The previous post is a lead-in to talking about alternative history and time travel stories. The argument of alternative history stories is that history is not fixed. It could have turned out other than it has. I tend to see history as having a broad trend. As one of the panelists at CONvergence said, this is the river theory. For the most part, history keeps to its bed. This view says that it's hard to make major changes to history. But hard does mean not impossible. Rivers do change their courses. And I may be wrong. History may be far more changeable than I believe.
The other view of history is the tree theory: history is full of branching points, at which it could have taken a different turn. (Terry Pratchett calls this "the trousers theory of time." At certain points, history bifurcates, and there is a choice as to which leg one goes down.)
It's possible that history took both branches, creating two separate universes. This is the multiverse theory. Some physicists are interested in this.
Time travel stories also tend to see history as unfixed. One steps on a butterfly in the Mesozoic and the future one came from no longer exists -- or, if it still exists, it is in another universe, which cannot be reached. (This is "The Sound of Thunder" by Ray Bradbury.)
There are time travel stories in which history does not change. "The Man Who Murdered Mohammed" by Alfred Bester is one example. The time traveler goes back and kills key people in history. But the effect is not to change history. Rather, it changes him. With each murder, he becomes less likely and real, until he finally vanishes.
I argue, in "Big Red Mama in Time and Morris, Minnesota," just out in Big Mama Stories
from Aqueduct Press, that time is hard to change. For the most part, history re-stabilizes. This puts me at the conservative end of time travel theories.
But this does not lead me to argue that the present and future are fixed. And I do believe that large changes in the past might well change the present and future. If you went back and introduced modern technology to imperial Rome, as L. Sprague DeCamp did in Lest Darkness Fall
, you might well avoid the dark and middle ages.
I think the likelihood of time travel is still up for grabs. According to Lee Smolin of the Perimeter Institute in Canada, current theories about physics do not include anything about time. It is simply ignored. According to Smolin, this is a mistake: a successful description of reality needs to include time.
If we don't have a description of reality that includes time, then we can hardly know whether or not time travel is possible.
One of the odd things about time is -- it appears to go in one direction only. But we don't know why, and we don't know for sure whether or not this is always true. Most physical phenomena go in both directions. You can make water (H2O) from O and H2. You can also split water into O and H2. This cannot be done with time. Until we have a theory that explains this, backed by good experiments, we simply don't know for certain if it's possible to reverse time or travel back in it.
The main argument against time travel is the grandfather paradox, which doesn't convince me. If you don't know it, it goes like this: if time travel is possible, then you can go back and kill you grandfather before he fathered your father or mother. But then you don't exist, so you can't commit the murder.
I'm reading a paper by a philosopher, who argues you cannot kill your grandfather precisely because of this paradox. For one reason or another -- a change of heart, a gun misfiring -- the murder will not happen, because it cannot happen.
This also does not convince me. I imagine something closer to the Bester story. If you kill your grandfather, then you don't exist and cannot kill your grandfather. Therefore he lives and fathers one of your parents. Therefore you exist and go back in time and kill him. He dies and you don't exist and cannot kill him. He lives and fathers your parent, you exist and kill him, and the entire cycle happens over and over. Why can't this happen? We don't know enough to say.
In any case, I seem to be writing time travel stories and some alternative histories, and most likely I will continue to.
The important thing about both is as an aid to thinking about the present: for the most part, the stories say history is contingent and thus can be changed. Right now, we can only make changes in the present. Well, then, we ought to assume that what we do matters and work to make changes now. Floss. Go on a diet. Join good cause organizations. Plant a garden. Put solar panels on your roof. Believe in change.