▁is ▁the ▁distance ▁between ▁the ▁mirror s ▁divided ▁by ▁the ▁speed ▁of ▁light . ▁But ▁if ▁the ▁observer ▁looks ▁at ▁clock ▁B , ▁he ▁sees ▁that ▁within ▁that ▁clock ▁the ▁signal ▁traces ▁out ▁a ▁longer , ▁ang led ▁path , ▁thus ▁clock ▁B ▁is ▁slower ▁than ▁A . ▁However , ▁for ▁the ▁observer ▁moving ▁alongside ▁with ▁B ▁the ▁situation ▁is ▁completely ▁in ▁reverse : ▁C lock ▁B ▁is ▁faster ▁and ▁A ▁is ▁slower . ▁Also ▁Lor ent z ▁( 1 9 1 0 – 1 9 1 2 ) ▁discussed ▁the ▁reci pro city ▁of ▁time ▁d ilation ▁and ▁analyz ed ▁a ▁clock ▁" par ado x ", ▁which ▁apparently ▁occurs ▁as ▁a ▁consequence ▁of ▁the ▁reci pro city ▁of ▁time ▁d ilation . ▁Lor ent z ▁showed ▁that ▁there ▁is ▁no ▁par ado x ▁if ▁one ▁cons iders ▁that ▁in ▁one ▁system ▁only ▁one ▁clock ▁is ▁used , ▁while ▁in ▁the ▁other ▁system ▁two ▁clock s ▁are ▁necessary , ▁and ▁the ▁relativ ity ▁of ▁simultane ity ▁is ▁fully ▁taken ▁into ▁account . ▁▁ ▁A ▁similar ▁situation ▁was ▁created ▁by ▁Paul ▁L ange vin ▁in ▁ 1 9 1 1 ▁with ▁what ▁was ▁later ▁called ▁the ▁" t win ▁par ado x ", ▁where ▁he ▁replaced ▁the ▁clock s ▁by ▁persons ▁( L ange vin ▁never ▁used ▁the ▁word ▁" tw ins " ▁but ▁his ▁description ▁contained ▁all ▁other ▁features ▁of ▁the ▁par ado x ). ▁L ange vin ▁solved ▁the ▁par ado x ▁by ▁all ud ing ▁to ▁the ▁fact ▁that ▁one