Forgive me for nerding out guys, this is kinda my thing. :)

A Tachyon - theorized superluminal particle can theoretically travel faster than light and backwards in time, and would theoretically let you communicate with the past. Unfortunately the operate word is >theoretically<

And this is ALL theoretic. Tachyons themselves are purely theoretical, science has yet to encounter anything like one. And their properties are equally theoretically, and based on what General and Special relativity predict >might< happen.

Another problem, is that theoretically a Tachyon would only travel back in until it met something that innately travels forward in time. At which point it would rebound forward along a time-stream. Kinda defeating the point.

Time Travel is permitted under Relativity and may become a thing in the future. But at this point the technology required is so beyond our capability that we don't even know what we don't know.

Gravity effects Time Flow. Time travels faster in intense gravity, I age slightly faster on Earth than an Astronaut on the International Space Station. But the difference is picoseconds over a period of years. And weightlessness has terrible health consequences, meaning you can only stay in orbit for a year or so before needing to return to Earth and recover.