, that the four-dimensional symmetry of electrodynamics is latently contained and mathematically applied in Herglotz' paper.[R 2]

### Poincaré (1905/6)

w:Henri Poincaré (June 1905[R 3]; July 1905, published 1906[R 4]) showed that the four quantities related to charge density ${\displaystyle \rho }$ are connected by a Lorentz transformation:

${\displaystyle {\begin{matrix}\rho ,\ \rho \xi ,\ \rho \eta ,\ \rho \zeta \\\hline \rho ^{\prime }={\frac {k}{l^{3}}}\rho (1+\epsilon \xi ),\quad \rho ^{\prime }\xi ^{\prime }={\frac {k}{l^{3}}}\rho (\xi +\epsilon ),\quad \rho ^{\prime }\eta ^{\prime }={\frac {\rho \eta }{l^{3}}},\ \quad \rho ^{\prime }\zeta ^{\prime }={\frac {\rho \zeta }{l^{3}}}&({\text{June}})\\\rho ^{\prime }={\frac {k}{l^{3}}}(\rho +\epsilon \rho \xi ),\quad \rho '\xi ^{\prime }={\frac {k}{l^{3}}}(\rho \xi +\epsilon \rho ),\quad \rho ^{\prime }\eta ^{\prime }={\frac {1}{l^{3}}}\rho \eta ,\quad \rho ^{\prime }\zeta ^{\prime }={\frac {1}{l^{3}}}\rho \zeta '&({\text{July}})\\\left(k={\frac {1}{\sqrt {1-\epsilon ^{2}}}},\ l=1\right)\end{matrix}}}$

and in his July paper he further stated the continuity equation and the invariance of Jacobian D:[R 5]

${\displaystyle {\begin{matrix}{\frac {d\rho ^{\prime }}{dt^{\prime }}}+\sum {\frac {d\rho ^{\prime }\xi ^{\prime }}{dx^{\prime }}}=0\\D_{1}^{'}={\frac {d\rho ^{\prime }}{dt^{\prime }}}+\sum {\frac {d\rho ^{\prime }\xi ^{\prime }}{dx^{\prime }}}=0,\ D_{1}={\frac {d\rho }{dt}}+\sum {\frac {d\rho \xi }{dx}}=0\end{matrix}}}$

Even though Poincaré didn't directly use four-vector notation in those cases, his quantities are the components of four-current (a).

### Marcolongo (1906)

Following Poincaré, w:Roberto Marcolongo defined the general Lorentz transformation ${\displaystyle \alpha ,\beta ,\gamma ,\delta }$ of the components of the four independent variables ${\displaystyle \mathbf {V} ,\varrho }$ and its continuity equation:[R 6]

${\displaystyle {\begin{matrix}(\xi ,\eta ,\zeta )=\mathbf {V} ,\ (\xi ',\eta ',\zeta ')=\mathbf {V} '\\\hline \varrho '\xi '=\varrho \left(\alpha _{1}\xi +\beta _{1}\eta +\gamma _{1}\zeta -i\delta _{1}\right)\\\dots \\\varrho '=\varrho \left(\alpha _{4}\xi +\beta _{4}\eta +\gamma _{4}\zeta -i\delta _{4}\right)\\\hline {\frac {\partial \varrho '}{\partial t'}}+{\frac {\partial \varrho '\xi '}{\partial x'}}+{\frac {\partial \varrho '\eta '}{\partial y'}}+{\frac {\partial \varrho '\zeta '}{\partial z'}}=0\\(t=iu)\end{matrix}}}$

equivalent to the components of four-current (a), and pointed out its relation to the components ${\displaystyle \mathbf {J} ,\varphi }$ of the four-potential

${\displaystyle {\begin{matrix}\Box \mathbf {J} '_{x}=-4\pi \varrho '\xi '=-4\pi \varrho \left(\alpha _{1}\xi +\beta _{1}\eta +\gamma _{1}\zeta -i\delta _{1}\right),\dots \\\Box \mathbf {J} =-4\pi \varrho \mathbf {V,\ \Box \varphi } =-4\pi \rho ,\ \Box \varphi '=-4\pi \varrho '\end{matrix}}}$

equivalent to the components of Maxwell's equations (b).

### Minkowski (1907/15)

w:Hermann Minkowski from the outset employed vector and matrix representation of four-vectors and six-vectors (i.e. antisymmetric second rank tensors) and their product. In a lecture held in November 1907, published 1915, Minkowski defined the four-current in vacuum with ${\displaystyle \varrho }$ as charge density and ${\displaystyle {\mathfrak {v}}}$ as velocity:[R 7]

${\displaystyle \left(\varrho _{1},\varrho _{2},\varrho _{3},\varrho _{4}\right)=(\varrho {\mathfrak {v}},\ i\varrho )}$

equivalent to (a), and the electric four-current in matter with ${\displaystyle \mathbf {i} }$ as current and ${\displaystyle \sigma }$ as charge density:[R 8]

${\displaystyle (\sigma )=\left(\sigma _{1},\ \sigma _{2},\ \sigma _{3},\ \sigma _{4}\right)=(i_{x},i_{y},i_{z},\ i\sigma )}$

In another lecture from December 1907, Minkowski defined the “space-time vector current” and its Lorentz transformation[R 9]

${\displaystyle {\begin{matrix}\left(\varrho \,{\mathfrak {w}}_{x},\ \varrho \,{\mathfrak {w}}_{y},\ \varrho \,{\mathfrak {w}}_{z},\ i\varrho \right)\Rightarrow \left(\varrho _{1},\ \varrho _{2},\ \varrho _{3},\ \varrho _{4}\right)\\\hline \varrho '_{3}=x_{3}\cos \ i\psi +\varrho _{4}\sin \ i\psi ,\quad \varrho '_{4}=-\varrho _{3}\sin \ i\psi +\varrho _{4}\cos \ i\psi ,\quad \varrho '_{1}=\varrho _{1},\quad \varrho '_{2}=\varrho _{2}\\\varrho '{\mathfrak {w}}'_{z'}=\varrho \left({\frac {{\mathfrak {w}}_{z}-q}{\sqrt {1-q^{2}}}}\right),\quad \varrho '=\varrho \left({\frac {-q\,{\mathfrak {w}}_{z}+1}{\sqrt {1-q^{2}}}}\right),\quad \varrho '{\mathfrak {w}}'_{x'}=\varrho \,{\mathfrak {w}}_{x},\quad \varrho '{\mathfrak {w}}'_{y'}=\varrho \,{\mathfrak {w}}_{y},\\\hline -(\varrho _{1}^{2}+\varrho _{2}^{2}+\varrho _{3}^{2}+\varrho _{4}^{2})=\varrho ^{2}(1-{\mathfrak {w}}_{x}^{2}-{\mathfrak {w}}_{y}^{2}-{\mathfrak {w}}_{z}^{2})=\varrho ^{2}(1-{\mathfrak {w}}^{2})\end{matrix}}}$

equivalent to (a). In moving media and dielectrics, Minkowski more generally used the current density vector “electric current” ${\displaystyle {\mathfrak {s}}}$ which becomes ${\displaystyle {\mathfrak {s}}=\sigma {\mathfrak {E}}}$ in isotropic media:[R 10]

${\displaystyle {\begin{matrix}\left({\mathfrak {s}}_{x},\ {\mathfrak {s}}_{y},\ {\mathfrak {s}}_{z},\ i\varrho \right)\Rightarrow \left(s_{1},\ s_{2},\ s_{3},\ s_{4}\right)\end{matrix}}}$

### Born (1909)

Following Minkowski, w:Max Born (1909) defined the “space-time vector of first kind” (four-vector) and its continuity equation[R 11]

${\displaystyle {\begin{matrix}\left({\frac {\varrho w_{x}}{c}},\ {\frac {\varrho w_{y}}{c}},\ {\frac {\varrho w_{z}}{c}},\ i\varrho \right)\Rightarrow \left(\varrho _{1},\ \varrho _{2},\ \varrho _{3},\ \varrho _{4}\right)\\{\frac {\partial \varrho w_{x}}{\partial x}}+{\frac {\partial \varrho w_{y}}{\partial y}}+{\frac {\partial \varrho w_{z}}{\partial z}}+{\frac {\partial \varrho }{\partial t}}=0\end{matrix}}}$

equivalent to (a), and pointed out its relation to Maxwell's equations as the product of the D'Alembert operator with the electromagnetic potential ${\displaystyle \Phi _{\alpha }}$:

${\displaystyle {\frac {\partial }{\partial x_{\alpha }}}\sum _{\beta =1}^{4}{\frac {\partial \Phi _{\beta }}{\partial x_{\beta }}}-\sum _{\beta =1}^{4}{\frac {\partial ^{2}\Phi _{\alpha }}{\partial x_{\beta }^{2}}}=\varrho _{\alpha }\quad \left(\sum _{\beta =1}^{4}{\frac {\partial \Phi _{\beta }}{\partial x_{\beta }}}=0\right)}$

equivalent to (c). He also expressed the four-current in terms of rest charge density and four-velocity

${\displaystyle {\begin{matrix}\varrho _{1}={\frac {\varrho ^{\ast }}{c}}\cdot {\frac {\partial x}{\partial \tau }},\ \varrho _{2}={\frac {\varrho ^{\ast }}{c}}\cdot {\frac {\partial y}{\partial \tau }},\ \varrho _{3}={\frac {\varrho ^{\ast }}{c}}\cdot {\frac {\partial z}{\partial \tau }},\ \varrho _{4}=i\varrho ^{\ast }{\frac {\partial t}{\partial \tau }}\\\left(\varrho ^{\ast }=\varrho {\sqrt {1-{\frac {w^{2}}{c^{2}}}}}={\sqrt {-\left(\varrho _{1}^{2}+\varrho _{2}^{2}+\varrho _{3}^{2}+\varrho _{4}^{2}\right)}},\ d\tau =dt{\sqrt {1-{\frac {w^{2}}{c^{2}}}}}\right)\\\varrho _{\alpha }=i\varrho ^{\ast }{\frac {\partial x_{\alpha }}{\partial \xi _{4}}}\quad \left(\xi _{4}=ic\tau \right)\end{matrix}}}$

equivalent to (b).

### Bateman (1909/10)

A discussion of four-current in terms of integral forms (even though in the broader context of w:spherical wave transformations), was given by w:Harry Bateman in a paper read 1909 and published 1910, who defined the Lorentz transformations of its components ${\displaystyle \left(\rho w_{x},\rho w_{y},\rho w_{z},\rho \right)}$[R 12]

${\displaystyle \rho w_{x}=\beta (\rho 'w'-v\rho '),\ \rho w_{y}=\rho 'w'_{y},\ \rho w_{z}=\rho 'w'_{z},\ -\rho =\beta (v\rho 'w'_{x}-\rho '),\ \left[\beta ={\frac {1}{\sqrt {1-v^{2}}}}\right]}$

forming the following invariant relations together with the differential four-position and four-potential:[R 13]

${\displaystyle {\begin{matrix}{\frac {1}{\lambda ^{2}}}\left[\rho w_{x}dx+\rho w_{y}dy+\rho w_{z}dz-\rho dt\right]\\{\frac {\rho ^{2}}{\lambda ^{2}}}\left(1-w^{2}\right)dx\ dy\ dz\ dt\\\rho \left[A_{x}w_{x}+A_{y}w_{y}+A_{z}w_{z}-\Phi \right]dx\ dy\ dz\ dt\end{matrix}}}$

with ${\displaystyle \lambda ^{2}=1}$ in relativity.

### Ignatowski (1910)

w:Wladimir Ignatowski (1910) defined the “vector of first kind” using charge density ${\displaystyle \varrho }$ and three-velocity ${\displaystyle {\mathfrak {v}}}$:[R 14]

${\displaystyle {\begin{matrix}\left(\varrho {\mathfrak {v}},\ \varrho \right)\\\hline \left[\varrho {\sqrt {1-n{\mathfrak {v}}^{2}}}=\varrho '{\sqrt {1-n{\mathfrak {v}}^{\prime 2}}}=\varrho _{0}\right]\end{matrix}}}$

equivalent to four-current (a).

### Sommerfeld (1910)

In influential papers on 4D vector calculus in relativity, w:Arnold Sommerfeld defined the four-current P, which he called four-density (Viererdichte):[R 15]

${\displaystyle {\begin{matrix}P_{x}=\varrho {\frac {{\mathfrak {v}}_{x}}{c}},\ P_{y}=\varrho {\frac {{\mathfrak {v}}_{y}}{c}},\ P_{z}=\varrho {\frac {{\mathfrak {v}}_{z}}{c}},\ P_{l}=i\varrho \\\hline \beta ^{2}={\frac {1}{c^{2}}}\left({\mathfrak {v}}_{x}^{2}+{\mathfrak {v}}_{y}^{2}+{\mathfrak {v}}_{z}^{2}\right)\quad \Rightarrow \quad \left|P\right|=i\varrho {\sqrt {1-\beta ^{2}}}\\{}[l=ict]\end{matrix}}}$

equivalent to (a). In the second paper he pointed out its relation to four-potential ${\displaystyle \Phi }$ and the electromagnetic tensor (six-vector) f together with the continuity condition:[R 16]

{\displaystyle {\begin{matrix}{\begin{aligned}P&={\mathfrak {Div}}\mathrm {Rot} \ \Phi ={\mathfrak {Div}}\ f\\-P&=\square \Phi ,\ (\mathrm {Div} \ \Phi =0)\\\mathrm {Div} \ P&=0\end{aligned}}\\\left[{\begin{aligned}\mathrm {Rot} &={\text{exterior product}}\\\mathrm {Div} &={\text{divergence four-vector}}\\{\mathfrak {Div}}&={\text{divergence six-vector}}\\\square &={\text{D'Alembert operator}}\end{aligned}}\right]\end{matrix}}}

equivalent to Maxwell's equations (c). The scalar product with the four-potential[R 17]

${\displaystyle (P\Phi )}$

he called “electro-kinetic potential” whereas the vector product with the electromagnetic tensor[R 18]

${\displaystyle (Pf)={\mathfrak {F}}}$

he called the electrodynamic force (four-force density).

### Lewis (1910), Wilson/Lewis (1912)

w:Gilbert Newton Lewis (1910) devised an alternative 4D vector calculus based on w:Dyadics which, however, never gained widespread support. The four-current is a “1-vector”:[R 19]

{\displaystyle {\begin{aligned}\mathbf {q} &={\frac {\varrho }{c}}\mathbf {v} +i\varrho \mathbf {k} _{4}\\&={\frac {\varrho }{c}}v_{1}\mathbf {k} _{1}+{\frac {\varrho }{c}}v_{2}\mathbf {k} _{2}+{\frac {\varrho }{c}}v_{3}\mathbf {k} _{3}+i\varrho \mathbf {k} _{4}\end{aligned}}}

equivalent to (a) and its relation to the four-potential ${\displaystyle \mathbf {m} }$ and electromagnetic tensor ${\displaystyle \mathbf {M} }$:

{\displaystyle {\begin{matrix}{\begin{aligned}\lozenge \lozenge \times \mathbf {m} &=\mathbf {q} \\\lozenge \mathbf {M} &=\mathbf {q} \\\lozenge ^{2}\mathbf {m} &=-\mathbf {q} \end{aligned}}\\{\begin{aligned}\left({\frac {\partial H_{12}}{\partial x_{2}}}+{\frac {\partial H_{13}}{\partial x_{3}}}+{\frac {\partial E_{14}}{\partial x_{4}}}\right)\mathbf {k} _{1}&={\frac {\varrho }{c}}v_{1}\mathbf {k} _{1}\\\left({\frac {\partial H_{21}}{\partial x_{1}}}+{\frac {\partial H_{23}}{\partial x_{3}}}+{\frac {\partial E_{24}}{\partial x_{4}}}\right)\mathbf {k} _{2}&={\frac {\varrho }{c}}v_{2}\mathbf {k} _{2}\\\left({\frac {\partial H_{31}}{\partial x_{1}}}+{\frac {\partial H_{32}}{\partial x_{2}}}+{\frac {\partial E_{34}}{\partial x_{4}}}\right)\mathbf {k} _{3}&={\frac {\varrho }{c}}v_{3}\mathbf {k} _{3}\\\left({\frac {\partial H_{41}}{\partial x_{1}}}+{\frac {\partial H_{42}}{\partial x_{2}}}+{\frac {\partial E_{43}}{\partial x_{4}}}\right)\mathbf {k} _{4}&={\frac {\varrho }{c}}i\mathbf {k} _{4}\end{aligned}}\\\left[{\begin{matrix}\lozenge =\mathbf {k} _{1}{\frac {\partial }{\partial x_{1}}}+\mathbf {k} _{2}{\frac {\partial }{\partial x_{2}}}+\mathbf {k} _{3}{\frac {\partial }{\partial x_{3}}}+\mathbf {k} _{4}{\frac {\partial }{\partial x_{4}}}\\\lozenge ^{2}={\frac {\partial ^{2}}{\partial x_{1}}}+{\frac {\partial ^{2}}{\partial x_{2}}}+{\frac {\partial ^{2}}{\partial x_{3}}}+{\frac {\partial ^{2}}{\partial x_{4}}}\end{matrix}}\right]\end{matrix}}}

equivalent to (c,d).

In 1912, Lewis and w:Edwin Bidwell Wilson used only real coordinates, writing the above expressions as[R 20]

{\displaystyle {\begin{matrix}{\begin{aligned}\lozenge \cdot \mathbf {M} &=4\pi \mathbf {q} \\\lozenge ^{2}\mathbf {m} &=-4\pi \mathbf {q} \end{aligned}}\\\left[{\begin{matrix}\lozenge =\mathbf {k} _{1}{\frac {\partial }{\partial x_{1}}}+\mathbf {k} _{2}{\frac {\partial }{\partial x_{2}}}+\mathbf {k} _{3}{\frac {\partial }{\partial x_{3}}}-\mathbf {k} _{4}{\frac {\partial }{\partial x_{4}}}\\\lozenge ^{2}={\frac {\partial ^{2}}{\partial x_{1}}}+{\frac {\partial ^{2}}{\partial x_{2}}}+{\frac {\partial ^{2}}{\partial x_{3}}}-{\frac {\partial ^{2}}{\partial x_{4}}}\end{matrix}}\right]\end{matrix}}}

equivalent to (c,d).

### Von Laue (1911)

In the first textbook on relativity in 1911, w:Max von Laue elaborated on Sommerfeld's methods and explicitly used the term “four-current” (Viererstrom) of density ${\displaystyle \varrho }$ in relation to four-potential ${\displaystyle \Phi }$ and electromagnetic tensor ${\displaystyle {\mathfrak {M}}}$:[R 21]

{\displaystyle {\begin{matrix}P\Rightarrow \left(P_{x}={\frac {\varrho {\mathfrak {q}}_{x}}{c}},\ P_{y}={\frac {\varrho {\mathfrak {q}}_{y}}{c}},\ P_{z}={\frac {\varrho {\mathfrak {q}}_{z}}{c}},\ P_{l}=i\varrho \right)\\\hline {\begin{aligned}P&=\varDelta iv\ ({\mathfrak {M}})\\-P&=\square \Phi \ (Div\ \Phi =0)\\Div\ (P)&=0\end{aligned}}\\\left[{\begin{aligned}{\mathfrak {Rot}}&={\text{exterior product}}\\Div&={\text{divergence four-vector}}\\\varDelta iv&={\text{divergence six-vector}}\\\square &={\text{D'Alembert operator}}\end{aligned}}\right]\end{matrix}}}

equivalent to (a,c,d). He went on to define four-force density F as vector-product with ${\displaystyle {\mathfrak {M}}}$, four-convection K and four-conduction ${\displaystyle \Lambda }$ using four-velocity Y:[R 22]

${\displaystyle {\begin{matrix}F=[P{\mathfrak {M}}],\ (PF)=(P[P{\mathfrak {M}}])=0\\K=-(YP)Y\\\Lambda =P+(YP)Y\end{matrix}}}$,

### Silberstein (1911)

w:Ludwik Silberstein devised an alternative 4D calculus based on w:Biquaternions which, however, never gained widespread support. He defined the “current-quaternion” (i.e. four-current) C and its relation to the “electromagnetic bivector” (i.e. field tensor) ${\displaystyle \mathbf {F} }$ and “potential-quaternion” (i.e. four-potential) ${\displaystyle \Phi }$[R 23]

{\displaystyle {\begin{matrix}{\begin{aligned}\mathrm {C} &=\rho \left(\iota +{\frac {1}{c}}\mathbf {p} \right)\\&=\iota \rho {\frac {dq}{dl}}\\\mathrm {C} &=\mathrm {D} \mathbf {F} =-\Box \Phi \\\mathrm {S} \mathrm {D} _{c}\mathrm {C} &=0\end{aligned}}\\\left[\mathrm {D} ={\frac {\partial }{\partial l}}-\nabla ,\ \mathrm {D} \mathrm {D} _{c}=\Box ={\frac {\partial ^{2}}{\partial x^{2}}}+{\frac {\partial ^{2}}{\partial y^{2}}}+{\frac {\partial ^{2}}{\partial z^{2}}}+{\frac {\partial ^{2}}{\partial l^{2}}}\right]\end{matrix}}}

### Kottler (1912)

w:Friedrich Kottler defined the four-current ${\displaystyle \mathbf {P} ^{(\alpha )}}$ and its relation to four-velocity ${\displaystyle V^{(\alpha )}}$, four-potential ${\displaystyle \Phi _{\alpha }}$, four-force ${\displaystyle F_{\alpha }}$, electromagnetic field-tensor ${\displaystyle F_{\alpha \beta }}$, stress-energy tensor ${\displaystyle S_{\alpha \beta }}$:[R 24]

${\displaystyle {\begin{matrix}P^{(1)}=\rho {\frac {{\mathfrak {v}}_{x}}{c}}=i\rho _{0}V^{(1)},\ P^{(2)}=\rho {\frac {{\mathfrak {v}}_{y}}{c}}=i\rho _{0}V^{(2)},\ P^{(3)}=\rho {\frac {{\mathfrak {v}}_{z}}{c}}=i\rho _{0}V^{(3)},\ P^{(4)}=i\rho =i\rho _{0}V^{(4)}\\\hline \sum _{h=1}^{4}{\frac {\partial F_{gh}}{\partial x^{(h)}}}=\mathbf {P} ^{(g)},\ \Box \Phi _{\alpha }=-\mathbf {P} ^{(\alpha )}\\F_{\alpha }(y)=\sum _{\beta }{\frac {F_{\alpha \beta }(y)\mathbf {P} ^{(\beta )}(y)}{\sqrt {1-{\mathfrak {w}}^{2}/c^{2}}}}\\\left[{\underset {\beta }{\sum }}F_{\alpha \beta }(y)\mathbf {P} ^{(\beta )}(y)={\underset {\beta }{\sum }}F_{\alpha \beta }(y){\underset {\gamma }{\sum }}{\frac {\partial }{\partial y^{(\gamma )}}}F_{\beta \gamma }(y)={\underset {\beta }{\sum }}{\frac {\partial }{\partial y^{(\beta )}}}S_{\alpha \beta },\ \rho _{0}=\rho {\sqrt {1-{\mathfrak {v}}^{2}/c^{2}}}\right]\end{matrix}}}$

equivalent to (a,b,c,d) and subsequently was the first to give the generally covariant formulation of Maxwell's equations using metric tensor ${\displaystyle c_{\alpha \beta }}$[R 25]

${\displaystyle {\begin{matrix}\sum c^{(1\alpha )}\sum _{\beta ,\gamma }c^{(\beta \gamma )}\Phi _{\alpha /\beta \gamma }=-\mathbf {P} ^{(\alpha )}\ {\text{etc}}.\\\left[\sum _{\beta ,\gamma }c^{(\beta \gamma )}\Phi _{\beta /\gamma }=0\right]\end{matrix}}}$

equivalent to (e).

### Einstein (1913)

Independently of Kottler (1912), w:Albert Einstein defined the general covariant four-current in the context of his Entwurf theory (a precursor of general relativity):[R 26]

${\displaystyle \varrho _{0}{\frac {dx_{\nu }}{ds}}={\frac {1}{\sqrt {-g}}}\varrho _{0}{\frac {dx_{\nu }}{dt}}}$

equivalent to (a), and the generally covariant formulation of Maxwell's equations

{\displaystyle {\begin{matrix}\sum _{\nu }{\frac {\partial }{\partial x_{\nu }}}\left({\sqrt {-g}}\cdot \varphi _{\mu \nu }\right)=\varrho _{0}{\frac {dx_{\mu }}{dt}}\\\hline {\begin{aligned}{\frac {\partial {\mathfrak {H}}_{x}}{\partial y}}-{\frac {\partial {\mathfrak {H}}_{y}}{\partial z}}-{\frac {\partial {\mathfrak {E}}_{x}}{\partial t}}&=u_{x}\\\dots \\\dots \\{\frac {\partial {\mathfrak {E}}_{x}}{\partial x}}+{\frac {\partial {\mathfrak {E}}_{y}}{\partial z}}+{\frac {\partial {\mathfrak {E}}_{x}}{\partial z}}&=\varrho \end{aligned}}\\\left[\varrho _{0}{\frac {dx_{\mu }}{dt}}=u_{\mu }\right]\end{matrix}}}

equivalent to (e) in the case of ${\displaystyle g_{\mu \nu }}$ being the Minkowski tensor.

## Historical sources

1. Herglotz (1904), p. 550f
2. Sommerfeld (1910a)
3. Poincaré (1905a), p. 1505
4. Poincaré (1905b), p. 133–134
5. Poincaré (1905b), p. 134
6. Marcolongo (1906), p. 348-349
7. Minkowski (1907/15), p. 929
8. Minkowski (1907/15), p. 933
9. Minkowski (1907/8), p. 57, 60, 67
10. Minkowski (1907/8), p. 71
11. Born (1909), p. 573-574, 576-577
12. Bateman (1910), p. 241
13. Bateman (1910), p. 252
14. Ignatowsky (1910), p. 24-25
15. Sommerfeld (1910a), p. 751
16. Sommerfeld (1910b), p. 651
17. Sommerfeld (1910a), p. 764
18. Sommerfeld (1910a), p. 770
19. Lewis (1910), p. 176ff
20. Wilson & Lewis (1910), p. 488ff
21. Laue (1911), p. 77f, 100f
22. Laue (1911), p. 80, 102, 119
23. Silberstein (1911), p. 796, 801f
24. Kottler (1912), p. 1661, 1686ff
25. Kottler (1912), p. 1688-1689
26. Einstein & Grossmann (1913), p. 241
• Laue, Max von (1911). Das Relativitätsprinzip. Braunschweig: Vieweg.
• Lewis, G. N. (1910). "On Four-Dimensional Vector Analysis, and Its Application in Electrical Theory". Proceedings of the American Academy of Arts and Sciences 43 (7): 165-181.
• Lewis, G. N. & Wilson, E. B. (1912). "The Space-time Manifold of Relativity. The Non-Euclidean Geometry of Mechanics and Electromagnetics". Proceedings of the American Academy of Arts and Sciences 48: 387–507.
• Marcolongo, R. (1906), "Sugli integrali delle equazioni dell'elettrodinamica" (PDF), Atti della Reale Accademia dei Lincei Rendiconti, 15: 344–349
• Minkowski, Hermann (1915) [1907], "Das Relativitätsprinzip", Annalen der Physik, 352 (15): 927–938