there are alot of theories...here are a few: if you go back in time and change something, you will forever replay the same events up until the time you went back in time. If you go back in time and change something, an alternate reality would be formed. there are alot more. killing your grandfather is not a paradox, you would simply cease to exist in any timeline. Harry Potter's time travel- that is one of the theories that people have. time is just something that is interpreted, you may hear someone say "the week went by slow" or "the week went by fast." It's all a perception, nothing more, nothing less.
No time travel doesn't work like that. When you go back in time, you have created a new dimension, a dimension where you exist in the past. So when you travel back to your present time. You don't actualy go back to your dimension, you are simply going forward in time in the new dimension you've created. Its bit confusing but you should see this chart on Wikipedia - Back to the Future timeline. I know the chart is based on Marty McFly's adventures in the trilogy but it makes my point.
