▁speed ▁of ▁light ▁is ▁invariant ▁for ▁all ▁obser vers ▁in ▁any ▁frame ▁of ▁reference ; ▁that ▁is , ▁it ▁is ▁always ▁the ▁same . ▁Time ▁d ilation ▁is ▁a ▁direct ▁consequence ▁of ▁the ▁in var iance ▁of ▁the ▁speed ▁of ▁light . ▁Time ▁d ilation ▁may ▁be ▁regarded ▁in ▁a ▁limited ▁sense ▁as ▁" time ▁travel ▁into ▁the ▁future ": ▁a ▁person ▁may ▁use ▁time ▁d ilation ▁so ▁that ▁a ▁small ▁amount ▁of ▁proper ▁time ▁passes ▁for ▁them , ▁while ▁a ▁large ▁amount ▁of ▁proper ▁time ▁passes ▁elsewhere . ▁This ▁can ▁be ▁achieved ▁by ▁travel ing ▁at ▁relativ istic ▁spe eds ▁or ▁through ▁the ▁effects ▁of ▁gravity . ▁ ▁For ▁two ▁identical ▁clock s ▁moving ▁relative ▁to ▁each ▁other ▁without ▁acceler ating , ▁each ▁clock ▁measures ▁the ▁other ▁to ▁be ▁tick ing ▁slower . ▁This ▁is ▁possible ▁due ▁to ▁the ▁relativ ity ▁of ▁simultane ity . ▁However , ▁the ▁symmetry ▁is ▁broken ▁if ▁one ▁clock ▁acceler ates , ▁allowing ▁for ▁less ▁proper ▁time ▁to ▁pass ▁for ▁one ▁clock ▁than ▁the ▁other . ▁The ▁tw in ▁par ado x ▁describes ▁this : ▁one ▁tw in ▁remains ▁on ▁Earth , ▁while ▁the ▁other ▁under go es ▁acceleration ▁to ▁relativ istic ▁speed ▁as ▁they ▁travel ▁into ▁space , ▁turn ▁around , ▁and ▁travel ▁back ▁to ▁Earth ; ▁the ▁travel ing ▁tw in ▁ages ▁less ▁than ▁the ▁tw in ▁who ▁stayed ▁on ▁Earth , ▁because ▁of ▁the ▁time ▁d ilation ▁experienced ▁during ▁their ▁acceleration . ▁General ▁relativ ity ▁tre ats ▁the ▁effects ▁of ▁acceleration ▁and ▁the ▁effects