# Too much horizontal space in an alignat environment

Move one of alignment signs & in front of the \phi and A.

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{mathtools}

\begin{document}

\begin{subequations}
\begin{alignat}{2}
\phi \quad &\Rightarrow \quad &\tilde{\phi} &= e^{-\, i q \vartheta} \, \phi, \\[1ex]
A_{\mu} \quad &\Rightarrow \quad &\tilde{A}_{\mu} &= A_{\mu} + \partial_{\mu} \, \vartheta,
\end{alignat}
\end{subequations}

\end{document}


You can move the &'s.

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{mathtools}

\begin{document}

\begin{subequations}
\begin{alignat}{2}
\phi    \quad & \Rightarrow \quad & \tilde{\phi}    & = e^{-\, i q \vartheta} \, \phi, \\[1ex]
A_{\mu} \quad & \Rightarrow \quad & \tilde{A}_{\mu} & = A_{\mu} + \partial_{\mu} \, \vartheta,
\end{alignat}
\end{subequations}

\end{document}