Hello holog,

Why do people think if you go faster than light you can immediately go
back in time?
i.e. if you could travel at twice the speed of light it would still take
two years to get to alpha centuri, right?

That´s from the earth´s point of view. If you could travel to alpha
centauri at twice the speed of ligtht, the events "departure from
earth" and "arrival at alpha centauri" would be spacelike seperated
events. This means that their temporal sequence depends on the frame
of reference you choose. The violation of causality becomes obvious if
you immediately travel back from alpha centauri to earth, again at
twice the speed of light: Then your arrival on earth would be *before*
your departure from earth. SR avoids such violations of causality by
limiting the maximum speed at which you possibly can travel to the
speed of light.

regards,
Jürgen