This is another version of The Fermi Paradox. Basically, if time travel backwards in time is ever to become possible where are the travellers? Stephen Hawking presented this question by wondering why we aren't awash with thousands of time travellers.
Possible replies are:
  • Time travel back in time isn't possible.
  • Time travelers are here.
  • They haven't gotten this far back yet.
  • They open a new dimension and don't appear in ours.
  • They died before inventing time travel.

Time travellers may be here but have technology that doesn't allow us to see them. It is also possible that the mystical creatures and UFO's some see are, in fact, time travellers from our future. Perhaps, because of the Grandfather Paradox the universe conspires to keep us from seeing them. For example, when they walk by we look the other way and when we turn around they duck.

Maybe their time machine doesn't allow them to come far enough back to see us. It could take too much power or only go back to a critical date. And it is always possible that the civilization dies before it ever creates the time machine.

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