▁ 1 9 1 2 ▁that ▁he ▁borrow ed ▁that ▁principle ▁from ▁Lor ent z ' s ▁station ary ▁a ether ▁( which ▁implies ▁valid ity ▁of ▁Max well ' s ▁equations ▁and ▁the ▁const ancy ▁of ▁light ▁in ▁the ▁a ether ▁frame ), ▁but ▁he ▁recognized ▁that ▁this ▁principle ▁together ▁with ▁the ▁principle ▁of ▁relativ ity ▁makes ▁any ▁reference ▁to ▁an ▁a ether ▁unnecessary ▁( at ▁least ▁as ▁to ▁the ▁description ▁of ▁elect rod ynam ics ▁in ▁in ert ial ▁frames ). ▁As ▁he ▁wrote ▁in ▁ 1 9 0 7 ▁and ▁in ▁later ▁papers , ▁the ▁apparent ▁contradiction ▁between ▁those ▁principles ▁can ▁be ▁resolved ▁if ▁it ▁is ▁admitted ▁that ▁Lor ent z ' s ▁local ▁time ▁is ▁not ▁an ▁auxili ary ▁quantity , ▁but ▁can ▁simply ▁be ▁defined ▁as ▁time ▁and ▁is ▁connected ▁with ▁signal ▁velocity . ▁Before ▁Ein stein , ▁Po inc ar é ▁also ▁developed ▁a ▁similar ▁physical ▁interpretation ▁of ▁local ▁time ▁and ▁noticed ▁the ▁connection ▁with ▁signal ▁velocity , ▁but ▁contrary ▁to ▁Ein stein ▁he ▁continued ▁to ▁argue ▁that ▁clock s ▁at ▁rest ▁in ▁the ▁station ary ▁a ether ▁show ▁the ▁true ▁time , ▁while ▁clock s ▁in ▁in ert ial ▁motion ▁relative ▁to ▁the ▁a ether ▁show ▁only ▁the ▁apparent ▁time . ▁Event ually , ▁near ▁the ▁end ▁of ▁his ▁life ▁in ▁ 1 9 5 3 ▁Ein stein ▁described ▁the ▁advantages ▁of ▁his ▁theory ▁over ▁that ▁of ▁Lor ent z ▁as ▁follows ▁( although ▁Po inc ar é ▁had ▁already ▁stated ▁in ▁ 1 9 0