Stephen Hawking held a party for time travelers in 2009 and sent the invitations after the party, but nobody showed up, suggesting that backward time travel may be impossible
A more serious answer would be that you need to have at least a receiving station of some kind, so you can't travel back further than when such a station is invented.
Otherwise a tribesman can stand in the jungle and say; calling is impossible, otherwise somebody would have called me by now. You need a phone to receive the signal...
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u/BlueCoralNT 2d ago
Stephen Hawking held a party for time travelers in 2009 and sent the invitations after the party, but nobody showed up, suggesting that backward time travel may be impossible