▁Alc ub ierre ▁drives . ▁The ▁theory ▁of ▁general ▁relativ ity ▁does ▁suggest ▁a ▁scientific ▁basis ▁for ▁the ▁possibility ▁of ▁back ward ▁time ▁travel ▁in ▁certain ▁unusual ▁scenarios , ▁although ▁arguments ▁from ▁sem ic lass ical ▁gravity ▁suggest ▁that ▁when ▁quantum ▁effects ▁are ▁incorpor ated ▁into ▁general ▁relativ ity , ▁these ▁loop hol es ▁may ▁be ▁closed . ▁These ▁sem ic lass ical ▁arguments ▁led ▁Stephen ▁Haw king ▁to ▁form ulate ▁the ▁chron ology ▁protection ▁conject ure , ▁suggesting ▁that ▁the ▁fundamental ▁laws ▁of ▁nature ▁prevent ▁time ▁travel , ▁but ▁phys ic ists ▁cannot ▁come ▁to ▁a ▁definite ▁judgment ▁on ▁the ▁issue ▁without ▁a ▁theory ▁of ▁quantum ▁gravity ▁to ▁join ▁quantum ▁mechan ics ▁and ▁general ▁relativ ity ▁into ▁a ▁completely ▁un ified ▁theory . ▁ ▁D ifferent ▁spac etime ▁geomet ries ▁ ▁The ▁theory ▁of ▁general ▁relativ ity ▁describes ▁the ▁universe ▁under ▁a ▁system ▁of ▁field ▁equations ▁that ▁determine ▁the ▁metric , ▁or ▁distance ▁function , ▁of ▁spac etime . ▁There ▁exist ▁exact ▁solutions ▁to ▁these ▁equations ▁that ▁include ▁closed ▁time - like ▁curves , ▁which ▁are ▁world ▁lines ▁that ▁intersect ▁themselves ; ▁some ▁point ▁in ▁the ▁caus al ▁future ▁of ▁the ▁world ▁line ▁is ▁also ▁in ▁its ▁caus al ▁past , ▁a ▁situation ▁that ▁can ▁be ▁described ▁as ▁time ▁travel . ▁Such ▁a ▁solution ▁was ▁first ▁proposed ▁by ▁Kurt ▁Gö del , ▁a ▁solution ▁known ▁as ▁the ▁Gö del ▁metric , ▁but ▁his ▁( and ▁others ') ▁solution ▁requires ▁the ▁universe ▁to ▁have ▁physical ▁characteristics ▁that ▁it ▁does ▁not ▁appear ▁to ▁have