▁For ▁example , ▁Arnold ▁Sommer feld ▁( 1 9 1 0 ) ▁replaced ▁M ink owski ' s ▁matrix ▁notation ▁by ▁an ▁elegant ▁vector ▁notation ▁and ▁co ined ▁the ▁terms ▁" four ▁vector " ▁and ▁" six ▁vector ". ▁He ▁also ▁introduced ▁a ▁trig on ometric ▁form ulation ▁of ▁the ▁relativ istic ▁velocity ▁addition ▁rule , ▁which ▁according ▁to ▁Sommer feld , ▁removes ▁much ▁of ▁the ▁str angen ess ▁of ▁that ▁concept . ▁Other ▁important ▁contributions ▁were ▁made ▁by ▁La ue ▁( 1 9 1 1 , ▁ 1 9 1 3 ), ▁who ▁used ▁the ▁spac etime ▁formal ism ▁to ▁create ▁a ▁relativ istic ▁theory ▁of ▁de form able ▁bodies ▁and ▁an ▁elementary ▁particle ▁theory . ▁He ▁extended ▁M ink owski ' s ▁expressions ▁for ▁elect romagnet ic ▁processes ▁to ▁all ▁possible ▁forces ▁and ▁thereby ▁clar ified ▁the ▁concept ▁of ▁mass - energy - equ ival ence . ▁La ue ▁also ▁showed ▁that ▁non - elect rical ▁forces ▁are ▁needed ▁to ▁ensure ▁the ▁proper ▁Lor ent z ▁transformation ▁properties , ▁and ▁for ▁the ▁stability ▁of ▁matter   – ▁he ▁could ▁show ▁that ▁the ▁" Po inc ar é ▁str esses " ▁( as ▁mentioned ▁above ) ▁are ▁a ▁natural ▁consequence ▁of ▁relativ ity ▁theory ▁so ▁that ▁the ▁electron ▁can ▁be ▁a ▁closed ▁system . ▁ ▁Lor ent z ▁transformation ▁without ▁second ▁post ulate ▁There ▁were ▁some ▁attempts ▁to ▁derive ▁the ▁Lor ent z ▁transformation ▁without ▁the ▁post ulate ▁of ▁the ▁const ancy ▁of ▁the ▁speed ▁of ▁light . ▁Vladimir ▁Ign at owski