Keep Latin Modern as the font used inside math equations

Question

In a Hebrew document, I changed the fonts as in the MWE below.

The thing is that when I have some text typed inside \text{…}, the font used to print it is not the font used for math.

How do I fix it?

I tried the no-math for fontspec but it doesn't help.

The output now:

whereas I want the "sec" to be printed with "Latin Modern Math":

MWE:

\documentclass{article}
\usepackage{amsmath}
\usepackage[no-math]{fontspec}
\usepackage{polyglossia}
\newfontfamily\hebrewfont{Arial}[Script=Hebrew]
\newfontfamily\englishfont{TeX Gyre Bonum}[Script=Latin, Language=English]
\setmathrm{Latin Modern Math} 
\setdefaultlanguage{hebrew}
\setotherlanguage{english}
\DeclareTextFontCommand{\emph}{\boldmath\bfseries}
\begin{document}
אבג דהו $\Delta t=1\ \text{sec}$ זחט יכל.
\end{document}

Answer 1

The simplest solution is to use \mathrm instead of \text. This defaults to the main document font, but you can change it with \setmathrm{Latin Modern Roman}. Also use ~ for a non-breaking space.

\documentclass{article}
\tracinglostchars=3 % Make it an error to use the wrong font for the current language.
\usepackage{amsmath}
\usepackage{polyglossia}

\defaultfontfeatures{ Scale=MatchLowercase, Ligatures=TeX }
\newfontfamily\hebrewfont{Arial}[Script=Hebrew]
\newfontfamily\englishfont{TeX Gyre Bonum}[Script=Latin, Language=English]
\setmathrm{Latin Modern Roman} 
\setdefaultlanguage{hebrew}
\setotherlanguage{english}
\DeclareTextFontCommand{\emph}{\boldmath\bfseries}

\begin{document}
אבג דהו $\Delta t=1\mathrm{~sec}$ זחט יכל.
\end{document}

In practice, you probably want to make commands for this, such as \sec or \unit{s}, but at that point you might seriously consider siunitx. You will seriously thank yourself later for using semantic markup that lets you change the formatting of your units or your vectors in a single place. Otherwise, hunting down all the \mathrm commands that format units, and only those that do, will be a major headache when you have to change it later.

i took the liberty of adding a command to stop you from trying to display Hebrew in a font that doesn’t support it, and to automatically rescale the fonts to the same x-height.

Side Notes

By the way, I answered the question as asked, but this isn’t a selection of fonts I would use by choice. You might get some more harmonious-looking traditional fonts with:

\documentclass{article}
\tracinglostchars=3 % Make it an error to use the wrong font for the current language.
\usepackage{unicode-math}
\usepackage{polyglossia}

\defaultfontfeatures{ Scale=MatchUppercase, Ligatures=TeX }
% The Culmus fonts are available at https://culmus.sourceforge.io/download.html
\newfontfamily\hebrewfont{Frank Ruehl CLM}[Script=Hebrew]
\newfontfamily\englishfont{TeX Gyre Schola}
\setmathfont{TeX Gyre Schola Math}
\setmathrm{TeX Gyre Schola}

\setdefaultlanguage{hebrew}
\setotherlanguage{english}

\DeclareTextFontCommand{\emph}{\boldmath\bfseries}

\begin{document}
אבג דהו $\Delta t=1\mathrm{~sec}$ זחט יכל.
\end{document}

This uses the fonts from the Culmus Project.

Or, if you want to stay with Arial and use a math font that matches it better:

\documentclass{article}
\tracinglostchars=3 % Make it an error to use the wrong font for the current language.
\usepackage{unicode-math}
\usepackage{polyglossia}

\defaultfontfeatures{ Scale=MatchLowercase, Ligatures=TeX }
\newfontfamily\hebrewfont{Arial}[Script=Hebrew]
\newfontfamily\englishfont{Fira Sans}
\setmathfont{Fira Math}
\setmathrm{fira Sans}

\setdefaultlanguage{hebrew}
\setotherlanguage{english}

\DeclareTextFontCommand{\emph}{\boldmath\bfseries}

\begin{document}
אבג דהו $\increment t=1\mathrm{~sec}$ זחט יכל.
\end{document}

(There is also [a fork of Fira, FiraGO, that supports Hebrew.)

Or, a good companion font for Computer Modern might be:

\documentclass{article}
\tracinglostchars=3 % Make it an error to use the wrong font for the current language.
\usepackage{fontspec}
\usepackage{polyglossia}

\defaultfontfeatures{ Scale=MatchLowercase, Ligatures=TeX }
% Frank Ruhl Libre is available from:
% https://opensiddur.org/wp-content/uploads/fonts/display-font-charmap.php?fnt=FrankRuhlLibre
\defaultfontfeatures[FrankRuhlLibre]{ UprightFont = *-Light ,
                                      BoldFont = *-Medium ,
                                      Extension = .ttf ,
                                      Scale = 0.85 }

\newfontfamily\hebrewfont{FrankRuhlLibre}[Script=Hebrew, Language=Hebrew]
\newfontfamily\englishfont{Latin Modern Roman}
\setmathrm{Latin Modern Roman}

\setdefaultlanguage{hebrew}
\setotherlanguage{english}

\DeclareTextFontCommand{\emph}{\boldmath\bfseries}

\begin{document}
\emph{ אבג דהו $\Delta t=1\mathrm{~sec}$ זחט יכל}

אבג דהו $ \Delta t=1\mathrm{~sec}$ זחט יכל.
\end{document}

Where you can download the Hebrew font from OpenSiddur.

As a historical aside, Hayyim Selig Slonimski, a leader of the nineteenth-century Maskilim and one of the first to publish scientific works in Hebrew, used Drugulin in the body of his books published in Warsaw. That would be an unusual choice today! (But if you were inclined to, the Taamey Ashkenaz font from Culmus with Times is a close approximation.)

Post-Post-Script

As requested, an example using siunitx:

\documentclass{article}
\tracinglostchars=3 % Make it an error to use the wrong font for the current language.
\usepackage{amsmath}
\usepackage{polyglossia}
\usepackage[mode=math,
            propagate-math-font=true,
            reset-math-version=false
           ]{siunitx}

\defaultfontfeatures{ Scale=MatchLowercase, Ligatures=TeX }
\newfontfamily\hebrewfont{Arial}[Script=Hebrew]
\newfontfamily\englishfont{TeX Gyre Bonum}
\setmathrm{Latin Modern Roman}
\setboldmathrm{LM Roman 10 Bold}
\setdefaultlanguage{hebrew}
\setotherlanguage{english}
\DeclareTextFontCommand{\emph}{\boldmath\bfseries}

\begin{document}
\emph{ אבג דהו $\Delta t=\qty{1}{\second}$ זחט יכל} \\
אבג דהו
\( \Delta t=\qty{1}{\second} \)
זחט יכל.
\end{document}

I turned off the reset-math-version package option because the redefinition of \emph suggests you’re using \boldmath and want it to propagate to your units. You might want to \setboldmathrm to make this work properly.

You can also reduce the spacing in some circumstances with the tight-spacing option, or use a font other than \mathrm with the unit-font-command= option.

Answer 2

\newfontfamily\lmmath{Latin Modern Math}
\NewCommandCopy{\oldtext}{\text}
\RenewDocumentCommand{\text}{m}{\oldtext{\lmmath #1}}

Add these to preamble.