Electric Field Strength Conservation in Direction of Motion

[StateOfTheEqn]

(Sorry for the typo in the title. I don't know how to get rid of it)

Recently I was looking at the Lorentz co-variance of F=dA (the Faraday 2-form) and *F=*dA (the Maxwell 2-form). I realized that Lorentz co-variance implies that the electric field strength and magnetic field strength are invariant in the direction of motion. In Einstein's notation in Zur Elektrodynamik bewegter Körper (Engl trans.:ON THE ELECTRODYNAMICS OF MOVING BODIES), X=X' and L=L'. It struck me as interesting that he had to assume the invariance in order to prove the co-variance of Maxwell's equations:

Maxwell's eqns:

transformed:

and now in the new variables:

Notice how he assumes X=X' and L=L'. This was no small assumption since it was at the time something Poincare was wanting to prove. The proof is, it seems, because it is necessary to establish co-variance, an established fact by then - otherwise you could measure the absolute velocity of the Earth through the luminiferous aether.

Further along he states explicitly:

 

[vanhees71]

It's simply the transformation law for a 2nd-rank tensor,
F'^{\mu \nu}={\Lambda^{\mu}}_{\rho} {\Lambda^{\nu}}_{\sigma} F^{\rho \sigma}.
Plugging in the components in 1+3-dimensional notation, i.e., for \mu \in \{1,2,3 \}
F^{0 \mu}=\partial^{0} A^{\mu}-\partial^{\mu} A^0=\partial_0 A^{\mu} + \partial_{\mu} A^0=\dot{A}^{\mu}+\partial_{\mu} \Phi=-E^{\mu},
and for \mu,\nu \in \{1,2,3 \}
F^{\mu \nu}=\partial^{\mu} A^{\nu} - \partial^{\nu} A^{\mu}=-\partial_{\mu} A^{\nu} + \partial_{\nu} A^{\mu}=\epsilon^{\nu \mu \rho} B^{\rho},
you find the transformation law in terms of (\vec{E},\vec{B}) for a boost,
<br /> \begin{split}<br /> \vec{E}&#039;&amp;=\gamma(\vec{E}+\vec{\beta} \times \vec{B})-\frac{\gamma^2}{1+\gamma} \vec{\beta}(\vec{\beta} \cdot \vec{E}),\\<br /> \vec{B}&#039;&amp;=\gamma(\vec{B}-\vec{\beta} \times \vec{E})-\frac{\gamma^2}{1+\gamma} \vec{\beta}(\vec{\beta} \cdot \vec{B}),<br /> \end{split}<br />
where \vec{\beta}=\vec{v}/c and \gamma=1/\sqrt{1-\vec{\beta}^2}. As you see, the components in boost direction are indeed unchanged.

Projecting to the parts in direction of \vec{\beta} and perpendicular to it gives
<br /> \begin{split}<br /> \vec{E}_{\parallel}&#039;&amp;=\vec{E}_{\parallel},\\<br /> \vec{B}_{\parallel}&#039;&amp;=\vec{B}_{\parallel},\\<br /> \vec{E}_{\perp}&#039;&amp;=\gamma(\vec{E}_{\perp}+\vec{\beta} \times \vec{B}),\\<br /> \vec{B}_{\perp}&#039;&amp;=\gamma(\vec{B}_{\perp}-\vec{\beta} \times \vec{E}).<br /> \end{split}<br />