Illegal unit of measure(pt inserted)

Question

I can't find my fault and appreciate any help for curing the error. Here is the code:

\documentclass[a4]{paper}
\usepackage{graphicx,slashed,color,amssymb}
\usepackage{xspace,colortbl,times,pause}
%\documentstyle[preprint,aps]{revtex}
\renewcommand{\baselinestretch}{1.6}
\begin{document}
\title{QFT}
\author{Athos\CZ}
\date{March 11, 2012}
\begin{eqnarray}
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\alpha\beta}g_\alpha^\nu&=&4i\gamma^\mu w_\mu^\nu g_\beta^\mu\nonumber\\
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\nu\beta}&=&4i\gamma^\mu w_\beta^\nu\\
\end{eqnarray}

I want to write two formula, but can do it. However, if I set the two formula apart as:

\begin{eqnarray}
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\alpha\beta}g_\alpha^\nu&=&4i\gamma^\mu w_\mu^\nu g_\beta^\mu\nonumber\\
\end{eqnarray}
\begin{eqnarray}
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\nu\beta}&=&4i\gamma^\mu w_\beta^\nu\\
\end{eqnarray}

there will be no problem. I'm so confused.

Answer 1

The analysis by @Stephan is correct: \\ is skipping spaces when looking for the optional argument and it finds the [ on the next line.

However is not correct to use \\{} to prevent this. It would stop \\ from looking for [ but {} introduces a "math atom" (kind of empty subformula) in the math formula and that messes up spacing or can mess up spacing. Example: $+a$ looks quite different from ${}+a$. As in this particular situation the {} is followed by [ the spacing will appear to be ok (as TeX will not add any space for this combination), but the moment the user changes [ to, say, \left[ without removing the {} things suddenly go wrong.

On possible solution is explicitly use an optional argument in this cases: \\[0pt]. An alternative is to use \\ \relax which also stops \\ to parse ahead for [.

As mentioned elsewhere eqnarray is defective in several respects and for serious math you should use the amsmathpackage and avoid this environment altogether. We would have taken it out of LaTeX2e if possible (it is really only in there for backward compatibility)

Answer 2

As written in the top-voted answer to the question "\eqnarray vs \align"

Although eqnarray may seem to work "well enough", Avoid eqnarray! Avoid eqnarray! Avoid eqnarray!

Use align and the rest of the ams environments. See texdoc amsldoc (PDF) or the short math guide for LaTeX for documentation on how to use them.

Your example compiles if you simply use the amsmath package and substitute align for eqnarray:

\begin{align*}
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\alpha\beta}g_\alpha^\nu&=4i\gamma^\mu w_\mu^\nu g_\beta^\mu\nonumber\\
[\gamma^\nu,\sigma_{\alpha\beta}]w^{\nu\beta}&=4i\gamma^\mu w_\beta^\nu\\
\end{align*}

(Notice that you only need one & to mark the alignment point.)

Answer 3

Your Problem is the \\ which is greedily looking for a [ holding its optional argument.

Try \\\relax (untested).

Edit: Thanks to @Frank for pointing out that {} is not the right thing to say.

Answer 4

I suggest you load the mathtools package. Loading the mathtools package will automatically fix the (well-documented but undesirable) property of the eqnarray and align environments to misinterpret a [ (left square bracket) that leads off an equation as the start of an instruction on how much extra vertical whitespace to leave below the preceding equation. Put differently, if you load the mathtools package, you don't have to remember to insert a [0pt] after \\ should the next equation lead off with [.

I have two additional comments on your MWE. First, I'd like to suggest you load the mathptmx package instead of the times package. If you load the times package, you'll get a document with Times New Roman as the text-mode font and Computer Modern as the math-mode font. The \mathptmx package, in contrast, loads Times-style text and and math fonts.

Second, don't set \renewcommand{\baselinestretch}{1.6}, because doing so will mess up the look of footnotes (which are usually set single-spaced) as well as the spacing around display-math material. Instead, include the instructions

\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.6}

in your document's preamble.

Finally, do heed the advice already given in the other answers regarding the use the eqnarray environment: don't use eqnarray, use align instead.

With all of these suggestions implemented, your MWE might look like this:

\documentclass[a4]{article}
\usepackage{graphicx,slashed,color,xspace,colortbl,pause}
\usepackage{mathptmx,mathtools}
\usepackage{setspace}
\setstretch{1.6}
\begin{document}
\begin{align}
[\gamma^\nu,\sigma_{\alpha\beta}]  w^{\alpha\beta} g_\alpha^\nu
  &= 4i\gamma^\mu w_\mu^\nu g_\beta^\mu\nonumber\\  % Observe: no need to insert [0pt]
[\gamma^\nu,\sigma_{\alpha\beta}] w^{\nu\beta}
  &= 4i\gamma^\mu w_\beta^\nu
\end{align}
\end{document}