“Miscellaneous § How Would You Prove You Are a Time-Traveler From the Past?”, Gwern2009-08-05 (; similar)⁠:

Misc thoughts, memories, proto-essays, musings, etc.

SF/F fiction frequently considers the case of a time-traveler from the future to the past, who can prove himself by use of advanced knowledge and items from the future. In the reverse case, a time-traveler from the past to the future wishes to prove he is from the past and that time-travel is real. How can he do this when all past knowledge is already known or whose chain of custody being broken is more likely than time-travel being real? I suggest 8 methods: carbon-14 nuclear isotope dating of their body as isotopes cannot be removed; sequencing of their genome to check consistency with pedigree as human genomes cannot be synthesized or edited on a large scale; selection & mutation clocks, likewise; immune system signatures of extinct or rare diseases such as smallpox, and accumulated pollution such as heavy metals, difficult & dangerous to fake. While these proofs may not offer conclusive proof since any human system can be subverted with enough effort, they can provide enough evidence to launch research into time travel and a definitive finding.

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