femtometre
{{short description|Unit of length}}
{{for|examples of things measuring between one and ten femtometres|1 femtometre}}
{{Infobox unit
| image = Helium atom QM.svg
| caption = The helium atom and perspective magnitudes
| symbol = fm
| standard = SI
| quantity = length
| units1 = SI base units
| inunits1 = {{val|1|e=-15|ul=m}}
| units2 = Natural units
| inunits2 = {{val|6.1877|e=19}} {{math|ℓP}}
{{val|1.8897|e=-5}} a0
| inunits3 = {{convert|1|fm|in|disp=out|lk=on|sigfig=5}}
}}
The femtometre (American spelling femtometer), symbol fm,{{cite web |url=http://www.unc.edu/~rowlett/units/dictF.html |title=Units: F |publisher=Unc.edu |access-date=2015-11-04 |archive-date=2018-07-09 |archive-url=https://web.archive.org/web/20180709210128/http://www.unc.edu/~rowlett/units/dictF.html |url-status=dead }}{{cite web|title=Nuclear Size and Shape |url=http://www.hep.phys.soton.ac.uk/hepwww/staff/D.Ross/phys3002/diffraction.pdf |url-status=bot: unknown |archive-url=https://web.archive.org/web/20120425092313/http://www.hep.phys.soton.ac.uk/hepwww/staff/D.Ross/phys3002/diffraction.pdf |archive-date=2012-04-25 }} (derived from the Danish and Norwegian word {{lang|no|femten}} 'fifteen', {{langx|grc|μέτρον|metron|lit=unit of measurement}}) is a unit of length in the International System of Units (SI) equal to 10−15 metres, which means a quadrillionth of one metre. This distance is sometimes called a fermi and was so named in honour of Italian naturalized to American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.
Definition and equivalents
1000000 zeptometres = 1 femtometre = 1 fermi = 0.000001 nanometre = {{val|e=-15|u=metres}}
{{val|1,000,000,000,000}} femtometres = 1 millimetre.
For example, the charge radius of a proton is approximately 0.841 femtometres{{cite web |url=http://perimeterinstitute.ca/news/case-shrinking-proton |title=The Case of the Shrinking Proton {{pipe}} Perimeter Institute |publisher=Perimeterinstitute.ca |date=2013-08-23 |access-date=2015-11-04 |archive-date=2014-04-23 |archive-url=https://web.archive.org/web/20140423003243/https://perimeterinstitute.ca/news/case-shrinking-proton |url-status=dead }} while
the radius of a gold nucleus is approximately 8.45 femtometres.Blatt, John M.; Weisskopf, Victor F. (1952), Theoretical Nuclear Physics, New York: Wiley, pp. 14–16.
1 barn = 100 fm2
History
The femtometre was adopted by the 11th Conférence Générale des Poids et Mesures, and added to the SI in 1964, using the Danish word for "15" and the similarity in spelling with fermi.
The fermi is named after the Italian physicist Enrico Fermi (1901–1954), one of the founders of nuclear physics. The term was coined by Robert Hofstadter in a 1956 paper published in Reviews of Modern Physics entitled "Electron Scattering and Nuclear Structure".[http://rmp.aps.org/abstract/RMP/v28/i3/p214_1 Hofstadter, Robert, Department of Physics, Stanford University, Stanford, California, "Electron Scattering and Nuclear Structure," Rev. Mod. Phys. 28, 214–254 (1956) The American Physical Society] The term is widely used by nuclear and particle physicists. When Hofstadter was awarded the 1961 Nobel Prize in Physics, it subsequently appeared in the text of his 1961 Nobel Lecture, "The electron-scattering method and its application to the structure of nuclei and nucleons" (December 11, 1961).[https://www.nobelprize.org/uploads/2018/06/hofstadter-lecture.pdf Hofstadter, Robert, "The electron-scattering method and its application to the structure of nuclei and nucleons," Nobel Lecture (December 11, 1961)]
References
{{reflist|25em}}
{{SI units of length}}
{{DEFAULTSORT:Femtometre}}