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A further unification of electromagnetism came with Einstein's [[Special relativity|special theory of relativity]]. According to special relativity, observers moving at different speeds relative to one another occupy different [[Frame of reference|observational frames of reference]]. If one observer is in motion relative to another observer then they experience [[length contraction]] where unmoving objects appear closer together to the observer in motion than to the observer at rest. Therefore, if an electron is moving at the same speed as the current in a neutral wire, then they experience the flowing electrons in the wire as standing still relative to it and the positive charges as contracted together. In the [[lab frame]], the electron is moving and so feels a magnetic force from the current in the wire but because the wire is neutral it feels no electric force. But in the electron's [[rest frame]], the positive charges seem closer together compared to the flowing electrons and so the wire seems positively charged. Therefore, in the electron's rest frame it feels no magnetic force (because it is not moving in its own frame) but it does feel an electric force due to the positively charged wire. This result from relativity proves that magnetic fields are just electric fields in a different reference frame (and vice versa) and so the two are different manifestations of the same underlying [[electromagnetic field]].<ref>{{Cite book| last=Purcell|first=Edward M. | title=Electricity and magnetism |date=2013 | isbn=978-1107014022| edition=Third| ___location=Cambridge |pages=235–68 |oclc=805015622}}</ref><ref>{{Cite web| title=The Feynman Lectures on Physics Vol. II Ch. 13: Magnetostatics | url=https://feynmanlectures.caltech.edu/II_13.html#Ch13-S6 |access-date=2018-10-30 |website=feynmanlectures.caltech.edu}}</ref><ref>A. French (1968) ''Special Relativity'', chapter 8 – Relativity and electricity, pp. 229–65, W.W. Norton.</ref>
== Conductors,
=== Conductors ===
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