text: time travelling problem...? if you went back in time, would you be able to change events that have already happened? what happens if you modified the past and got stuck in a loop. for example: you hate ure grand-father, and go in the past and kill him. but if he was killed, then your father cannot have been borned and neither you. if you couldn't have been born then how did you go back in the past and killed your grand-father, meaning you didn't so if you didn't.... (you get the cycle) this paradigm is the plot for lots of movies, (back to the future, somewhere in time, dr. who, etc). however it's not a question worth considering because time travel isn't physically possible nor will the advancement of technology ever make time travel possible.  here's a couple of reason's why:  1.  if time travel was possible, it would make logical sense that we would have had 'vistors' from the future already.  i'm sure they would have loved to come back and 'fix' a few things. 2.  the physcial science will not allow it based on time itself.  for you see.. time is relative to the object and/or observer.  this is the addtion to einstien's theory of relativety where as time consided by einstein was a set unchangeable measurable 'dimension'.  in fact, time is not only 'bend able' but time is relative and different for each observer.  2.  now let's talk about 'imaginary time'.  this is a proven physic theory that states that each object/person has it's own 'normal time' in which their 'journey' has progressed but their history is unknown because their 'travel' could have gone down multiple paths to reach the point they are at today.  3.  now let's talk about the physical location.  so.. you want to go back 70 years.  you get this machine set up in your back yard that can time in an instant to 70 years ago.  poof!  your sitting out in the middle of space wondering where the oxygen is!   for you see.. not only is the earth spinning and rotating and wobbleing at a somewhat non uniform rate... it's also rotating around the sun... while spinning around the galaxy ... while the galaxy is blasting through space at a million miles and hours.  your machine would have to be able to calculate exactly where the earth was in it's rotation, wobble, orbit and location in space.  trow in the time factor... and your screwed.  sorry.. no need to worry our brains about this kind of paradigm as it's not possible now.. nor will be in the future.  hope this helps!