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Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.


  • 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}


The motto in this section is: the higher the level of abstraction, the better.

Why is it interesting?

basic_notions/units.txt · Last modified: 2018/04/06 15:37 by jakobadmin