time travel to the past is completely possible. (ex. move one end of a wormhole at speeds close to c, then go through the wormhole. this time machine can only take you back to when you started to move the wormhole. if someone from the other side jumped in, they would travel hundreds, thousands, or millions of years into the future, depending on how fast and how long you moved the wormhole.)
however, it is impossible to CHANGE the past. you may say that going back in time is changing the past, but it isn't. basically, if time travel occurred, then our present is built on that time travel. the fact that you are alive proves that if you went back in time to kill your parents, that you failed to kill them.