Igor Novikov

... if one were to travel back in time, the laws of nature, or other intervening cause, would simply forbid the traveler from doing anything that could later result in their time travel not occurring. For example, a shot fired at the traveler's grandfather will miss, or the gun will jam, or misfire, or the grandfather will be injured but not killed, or the person killed will turn out to be not the real grandfather, or some other event will occur to prevent the attempt from succeeding. No action the traveller takes to affect change will ever succeed ...