We present the quantum-mechanical calculations of the dimensionless sensitivity coefficients $Q$ for small changes in the fundamental physical constant $\mu =m_e/m_p$ (the electron-to-proton mass ratio) for a number of low-frequency (1–50 GHz) transitions of the acetaldehyde (${\mathrm{CH}}_3\mathrm{CHO}$) molecule. The calculations show that $Q$ varies in the range from 0.62 to 3.61. An example of the practical use of the ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3\mathrm{OH}$ lines tracing the same regions in three molecular clouds, located at large galactocentric distances ($D_{\mathrm{GC}}\sim 8$ kpc), is considered. This results in a limit on the $\mu$ variations of $\mathrm{\Delta }\mu /\mu =(0.1\pm 0.4)\times {10}^{-7}$, which is in line with the most stringent upper limits on changes in $\mu$ previously obtained based on other molecules and methods. The limit obtained restricts hypothetical violations of the Einstein principle of the local position invariance at the level of $4\times {10}^{-8}$ in the Galactic disk at large galactocentric distances.

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• Atomic & molecular structure
• Molecular spectra
• Radio, microwave, & sub-mm astronomy