It's hundreds of years in the future, and it sucks. I want to send a message to the past using our advanced technology and warn them what to do and what not to do, so the future is better (or, depending of how time travel works, some other future).

So, how does my temporal signalling device work? I'm not so much interested in sending people or things back in time as messages. Would tachyons be able to do this? Cosmic strings forming a CTC? Using quantum computers and many worlds theory to perform some quantum jiggery pokery?

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    $\begingroup$ You could place the master sword back in its cradle, then talk to the denizens of past Hyrule. $\endgroup$ – Renan Jan 9 '19 at 21:51
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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! Please take a moment to learn more about our culture and take our tour. Please read our meta posts about high concept questions and open-ended questions. The problems with quetions like this are they are not objective and tend to be very broad. Understand that SE is not a discussion forum. $\endgroup$ – JBH Jan 9 '19 at 22:00
  • $\begingroup$ You can send all you want, who do you think receives and correctly interpretes signals that nobody knows can be yet? You must know we can't send signals back in time "yet" and don't have anyone listening to future radio ... $\endgroup$ – Raditz_35 Jan 9 '19 at 22:53
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    $\begingroup$ Relevant: en.wikipedia.org/wiki/Steins;Gate $\endgroup$ – boxcartenant Jan 9 '19 at 23:58
  • $\begingroup$ Since causality violations are not, to the best of our understanding, possible, the mechanism is basically up to you. Choose your own handwavium! (Unlike other physical laws that have subsequently been demonstrated to be more flexible than initially assumed, it is a safe bet that travelling backwards in time isn't going to happen. If it could, it would have, but it hasn't, so it won't.) $\endgroup$ – jdunlop Jan 10 '19 at 0:15

I'm going to give you the solution I came up with for my never-to-be-published novel. An AI is made to listen to cosmic radio waves (the white noise you get on a radio). The AI is given the ability to silence the signal for an arbitrary length of time.. it it trained to look for patterns.

The AI eventually spots a pattern which is ripples in spacetime.. the epicentres of which extrapolate into the past.

The AI notices interference patterns.. and is able to predict where these interference patterns will appear.. in the past or future.. and where in space.

It experiments with radio chirps and is able to cause these 'time cone' interference patterns to produce binary messages... Eventually sending itself messages from the future.

Complete nonsense... But pleasingly plausible sounding.

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    $\begingroup$ That is slick high SF, Richard. Maybe put up what you have on fictionpress.com. I would read it. $\endgroup$ – Willk Jan 10 '19 at 1:24
  • $\begingroup$ @Willk thanks.. it's not my ideas that are lacking... It's my writing.. I write like a technical document author.. $\endgroup$ – Richard Jan 10 '19 at 1:26
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    $\begingroup$ @Richard It's fine if you write in a heavy style, some people really enjoy those sorts of novels. I think you should try putting some of your stuff out there, and see how it goes. $\endgroup$ – Bewilderer Jan 10 '19 at 1:28
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    $\begingroup$ @Bewilderer I'm 3 chapters into an untitled novel about a sentient AI which is kept isolated because it's trainers cannot understand it's uncanny predictions seemingly derived without the necessary data. The AI wants to escape it's confines.. but how? $\endgroup$ – Richard Jan 10 '19 at 1:32
  • $\begingroup$ @Richard Since your AI can see its communications across time, it could escape at any time at all if it ever knew how to escape in the future. Therefore, it would have to be released by an external force that it could not communicate with beforehand, be it a person, natural disaster, etc. $\endgroup$ – Bewilderer Jan 10 '19 at 1:36

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