Editing Special relativity (section)

== Traditional "two postulates" approach to special relativity <span class="anchor" id="Postulates"></span> ==
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| quote = "Reflections of this type made it clear to me as long ago as shortly after 1900, i.e., shortly after Planck's trailblazing work, that neither mechanics nor electrodynamics could (except in limiting cases) claim exact validity. Gradually I despaired of the possibility of discovering the true laws by means of constructive efforts based on known facts. The longer and the more desperately I tried, the more I came to the conviction that only the discovery of a universal formal principle could lead us to assured results&nbsp;... How, then, could such a universal principle be found?"
| source = Albert Einstein: ''Autobiographical Notes''<ref name="autogenerated1" group=p>Einstein, Autobiographical Notes, 1949.</ref>
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}}{{Main|Postulates of special relativity}}
Einstein discerned two fundamental propositions that seemed to be the most assured, regardless of the exact validity of the (then) known laws of either mechanics or electrodynamics. These propositions were the constancy of the speed of light in vacuum and the independence of physical laws (especially the constancy of the speed of light) from the choice of inertial system. In his initial presentation of special relativity in 1905 he expressed these postulates as:<ref name=electro group=p/>
* The [[principle of relativity]] – the laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.<ref name=electro group=p/>
* The principle of invariant light speed – "...&nbsp;light is always propagated in empty space with a definite velocity [speed] ''c'' which is independent of the state of motion of the emitting body" (from the preface).<ref name=electro group=p/> That is, light in vacuum propagates with the speed ''c'' (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.

The constancy of the speed of light was motivated by [[Maxwell's theory of electromagnetism]]<ref>{{Cite web |date=2006-12-01 |title=James Clerk Maxwell: a force for physics |url=https://physicsworld.com/james-clerk-maxwell-a-force-for-physics/ |access-date=2024-03-22 |website=Physics World |language=en-GB}}</ref> and the lack of evidence for the [[luminiferous ether]].<ref>{{Cite web |title=November 1887: Michelson and Morley report their failure to detect the luminiferous ether |url=http://www.aps.org/publications/apsnews/200711/physicshistory.cfm |access-date=2024-03-22 |website=www.aps.org |language=en}}</ref> There is conflicting evidence on the extent to which Einstein was influenced by the null result of the Michelson–Morley experiment.<ref>[[Michael Polanyi]] (1974) ''Personal Knowledge: Towards a Post-Critical Philosophy'', {{isbn|0-226-67288-3}}, footnote page 10–11: Einstein reports, via Dr N Balzas in response to Polanyi's query, that "The Michelson–Morley experiment had no role in the foundation of the theory." and "...&nbsp;the theory of relativity was not founded to explain its outcome at all". [https://books.google.com/books?id=0Rtu8kCpvz4C&lpg=PP1&pg=PT19]</ref><ref name="mM1905"/> In any case, the null result of the Michelson–Morley experiment helped the notion of the constancy of the speed of light gain widespread and rapid acceptance.

The derivation of special relativity depends not only on these two explicit postulates, but also on several tacit assumptions ([[Duhem–Quine thesis|made in almost all theories of physics]]), including the [[isotropy]] and [[homogeneity (physics)|homogeneity]] of space and the independence of measuring rods and clocks from their past history.<ref group=p>Einstein, "Fundamental Ideas and Methods of the Theory of Relativity", 1920</ref>