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?