▁exceed ▁the ▁speed ▁of ▁light . ▁Also ▁for ▁Lor ent z ▁( 1 8 9 9 ), ▁the ▁integration ▁of ▁the ▁speed - dep end ence ▁of ▁masses ▁recognized ▁by ▁Thom son ▁was ▁especially ▁important . ▁He ▁noticed ▁that ▁the ▁mass ▁not ▁only ▁varied ▁due ▁to ▁speed , ▁but ▁is ▁also ▁dependent ▁on ▁the ▁direction , ▁and ▁he ▁introduced ▁what ▁Abraham ▁later ▁called ▁" long itud inal " ▁and ▁" trans verse " ▁mass . ▁( The ▁trans verse ▁mass ▁corresponds ▁to ▁what ▁later ▁was ▁called ▁relativ istic ▁mass .) ▁ ▁Wilhelm ▁Wien ▁( 1 9 0 0 ) ▁assumed ▁( follow ing ▁the ▁works ▁of ▁Thom son , ▁He av is ide , ▁and ▁S ear le ) ▁that ▁the ▁entire ▁mass ▁is ▁of ▁elect romagnet ic ▁origin , ▁which ▁was ▁form ulated ▁in ▁the ▁context ▁that ▁all ▁forces ▁of ▁nature ▁are ▁elect romagnet ic ▁ones ▁( the ▁" E lect romagnet ic ▁World ▁View "). ▁Wien ▁stated ▁that , ▁if ▁it ▁is ▁assumed ▁that ▁grav itation ▁is ▁an ▁elect romagnet ic ▁effect ▁too , ▁then ▁there ▁has ▁to ▁be ▁a ▁proportional ity ▁between ▁elect romagnet ic ▁energy , ▁in ert ial ▁mass ▁and ▁gravit ational ▁mass . ▁In ▁the ▁same ▁paper ▁Henri ▁Po inc ar é ▁( 1 9 0 0 b ) ▁found ▁another ▁way ▁of ▁combining ▁the ▁concepts ▁of ▁mass ▁and ▁energy . ▁He ▁recognized ▁that ▁elect romagnet ic ▁energy ▁behav es ▁like ▁a ▁fict it ious ▁fluid ▁with ▁mass ▁density ▁of ▁ ▁( or ▁) ▁and ▁defined ▁a