Mention of economists reminds me of a proof of the non-existence of time travelers that employs Fermi paradox-like reasoning (Reinganum 1986–7): if time travelers existed, then interest rates would not be positive! In fact, if people could travel back in time, then interest rates would have to be 0%—otherwise savers could use banks as bottomless ATM machines. Savers could simply travel back in time a few thousand years, deposit a few dollars, then return to the present; compound interest on even a small sum would guarantee riches.
Wow
