<...If you go at the speed of light, you will go through time slower...>
According to Relativity theory, the faster you travel the slower you will appear to experience the rate of time's passage as measured by an observer not moving as fast as you.
Also, according to Relativity theory, the faster one moves the more massive you become (analogous to weighing more, but not the same thing.)As you approach the speed of light your mass approaches infinity. Theoretically, there is not enough energy in the universe to to move an infinite mass passed the speed of light.
<...if you go past the speed of light, you go backwards in time...>
Relativity says that as a person -- an astronaut, for instance -- approaches the speed of light, time passes the same speed it always has for the astronaut but an observer not travelling as fast as the astronaut sees the astronaut's rate of time slow toward zero.
Presumably, were the astronaut to actually pass the speed of light, the outside observer might see the rate of time's passage for the astronaut move backwards. It would not for the astronaut. That's why it's called Relativity.
The Tachyon is a theoretical particle that is said to be unable to travel slower than the speed of light. Presumably, tv characters use them to travel backwards in time.
