En tidsrejse betegner dét at kunne "rejse" enten frem eller tilbage i tiden[1][2]
på en måde, der i en eller anden grad adskiller sig fra det normale intersubjektive tidsforløb. Kosmonauten Sergei Avdeyev var i 2015 den person, som havde rejst længst frem i tid. Sergei Avdeyev har rejst 1/50 sekund frem i tiden over en periode på 748 døgn i forhold til mennesker på jorden.
Ifølge relativitetsteorien er det muligt, ved hjælp af tilført energi, at bremse tidshastigheden lokalt. Rejser tilbage i tiden er dog stadig science fiction, og nogen mener, at det ligefrem kan bevises, at det projekt for altid vil forblive umuligt.
Andre videnskabsfolk mener dog, at det kan lade sig gøre.
[3]
^November 14, 2001, Princeton University: Time travel: Truth not always stranger than science fiction Citat: "..."We have had time travelers already," Gott said in an interview at his Peyton Hall office. "The greatest time traveler so far is (cosmonaut) Sergei Avdeyev, who, by virtue of being on space flights for 748 days, is one-fiftieth of a second younger than if he had stayed home. So that man has traveled one-fiftieth of a second into the future."...Regarding time travel into the past, Gott said it may be possible, but much more difficult...", backup
^18 December 2003, BBC two: Time Trip – transcript Citat: "...Prof RICHARD GOTT: If the cosmic strings moved rapidly enough but still slower than the speed of light, the solution was sufficiently twisted so that you could start here, circle the two cosmic strings whilst they were passing and come back to an event in your own past...You could leave Planet B at noon here and travel over here to arrive back at Planet A at noon. In fact you could arrive back in time to shake hands with your younger self and wish yourself a nice trip around the cosmic string...Prof RICHARD GOTT: So that makes a time machine...", backup