, ▁and ▁ 6 0 ▁years ▁later ▁the ▁same ▁watch ▁is ▁brought ▁back ▁in ▁time ▁and ▁given ▁to ▁the ▁same ▁character . ▁Ross ▁states ▁that ▁entropy ▁of ▁the ▁watch ▁will ▁increase , ▁and ▁the ▁watch ▁carried ▁back ▁in ▁time ▁will ▁be ▁more ▁worn ▁with ▁each ▁repet ition ▁of ▁its ▁history . ▁The ▁second ▁law ▁of ▁therm od ynam ics ▁is ▁understood ▁by ▁modern ▁phys ic ists ▁to ▁be ▁a ▁statistical ▁law , ▁so ▁decre asing ▁entropy ▁or ▁non - in cre asing ▁entropy ▁are ▁not ▁impossible , ▁just ▁impro b able . ▁Additionally , ▁entropy ▁statist ically ▁increases ▁in ▁systems ▁which ▁are ▁isolated , ▁so ▁non - is ol ated ▁systems , ▁such ▁as ▁an ▁object , ▁that ▁interact ▁with ▁the ▁outside ▁world , ▁can ▁become ▁less ▁worn ▁and ▁decrease ▁in ▁entropy , ▁and ▁it ' s ▁possible ▁for ▁an ▁object ▁whose ▁world - line ▁forms ▁a ▁closed ▁loop ▁to ▁be ▁always ▁in ▁the ▁same ▁condition ▁in ▁the ▁same ▁point ▁of ▁its ▁history . ▁ ▁Daniel ▁Green berger ▁and ▁Karl ▁S vo zil ▁proposed ▁that ▁quantum ▁theory ▁gives ▁a ▁model ▁for ▁time ▁travel ▁where ▁the ▁past ▁must ▁be ▁self - cons istent . ▁ ▁In ▁fiction ▁▁ ▁Time ▁travel ▁them es ▁in ▁science ▁fiction ▁and ▁the ▁media ▁can ▁generally ▁be ▁grouped ▁into ▁three ▁categories : ▁imm utable ▁tim eline ; ▁mutable ▁tim eline ; ▁and ▁alternate ▁histor ies , ▁as ▁in ▁the ▁interact ing - many - world s ▁interpretation . ▁Fre qu ently ▁in ▁fiction , ▁tim eline ▁is ▁used ▁to ▁refer