I sometimes think about the problem of sending a message to someone in the far future such that they'd be guaranteed to receive it, regardless of their location on earth or attentiveness to the arrival time of the message.

It's a fun creative thinking exercise.

You can 'cheat' a bit by thinking of ways of sending a message such that ~everyone on earth would get it at the delivery time, including the intended target. But personalizing the delivery (thus adding privacy) makes the problem far, far more difficult.