"Units in particle physics". Associate Teacher Institute Toolkit. Fermilab. 22 March 2002. Archived from the original on 14 May 2011. Retrieved 13 February 2011.
harvard.edu
ui.adsabs.harvard.edu
Barrow, J. D. (1983). "Natural Units Before Planck". Quarterly Journal of the Royal Astronomical Society. 24: 24. Bibcode:1983QJRAS..24...24B.
"Units in particle physics". Associate Teacher Institute Toolkit. Fermilab. 22 March 2002. Archived from the original on 14 May 2011. Retrieved 13 February 2011.