TikZ/pgf: Why does `\newline` and `\\` not split text in node?

Question

The MWE:

\documentclass{standalone}

%======================================
\usepackage{tikz}
\usetikzlibrary{graphdrawing}
\usetikzlibrary{arrows}
\usetikzlibrary{graphs}
\usegdlibrary{force, layered, trees}
%======================================

\begin{document}

\tikz 
\graph [spring layout, nodes = draw]
{
    a/"dispersive\newline collision";
    b/"non-dispersive\newline  collision";

    a -> b
};

\end{document}

The output:

The question:

As you can see above, the \newline command does nothing. Trying to use \\ instead of \newline, like how the pgfmanual does in some examples (see pg. 386) has the same effect. Why? What can I do in order to have the words appear on separate "lines"?

Answer 1

By default nodes are set in a box that doesn't allow line breaks. Adding an alignment (e.g. align=center) or text width to the node sets it in a paragraph box instead, where line breaks can occur. For some reason \newline still doesn't work (no idea why), but \\ does.

\documentclass{standalone}

%======================================
\usepackage{tikz}
\usetikzlibrary{graphdrawing}
\usetikzlibrary{arrows}
\usetikzlibrary{graphs}
\usegdlibrary{force, layered, trees}
%======================================

\begin{document}

%simpler example
\begin{tikzpicture}
\node {a\\b};
\node [align=center] at (1,0) {a\\b};
\end{tikzpicture}

% your code, modified
\tikz 
\graph [spring layout, nodes = {draw,align=center}]
{
    a/"dispersive\\ collision";
    b/"non-dispersive\\  collision";

    a -> b
};

\end{document}