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 by 'britannica' (www.britannica.com) relativity , wide-ranging physical theories formed by the German-born physicist  Albert Einstein . With his theories of  special relativity  (1905) and  general relativity  (1915), Einstein overthrew many assumptions  underlying  earlier physical theories, redefining in the process the fundamental concepts of  space ,  time ,  matter ,  energy , and  gravity . Along with  quantum mechanics , relativity is central to modern  physics . In particular, relativity provides the basis for understanding cosmic processes and the geometry of the  universe  itself. (Read Einstein’s 1926 Britannica essay on space-time.) Explaining E = mc 2 See all videos for this article “Special relativity” is limited to objects that are moving with respect to inertial frames of reference—i.e, in a state of  uniform motion  with respect to one another such that an observer cannot, by purely mechanical experiments, distinguish one from the other. Beginning with the beh