Hm...I must say, this is an awesome thread idea.

...Okay, gotta get started and all worked up. *ahem*

As the parallel universe theory seems in itself impossible, there must be some other explanation. And so, there must be only one universe, but how do we explain certain paradoxes arising from time travel? What would happen if you attempted to shoot your grandma in the past? According to the parallel universe theory, your grandma would die, but you would continue to exist in a modified branch of the universe. But again, we must seek another explanation disregarding the possibility of two universes. I'd speculate that you could not kill your grandmother--something would prevent it. This prevention must be traced to the source, yourself. There is no concievable way that a paradox could stop your free will and cognition, so we must take one step ahead, to the weapon. Say one used a gun--the gun must therefore cease to exist, because it cannot kill that which created the being that unleashes its potential. But Conservation of Matter doesn't allow the gun to simply dissapear, so it must change form. It couldn't change to energy--the energy released would kill both you and your grandmother as well as anybody else within a few kilometers of your location. So it must stay as matter. Since no one form seems more likely to take the gun's place than any other form, anything non-lethal is possible.

And hence i show ye undeniable proof that if you try to shoot your grandmother in the past, you'll likely be left holding a chicken rather than a gun.