Introduction to special relativity: Difference between revisions

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[[File:Einstein1921 by F Schmutzer 2.jpg|right|thumb|236px|Albert Einstein during a lecture in Vienna in 1921.]]

In [[physics]], '''[[special relativity]]''' is a fundamental [[theory]] concerning [[space]] and [[time]], developed by [[Albert Einstein]] in 1905<ref>"On the Electrodynamicslectrodynamics of Moving Bodies". (fourmilab.ch web site): [http://www.fourmilab.ch/etexts/einstein/specrel/www/ Translation from the German article]: "Zur Elektrodynamik bewegter Körper", ''Annalen der Physik''. '''17''':891-921. (June 30, 1905)</ref> as a modification of [[Galilean invariance|Galilean relativity]]. (See "[[History of special relativity]]" for a detailed account and the contributions of [[Hendrik Lorentz]] and [[Henri Poincaré]].) The theory was able to explain some pressing [[theoretical physics|theoretical]] and [[experimental physics|experimental]] issues in the physics of the time involving [[light]] and [[electrodynamics]], such as the failure of the 1887 [[Michelson–Morley experiment]], which aimed to measure differences in the relative speed of light due to the [[Earth's orbit|Earth's motion]] through the hypothetical, and now discredited, [[luminiferous aether]]. The aether was then considered to be the medium of propagation of [[electromagnetic wave]]s such as light.

Einstein postulated that the [[speed of light]] in [[free space]] is the same for all [[observer (special relativity)|observer]]s, regardless of their motion relative to the light source, where we may think of an observer as an imaginary entity with a sophisticated set of measurement devices, at rest with respect to itself, that perfectly records the positions and times of all events in space and time. This postulate stemmed from the assumption that [[Maxwell's equations]] of [[electromagnetism]], which predict a specific speed of light in a vacuum, hold in any [[inertial frame of reference]]<ref name=Gabriel>{{cite book |title=Introduction to the Theory of Relativity |author=Peter Gabriel Bergmann |url=http://books.google.com/?id=3cE9jXr_QhwC&pg=PA3&dq=reference+frame+%22coordinate+system%22+choose |isbn= 0-486-63282-2 |others=Reprint of first edition of 1942 with a forward by A. Einstein |publisher=Courier Dover Publications |pages=''xi'' |year=1976}}</ref> rather than, as was previously believed, just in the frame of the aether. This prediction contradicted the laws of [[classical mechanics]], which had been accepted for centuries, by arguing that time and space are not fixed and in fact change to maintain a constant speed of light regardless of the relative motions of sources and observers. Einstein's approach was based on [[thought experiment]]s, calculations, and the [[principle of relativity]], which is the notion that all physical laws should appear the same (that is, take the same basic form) to all inertial observers. Today, the result is that the speed of light defines the [[metre]] as "the length of the path travelled by light in vacuum during a time interval of {{nowrap|1/299 792 458}} of a second."<ref>{{cite web |url=http://www.bipm.org/fr/CGPM/db/17/1/ |title=Définition du mètre |accessdate=2008-10-03 |year=1983 |work={{lang|fr|Résolution 1 de la 17^e réunion de la CGPM}} |publisher=[[Bureau International des Poids et Mesures]] |___location=Sèvres |language=French |quote={{lang|fr|Le mètre est la longueur du trajet parcouru dans le vide par la lumière pendant une durée de {{nowrap|1/299 792 458}} de seconde.}}}} English translation: {{cite web |url=http://www.bipm.org/en/CGPM/db/17/1/ |title=Definition of the metre |accessdate=2008-10-03 |work=Resolution 1 of the 17th meeting of the CGPM}}</ref> This relates that the speed of light is ''by convention'' {{nowrap|299 792 458 m/s}} (approximately 1.079&nbsp;billion kilometres per hour, or 671&nbsp;million miles per hour).