If you ever meet anyone who is planning on creating a time machine, and has a reasonable chance of success, you should kill them straight away. This is not to prevent the grandfather paradox. As far as I’m concerned, there is no reason not to go back in time and go gunning for grandpa. I’m more concerned about the act of time travel itself.
I am of course only concerning myself with backwards time travel. Forward time travel is fairly harmless and we have the technology right now to do it. (You just need to slow down personal time, and that is automatically done in the presence of any sufficiently dense object. [Insert fat momma jokes here.])
There are two theories of how you can get past time travel paradoxes. The first is the Novikov self-consistency principle. I like this one for fictional purposes, because it maintains consistency while still allowing fun things to happen. As long as you can make it look like nothing has changed, you can change as much as you want. i.e. Rescue every drowning person on the Titanic, as long as you replace them with identical drowned corpses, and then take them back with you to wherever you left from. But it also implies the idea of some greater force controlling things and making sure causality is maintained. As an atheist, I have trouble believing that. Physics doesn’t have a plan.
I’m more concerned about the parallel universe idea. Basically, when you go back in time, you create a parallel universe that you are free to mess up, but you are stuck there. So if you kill your grandfather he is dead and in the future of that universe you will not be born. But that isn’t the problem of your previous universe.
But let’s think about that: You are creating ANOTHER universe! Well, mass and energy are the same thing. Where is the energy coming from to create this universe? To create a parallel universe would require all the energy of this universe. And with entropy, you would actually need more than the energy of this universe. So if you built a time machine and went back, you would need to destroy this universe.
I’m opposed to that. This universe is where I keep all my stuff.