Stories of time travel to the past always run up against a possible very big problem. The grandfather paradox. Here's a way to imagine it:

You've traveled back in time, and you're in the past, and you get in an argument with a man about your age. You punch him in the nose, and he falls backward and hits his head and dies. Boom.

Little did you know, this young man was your own grandfather. He hadn't even met your grandmother yet, and your father hadn't even been born yet, and you killed him.

So your father will never be born soo?how will you be born?

But if you were never born, then logically, you couldn't kill your own grandpa.

So if he doesn't die, then you do get born, then you do kill him, then you don't get born, then you don't kill him, then you do, don't, do don'tdodon'tdodon't?ahhrrghh!

That's the grandfather paradox. If you travel to the past, you might affect the past. Then there will be an effect on the future, which is your present and you're caught in a paradox.

Different authors have resolved this paradox in different ways. In some stories, any effect on the past can multiply to cause this paradox (like in Ray Bradbury's "A Sound of Thunder"...you go hunting dinosaurs, you step on a bug, that bug was the ancestor of all humans, your entire world disappears, ahhrrghh!) so time travellers have to be isolated in some sort of protective field, or sometimes time travel to the past is just plain impossible.

Some authors work with a kind of Time Corps--some military or police organziation that keeps people from interfering too much in the past.

Some authors use a theory many (infinite) alternate pasts, presents, and futures--parallel world lines. In this model, you just visit one of many possible pasts. There's one where your grandpa dies, one where he lives, one where Hitler wins, one where Stalin wins, one where you ate that Turkey Sandwich for lunch, one where you just had a cup of lukewarm tea. And so on and so on.

Some authors let the paradox happen, and have fun with imagining the consequences.

And other authors just ignore the whole problem, or use that handwaving technique again.

All these possibilities, except for the last one, allow for the kind of mental agility and speculation about "what if?" that is at the core of SF's fun. Thinking through, and teasing out the implications and consequences of one particular technological or scientific advance or idea...that's SF at it's best. The grandfather paradox is not really an obstacle in good SF, it's an advantage ("it's not a bug, it's a feature").

No specific discussion board for this section. Go back and add some responses to earlier discussions.