Editing Length contraction (section)

== Magnetic forces ==
{{Main|Relativistic electromagnetism}}
Magnetic forces are caused by relativistic contraction when electrons are moving relative to atomic nuclei. The magnetic force on a moving charge next to a current-carrying wire is a result of relativistic motion between electrons and protons.<ref>{{Cite web|url=https://www.feynmanlectures.caltech.edu/II_13.html|title=The Feynman Lectures on Physics Vol. II Ch. 13: Magnetostatics|website=www.feynmanlectures.caltech.edu}}</ref><ref name=Landau>{{cite book |author=E M Lifshitz, L D Landau |title=The classical theory of ields |volume=2 |series=[[Course of Theoretical Physics]] |edition=Fourth |publisher= Butterworth-Heinemann |location=Oxford UK |year=1980 |isbn=0-7506-2768-9 |url=http://worldcat.org/isbn/0750627689}}</ref>

In 1820, [[André-Marie Ampère]] showed that parallel wires having currents in the same direction attract one another. In the electrons' frame of reference, the moving wire contracts slightly, causing the protons of the opposite wire to be locally ''denser''.  As the electrons in the opposite wire are moving as well, they do not contract (as much). This results in an apparent local imbalance between electrons and protons; the moving electrons in one wire are attracted to the extra protons in the other. The reverse can also be considered.  To the static proton's frame of reference, the electrons are moving and contracted, resulting in the same imbalance. The electron [[drift velocity]] is relatively very slow, on the order of a meter an hour but the force between an electron and proton is so enormous that even at this very slow speed the relativistic contraction causes significant effects.

This effect also applies to magnetic particles without current, with current being replaced with electron spin.{{citation needed|date=December 2016}}