▁shape ▁of ▁the ▁pseud os phere .) ▁ ▁The ▁brown ▁line ▁on ▁the ▁diagram ▁is ▁the ▁world line ▁of ▁the ▁Earth ▁( or , ▁at ▁earlier ▁times , ▁of ▁the ▁matter ▁which ▁cond ensed ▁to ▁form ▁the ▁Earth ). ▁The ▁yellow ▁line ▁is ▁the ▁world line ▁of ▁the ▁most ▁distant ▁known ▁qu as ar . ▁The ▁red ▁line ▁is ▁the ▁path ▁of ▁a ▁light ▁beam ▁em itted ▁by ▁the ▁qu as ar ▁about ▁ 1 3 ▁billion ▁years ▁ago ▁and ▁reaching ▁the ▁Earth ▁in ▁the ▁present ▁day . ▁The ▁orange ▁line ▁shows ▁the ▁present - day ▁distance ▁between ▁the ▁qu as ar ▁and ▁the ▁Earth , ▁about ▁ 2 8 ▁billion ▁light ▁years , ▁which ▁is , ▁not ably , ▁a ▁larger ▁distance ▁than ▁the ▁age ▁of ▁the ▁universe ▁multip lied ▁by ▁the ▁speed ▁of ▁light : ▁c t . ▁ ▁According ▁to ▁the ▁equivalence ▁principle ▁of ▁general ▁relativ ity , ▁the ▁rules ▁of ▁special ▁relativ ity ▁are ▁locally ▁valid ▁in ▁small ▁regions ▁of ▁spac etime ▁that ▁are ▁approximately ▁flat . ▁In ▁particular , ▁light ▁always ▁travel s ▁locally ▁at ▁the ▁speed ▁c ; ▁in ▁our ▁diagram , ▁this ▁means , ▁according ▁to ▁the ▁convention ▁of ▁construct ing ▁spac etime ▁diag rams , ▁that ▁light ▁be ams ▁always ▁make ▁an ▁angle ▁of ▁ 4 5 ° ▁with ▁the ▁local ▁grid ▁lines . ▁It ▁does ▁not ▁follow , ▁however , ▁that ▁light ▁travel s ▁a ▁distance ▁c t ▁in ▁a ▁time ▁t , ▁as ▁the ▁red ▁world line ▁illustr ates . ▁While ▁it ▁always