▁of ▁gravity ▁as ▁equivalent , ▁and ▁shows ▁that ▁time ▁d ilation ▁also ▁occurs ▁in ▁gravity ▁well s , ▁with ▁a ▁clock ▁deeper ▁in ▁the ▁well ▁tick ing ▁more ▁slowly ; ▁this ▁effect ▁is ▁taken ▁into ▁account ▁when ▁cal ibr ating ▁the ▁clock s ▁on ▁the ▁sat ell ites ▁of ▁the ▁Global ▁Position ing ▁System , ▁and ▁it ▁could ▁lead ▁to ▁significant ▁differences ▁in ▁rates ▁of ▁ag ing ▁for ▁obser vers ▁at ▁different ▁distances ▁from ▁a ▁large ▁gravity ▁well ▁such ▁as ▁a ▁black ▁hole . ▁ ▁A ▁time ▁machine ▁that ▁util izes ▁this ▁principle ▁might ▁be , ▁for ▁instance , ▁a ▁spher ical ▁shell ▁with ▁a ▁diameter ▁of ▁five ▁meters ▁and ▁the ▁mass ▁of ▁Jup iter . ▁A ▁person ▁at ▁its ▁center ▁will ▁travel ▁forward ▁in ▁time ▁at ▁a ▁rate ▁four ▁times ▁that ▁of ▁distant ▁obser vers . ▁S que ez ing ▁the ▁mass ▁of ▁a ▁large ▁planet ▁into ▁such ▁a ▁small ▁structure ▁is ▁not ▁expected ▁to ▁be ▁within ▁human ity ' s ▁techn ological ▁capabilities ▁in ▁the ▁near ▁future . ▁With ▁current ▁techn ologies , ▁it ▁is ▁only ▁possible ▁to ▁cause ▁a ▁human ▁travel er ▁to ▁age ▁less ▁than ▁compan ions ▁on ▁Earth ▁by ▁a ▁few ▁mill iseconds , ▁the ▁current ▁record ▁being ▁about ▁ 2 0 ▁mill iseconds ▁for ▁the ▁cos mon aut ▁Serge i ▁K rik ale v . ▁ ▁Philosoph y ▁▁ ▁Philosoph ers ▁have ▁discussed ▁the ▁nature ▁of ▁time ▁since ▁at ▁least ▁the ▁time ▁of ▁ancient ▁Greece ; ▁for ▁example , ▁Par men ides ▁presented ▁the ▁view ▁that ▁time