▁of ▁the ▁body . ▁However , ▁many ▁modern ▁text books ▁on ▁relativ ity ▁don ' t ▁use ▁the ▁concept ▁of ▁relativ istic ▁mass ▁anymore , ▁and ▁mass ▁in ▁special ▁relativ ity ▁is ▁considered ▁as ▁an ▁invariant ▁quantity . ▁ ▁Mass ▁and ▁energy ▁Ein stein ▁( 1 9 0 6 ) ▁showed ▁that ▁the ▁in ert ia ▁of ▁energy ▁( mass - energy - equ ival ence ) ▁is ▁a ▁necessary ▁and ▁sufficient ▁condition ▁for ▁the ▁conservation ▁of ▁the ▁center ▁of ▁mass ▁theorem . ▁On ▁that ▁occasion , ▁he ▁noted ▁that ▁the ▁formal ▁mathematical ▁content ▁of ▁Po inc ar é ▁paper ▁on ▁the ▁center ▁of ▁mass ▁( 1 9 0 0 b ) ▁and ▁his ▁own ▁paper ▁were ▁mainly ▁the ▁same , ▁although ▁the ▁physical ▁interpretation ▁was ▁different ▁in ▁light ▁of ▁relativ ity . ▁ ▁Kur d ▁von ▁Mos en ge il ▁( 1 9 0 6 ) ▁by ▁extending ▁Has en ö hr l ' s ▁calculation ▁of ▁black - body - rad iation ▁in ▁a ▁cav ity , ▁derived ▁the ▁same ▁expression ▁for ▁the ▁additional ▁mass ▁of ▁a ▁body ▁due ▁to ▁elect romagnet ic ▁radiation ▁as ▁Has en ö hr l . ▁Has en ö hr l ' s ▁idea ▁was ▁that ▁the ▁mass ▁of ▁bodies ▁included ▁a ▁contribution ▁from ▁the ▁elect romagnet ic ▁field , ▁he ▁imag ined ▁a ▁body ▁as ▁a ▁cav ity ▁containing ▁light . ▁His ▁relationship ▁between ▁mass ▁and ▁energy , ▁like ▁all ▁other ▁pre - E in stein ▁ones , ▁contained ▁incorrect ▁numerical ▁pre fact ors ▁(