Editing Principle of locality (section)

== Pre-quantum mechanics ==
{{main article|Action at a distance}}

During the 17th century, Newton's principle of [[Newton's law of universal gravitation|universal gravitation]] was formulated in terms of "action at a distance", thereby violating the principle of locality. Newton himself considered this violation to be absurd:
{{quote|It is inconceivable that inanimate Matter should, without the Mediation of something else, which is not material, operate upon, and affect other matter without mutual Contact…That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro' a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers.<ref name="sep-qm-action-distance">{{cite encyclopedia |first=Joseph |last=Berkovitz |editor=Edward N. Zalta |year=2008 |title=Action at a Distance in Quantum Mechanics |encyclopedia=The Stanford Encyclopedia of Philosophy |edition=Winter |url=http://plato.stanford.edu/archives/win2008/entries/qm-action-distance/#ActDisCoExiNonSepHol}}</ref>|Isaac Newton|Letters to [[Richard Bentley|Bentley]], 1692/3}}

[[Coulomb's law]] of electric forces was initially also formulated as instantaneous action at a distance, but in 1880, [[James Clerk Maxwell]] showed that [[Maxwell's equations| field equations]] – which obey locality – predict all of the phenomena of electromagnetism.{{cn|date=October 2023}} These equations show that electromagnetic forces propagate at the speed of light.

In 1905, [[Albert Einstein]]'s [[special theory of relativity]] postulated that no matter or energy can travel faster than the speed of light, and Einstein thereby sought to reformulate physics in a way that obeyed the principle of locality. He later succeeded in producing an alternative theory of gravitation, [[general relativity]], which obeys the principle of locality.

However, a different challenge to the principle of locality developed subsequently from the theory of [[quantum mechanics]], which Einstein himself had helped to create.