▁to . ▁K eller ▁and ▁Nelson ▁have ▁argued ▁that ▁even ▁if ▁past ▁and ▁future ▁objects ▁do ▁not ▁exist , ▁there ▁can ▁still ▁be ▁definite ▁truth s ▁about ▁past ▁and ▁future ▁events , ▁and ▁thus ▁it ▁is ▁possible ▁that ▁a ▁future ▁truth ▁about ▁a ▁time ▁travel er ▁dec iding ▁to ▁travel ▁back ▁to ▁the ▁present ▁date ▁could ▁explain ▁the ▁time ▁travel er ' s ▁actual ▁appearance ▁in ▁the ▁present ; ▁these ▁views ▁are ▁cont ested ▁by ▁some ▁authors . ▁ ▁Pres ent ism ▁in ▁classical ▁spac etime ▁de ems ▁that ▁only ▁the ▁present ▁exists ; ▁this ▁is ▁not ▁recon cil able ▁with ▁special ▁relativ ity , ▁shown ▁in ▁the ▁following ▁example : ▁Alice ▁and ▁Bob ▁are ▁simultane ous ▁obser vers ▁of ▁event O . ▁For ▁Alice , ▁some ▁event E ▁is ▁simultane ous ▁with O , ▁but ▁for ▁Bob , ▁event E ▁is ▁in ▁the ▁past ▁or ▁future . ▁Therefore , ▁Alice ▁and ▁Bob ▁disag ree ▁about ▁what ▁exists ▁in ▁the ▁present , ▁which ▁contradict s ▁classical ▁present ism . ▁" Here - now ▁present ism " ▁attempts ▁to ▁recon cile ▁this ▁by ▁only ▁acknowled ging ▁the ▁time ▁and ▁space ▁of ▁a ▁single ▁point ; ▁this ▁is ▁uns atisf actory ▁because ▁objects ▁coming ▁and ▁going ▁from ▁the ▁" here - now " ▁alternate ▁between ▁real ▁and ▁un real , ▁in ▁addition ▁to ▁the ▁lack ▁of ▁a ▁privile ged ▁" here - now " ▁that ▁would ▁be ▁the ▁" real " ▁present . ▁" Rel ativ ized ▁present ism " ▁acknowled ges ▁that ▁there ▁are