Often in physics, the symbol \sim is used as a unary operator to mean "approximately."
For example, one might say "the energy of these processes is $\sim 4$ eV." However, this looks awkward because \sim is a binary operator and the spacing is incorrect. How can one use \sim act as a unary operator?
Just define a new command for it:
\newcommand{\ca}{{\sim}} I suggest also to use siunitx, if you have units of measure to typeset in your document: it ensures uniform setting.
\documentclass{article} \newcommand{\ca}{{\sim}} \usepackage{siunitx} \sisetup{input-protect-tokens=\ca,input-symbols=\ca} \begin{document} \SI{\ca 4}{eV} $\ca 4$\,eV \end{document} The input might seem more difficult, but it's surely rewarding.
If you load amssymb you can change the definition to
\usepackage{amssymb} \newcommand{\ca}{{\thicksim}} and the result would be
${\sim}4$or$\mathord{\sim}4$\simis a relation symbol, rather than a binary operation; if it were then$\sim 4$would give no space. It's probably better to define a new command for it as an "Ord":\newcommand{\appx}{{\sim}}. Consider usingsiunitxfor typesetting units of measure.\simsymbol as the minus symbol, that would automatically solve the OP's question.