Intuitive

Concrete

• The International System of Units (SI) - the modern metric system of measurement.
• Gaussian - commonly used in electromagnetism.
• Heaviside-Lorentz (rationalized Gaussian)
• Natural units - commonly used in quantum electrodynamics and particle physics in general.

• A great discussion can be found in Section 2.1 in Klauber's "Student-friendly Quantum Field Theory".
• See also BABEL OF UNITS by Neal J. Carron

Any physical quantity can be expressed by using as fundamental unit either energy, mass, length or time.

\begin{align} ct&\approx \ell\Longrightarrow {\rm time}\approx{\rm length},\\ E&\approx mc^2\Longrightarrow {\rm energy}\approx{\rm mass},\\ E&\approx pv\Longrightarrow {\rm energy}\approx{\rm momentum},\\ Et&\approx \hbar\Longrightarrow {\rm energy}\approx{\rm (time)^{-1}}\approx {\rm (length)^{-1}}. \end{align}

Abstract

Why is it interesting?