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.