This argument: If time travel is possible, then it’s already happened and they’ve come back.
Yeah, sure. I mean…if someone comes up to me and says “I’m from the future”…the first thing I think is “oh, time travel is possible” not “oh, an escape mental patient.”
And even if time travellers did come back, how would they prove it to us?
“I am from the future! Here, come with me into my spaceship.”
Time traveller? Or an episode of Dateline: To Catch A Predator?
This argument: If time travel is possible, then it’s already happened and they’ve come back.
