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AB: Somebody recently said, and I have been considering since I heard it, a very simple question: if time travel is possible, then where are the time travelers?
TM: Well, when I asked that question to my sources, they said you can only travel as far back into the past as the moment of the invention of the first time machine. Because before that there were no time machines.
AB: Uh, let me think about that: you can only travel back as far as the invention of the first time machine, because before that there was no capability…
TM: It’s like trying to drive where there are no roads. It also means that when you invent the first time machine, instantly time machines will appear by the tens of thousands, having come through time back to see the first flight into time.

(Art Bell interviews Terence McKenna on Coast to Coast AM, May 22nd 1997)