Consider a connection on a principal $U(1)$-bundle $A_\mu$ over the flat base manifold $M_4$. The action of the theory is described in terms of the curvatures of such connection coupled to some source $j^\mu$ in the following way
\begin{align}
S=\int_{M_4}d^4x\left(-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}-A_\mu j^\mu\right),
\end{align}
where $F_{\mu\nu}=\partial_\mu A_\nu-\partial_\nu A_\mu$ and with sign convention $(+,-,-,-)$. The associated equations of motion are given by
\begin{align}
\partial_\mu F^{\mu\nu}=j^\nu.
\end{align}
Both the action and the equations of motion enjoy an invariance under the gauge transformations
\begin{align}
A'_\mu(x)=A_\mu(x)+\partial_\mu\Lambda(x).
\end{align}
We now decide to use this invariance to fix a certain condition on the gauge field. For example, we can choose the gauge fixing condition $$ A'_0=0, $$
which is satisfied by the following Cauchy problem
\begin{align}\label{condlambda}
\begin{cases}
\partial_0\Lambda (x)=- A_0(x),\\
\Lambda(\bar x^0,x^i)=\varphi(x^i).
\end{cases}
\end{align}
This is solved by
\begin{align}
\Lambda(x)=\varphi(x^i)-\int_{\bar x^0}^{x^0}d\tilde x^0 A_0(\bar x^0,x^i).
\end{align}
A gauge transformation allowed to fix one component of the gauge field to zero.
A second gauge transformation
\begin{align}
A_\mu''(x)=A'_\mu(x)+\partial_\mu\alpha(x)
\end{align}
maintaining the gauge fixing condition $A''_0=0$ is still possible if we require
\begin{align}
\alpha=\alpha(x^i).
\end{align}
This residual gauge has no effect in terms of fixing another component of the gauge field.
We then focus on the equations of motion: these can be split into
\begin{align}
\begin{cases}
\partial_j\partial^jA''^0(x)-\partial_j\partial^0A''^j(x)=j^0(x),\\
\partial_0\partial^0A''^i(x)+\partial_j(\partial^jA''^i(x)-\partial^iA''^j(x))=j^i(x).
\end{cases}
\end{align}
The first equation can be further simplified $$\partial_j\partial^0A''^j(x)=-j^0(x)\implies \partial_1\partial^0A''^1(x)=-j^0(x)-\partial_a\partial^0A''^a(x),$$ where $a=2,3$. By integrating two times, we get
\begin{align}\nonumber
\int_{\bar x^0}^{x^0}d\tilde x^0\int_{\bar x^1}^{x^1}d\tilde x^1&\tilde\partial_1\tilde\partial^0A''^1(\tilde x^0,\tilde x^1,x^a)=\\ \nonumber
&=-\int_{\bar x^0}^{x^0}d\tilde x^0\int_{\bar x^1}^{x^1}d\tilde x^1\bigg(j^0(\tilde x^0,\tilde x^1,x^a)+\partial_a\tilde\partial^0A''^a(\tilde x^0,\tilde x^1,x^a)\bigg)\\
&\equiv G(\bar x^0, \bar x^1; x^0,x^1, x^a)
\end{align}
which implies
\begin{align}
A''^1(x^0,x^1,x^a)-A''^1(\bar x^0,x^1,x^a)-A''^1(x^0,\bar x^1,x^a)+A''^1(\bar x^0,\bar x^1,x^a)=G(\bar x^0, \bar x^1; x^0,x^1, x^a).
\end{align}
Since $G(\bar x^0, \bar x^1; x^0,x^1, x^a)$ is a function of the remaining directions of the gauge field and the sources, we seem to have determined $A''^1(x^0,x^1,x^a)$ up to terms evaluated at the boundary. Is it correct?
Are these terms part of the Cauchy problem for our system of partial differential equations $\partial_\mu F^{\mu\nu}=j^\nu$?
$\begingroup$
$\endgroup$
Add a comment
|