A slight modification may not seem like much, but what if it leads to your father thrusting at a different time when he impregnates your mother? You will cease to either exist or immediately turn into a completely different individual. This chain reaction makes traveling back through time risky.
This is another reason why if tiem travel were possible, it would have to mean everything has already ahppened that will happen. Either you assume EVERY time you travel back in time you reset everything, so there are numerous alt timelines running around, or you have to assume that nothing you do can alter anything that will happen, because it has already happened and has to happen that way.
There are way to many random things that can happen, like weather, sex of a baby, someone forgetting to do something one day, etc.
One good example of that is South Park in Go God Go. Where even the tiniest change Cartmen made to the past resulted in a different future (present), or you have Time Machine, where no matter what he tried to do to prevent the future from happening, and the past from happening the way it did, he couldn't.
In conclusion, to assuem time travel is possible, you have to either assume
a. Each time you travel, you create and alt timeline and an alternate universe essentially
b. Nothing you do will change anything, because everything that will or has happened, has to happen that way and will happen that way.
This post was edited on 11/28 at 12:09 pm