5 ▁that ▁Lor ent z ▁in var iance ▁is ▁an ▁exact ▁condition ▁for ▁any ▁physical ▁theory ): ▁ ▁Mass - energy ▁equivalence ▁ ▁Al ready ▁in ▁§ 1 0 ▁of ▁his ▁paper ▁on ▁elect rod ynam ics , ▁Ein stein ▁used ▁the ▁formula ▁ ▁for ▁the ▁kin etic ▁energy ▁of ▁an ▁electron . ▁In ▁elabor ation ▁of ▁this ▁he ▁published ▁a ▁paper ▁( rece ived ▁September ▁ 2 7 , ▁November ▁ 1 9 0 5 ), ▁in ▁which ▁Ein stein ▁showed ▁that ▁when ▁a ▁material ▁body ▁lost ▁energy ▁( e ither ▁radiation ▁or ▁heat ) ▁of ▁amount ▁E , ▁its ▁mass ▁decre ased ▁by ▁the ▁amount ▁ ▁This ▁led ▁to ▁the ▁famous ▁mass – energy ▁equivalence ▁formula : ▁E   =   mc 2 . ▁Ein stein ▁considered ▁the ▁equival ency ▁equation ▁to ▁be ▁of ▁param ount ▁importance ▁because ▁it ▁showed ▁that ▁a ▁massive ▁particle ▁poss esses ▁an ▁energy , ▁the ▁" rest ▁energy ", ▁distinct ▁from ▁its ▁classical ▁kin etic ▁and ▁potential ▁energ ies . ▁As ▁it ▁was ▁shown ▁above , ▁many ▁authors ▁before ▁Ein stein ▁arrived ▁at ▁similar ▁formulas ▁( including ▁a ▁ 4 / 3 - factor ) ▁for ▁the ▁relation ▁of ▁mass ▁to ▁energy . ▁However , ▁their ▁work ▁was ▁focused ▁on ▁elect romagnet ic ▁energy ▁which ▁( as ▁we ▁know ▁today ) ▁only ▁represents ▁a ▁small ▁part ▁of ▁the ▁entire ▁energy ▁within ▁matter . ▁So ▁it ▁was ▁Ein stein ▁who ▁was ▁the ▁first ▁to : ▁( a ) ▁as cribe ▁this ▁relation ▁to ▁all ▁forms ▁of ▁energy