Time travel is the concept of moving between different moments in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to a moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).
In her senior thesis, Einstein's Twin Paradox: A New Interpretation, Dana Scully theorized that, although time travel was a possibility, the extreme temperatures required for such a journey would be beyond human endurance.
Some years after 1997, tachyons were discovered, and thus time travel became possible. The research of Lisa Ianelli, Jason Nichols and Yonechi into cryobiology would provide a method of instantly preserving a human, thus making time travel safe for humans.
Distraught at the world that accessible Time Travel creates, Nichols would travel to 1997 in order to attempt to sabotage his past self's work. While he succeeds in murdering Yonechi and his past self, it is unclear if Inelli continues the work. (TXF: "Synchrony")
- The fact that some of the individuals met by Mulder in the "past" shared physical and character traits with individuals linked to him in the present suggests that the phenomena was hallucinatory or parapsychological.