▁one ▁can ▁define ▁an ▁in ert ial ▁reference ▁frame ▁in ▁which ▁the ▁acceler ated ▁body ▁is ▁tempor arily ▁at ▁rest . ▁It ▁follows ▁that ▁in ▁acceler ated ▁frames ▁defined ▁in ▁this ▁way , ▁the ▁application ▁of ▁the ▁const ancy ▁of ▁the ▁speed ▁of ▁light ▁to ▁define ▁simultane ity ▁is ▁restricted ▁to ▁small ▁local ities . ▁However , ▁the ▁equivalence ▁principle ▁that ▁was ▁used ▁by ▁Ein stein ▁in ▁the ▁course ▁of ▁that ▁investigation , ▁which ▁express es ▁the ▁equality ▁of ▁in ert ial ▁and ▁gravit ational ▁mass ▁and ▁the ▁equivalence ▁of ▁acceler ated ▁frames ▁and ▁hom ogeneous ▁gravit ational ▁fields , ▁trans c ended ▁the ▁limits ▁of ▁special ▁relativ ity ▁and ▁resulted ▁in ▁the ▁form ulation ▁of ▁general ▁relativ ity . ▁ ▁Near ly ▁simultaneously ▁with ▁Ein stein , ▁also ▁M ink owski ▁( 1 9 0 8 ) ▁considered ▁the ▁special ▁case ▁of ▁uniform ▁acceler ations ▁within ▁the ▁framework ▁of ▁his ▁space - time ▁formal ism . ▁He ▁recognized ▁that ▁the ▁world - line ▁of ▁such ▁an ▁acceler ated ▁body ▁corresponds ▁to ▁a ▁hyper bol a . ▁This ▁notion ▁was ▁further ▁developed ▁by ▁Born ▁( 1 9 0 9 ) ▁and ▁Sommer feld ▁( 1 9 1 0 ), ▁with ▁Born ▁introdu cing ▁the ▁expression ▁" hyper bol ic ▁motion ". ▁He ▁noted ▁that ▁uniform ▁acceleration ▁can ▁be ▁used ▁as ▁an ▁approximation ▁for ▁any ▁form ▁of ▁acceleration ▁within ▁special ▁relativ ity . ▁In ▁addition , ▁Harry ▁Bat eman ▁and ▁Eb ene zer ▁C unning ham ▁( 1 9 1 0 ) ▁showed ▁that