I take the following view of the past:  Anything that happens stays happened.  Once an event has passed, it cannot be altered, even if time travel is something that eventually comes to be.  If one was to try and go back in time and (for example) assassinate Hitler, one would be unable to because it is an even that has already taken place.  I like multiple universes as a fictional idea (particularly Heinlein’s fictons idea, where any book you read is actually telling a story about real people in some other dimension) but I think the real world is more solid than that.  This leads me to the following conclusion:

Since I have no recollection of somebody who looks like an older me coming back and punching me in the head and telling me not to make <mistake x>, I’m pretty sure time travel is not invented/discovered in my lifetime.  I expounded on this idea to Amy earlier in the week and she suggested that maybe I just haven’t made a mistake big enough yet.  I guess it is also possible that, since this face-punching never took place, even if time travel were invented, the face-punching could/would not have taken place.  You start to run out of verb tenses in a hurry when discussing non-linear time events.  I do know this.  In the event that I meet somebody with my name who looks just like an older me, I will pre-emptively  punch him and then steal his time machine.  Teenage Adam has some straightening out to do.  

no turning back, no turning back