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A tachyonic antitelephone is a hypothetical device in theoretical physics that could be used to send signals into one’s own past.

The paradoxes of backward-in-time communication are well known. Suppose A and B enter into the following agreement: A will send a message at three o’clock if and only if he does not receive one at one o’clock. B sends a message to reach A at one o’clock immediately on receiving one from A at three o’clock. Then the exchange of messages will take place if and only if it does not take place. This is a genuine paradox, a causal contradiction.