Excerpt taken from BBC h2g2 http://www.bbc.co.uk/dna/h2g2/A398955
"What if, when we had invented time machines, you went back in time, and accidentally landed on your grandfather, who had not yet had children. You would simply pop out of existence because you could not have been born. Then, you could not have killed Grandpa, and so he survives and you are eventually born and go back in time and land on him again, so you instantly pop out of existence... you would be stuck in an endless loop. There are a few theories around how this could be resolved."
What if the nemesis is from the future, and Jacob from the past. The nemesis travels backwards in time as a result of an as yet unidentified need to destroy Jacob. What isn't known to the nemes…Read more >