From: ingles@engin.umich.edu (Ray Ingles)
Subject: Time Travel FAQ Version .0001 (long)
Date: Sat, 19 Dec 92 21:40:38 EST
Organization: University of Michigan Engineering, Ann Arbor
Keywords: FAQ, time travel, paradox, plebny

Okay, I'm going to try for a 'FAQ' on time travel, as it pops up from time to
time (no pun intended) and we keep covering the same ground.
 
  Definition: The "Grandfather Paradox" is the classic time travel paradox.
You go back and kill your grandfather before he meets your grandmother.
Therefore, you were never born. Therefore, you never went back to kill
him. Therefore, he's alive. Therefore, he's dead. Etc. How this paradox is
resolved is the main feature of any universe that involves time travel.

 Something needs to be cleared up before we go any further. There is no
such thing as a 'common-sense' type of time travel. No matter what version
you pick, *something* that everyone takes for granted will be hurled out
the window at the speed of light. You just can't stay normal when you have
a technology that lets you say, "I'll meet you in half an hour at the
battle of Pearl Harbor, with the weapon I got 300 years from now, and we'll
make sure that you and I will never have been having this conversation."
The various versions of time travel are attempts to preserve what the authors
consider most important. I can think of three main branches...
 
	I. So, Was Kennedy Assassinated in *Your* Timeline?
 The "many worlds" idea. You can travel into the past, but you immediately
create a whole new universe. Any changes you make apply only to this new
universe's history. If you kill your grandad, it isn't *your* grandad, it's
the guy who would have been the grandad of someone like you in this other
universe.
 Examples: Stories: H. Beam Piper's "Paratime" series, Niven's stuff in
"Flight of the Horse," David Gerrold's "The Man Who Folded Himself," L.
Sprague De Camp's "Lest Darkness Fall" Movies: "Back to the Future" parts
2 and 3.
 The 'many worlds' idea preserves a nearly normal sense of causality. I can
go back and construct the steam engine before one was ever built, but it was
still invented at some point, just in another universe, and I 'imported' it.
If you traces any chain of events backward, it may jump around any number of
universes at different times, but it will always go back, cause preceding
effect, to the beginning of time, if any.
 But the 'mw' idea cannot preserve both free will and the Law of Conservation
of Energy. If you assume that time-travel 'creates' universes, then where the
heck do they come from? You have to make the L.C.E. a local phenomenon. If
you assume that the universes were there and you traveled to them, then free
will takes a strange turn. Either every decision you could make you make
(Niven's "All the Myriad Ways") or only the universes you actually travel to
exist... and how did they know to be there?
 Finally, the 'many worlds' idea requires several more dimensions than the
ones we're familiar with in order to operate. You need at least five. Three
spatial, one traditional time, and another spatial to pile all these universes
on top of each other. While this is not necessarily a problem, it *does* make
it more complicated than some other versions.
 
	II. I Never Liked Grandpa Anyway...
 (a) *That* Was Easy...
 Well, you don't make alternate universes, you just change history.
Paradoxes are ignored, and easy to make. Grandad's toast.
 Examples: "Terminator 2,"  (I *hated* the plot of T2...) "Back to the
Future" (part 1)
 
 (b) Okay, *Next* Time We Try Nukes...
 You *can* change history, but it's rough. The universe changes odds
to protect its timeline. There's a sort of "historical inertia" to things. (I
think this should be called 'historisis,' if that's not too awful a pun...)
As Niven writes in his "Theory and Practice of Time Travel"... "If you
try to save Jesus with a machine gun, it will *positively* jam." Taking out
Grandad will require "fore"sight and  planning, and you have to watch he
doesn't get *you.*
 Examples: Niven's "Rotating Cylinders and the Possibility...," "Replay" by
someone or other, Anderson's (?) "Time Patrol" series, etc. Fairly common.

 The 'historisis' model preserves free will, but now there's a 'cosmic censor'
working against time-travellers, in some stories so deviously that it seems
conscious, and that has to be explained. In addition, this version falls
under Niven's Law. "Any universe that permits both time travel and changing
the past will not develop time travel." Either the Censor will forbid it, or
time travel will eventually be 'changed' out of ever having existed.
 Also, a not-so-obvious complication is that this model, too, requires an
extra dimension, one of time. You need a time for stuff to happen in, and
a 'hypertime' for changes in history to happen in. Consider, at one
'hypermoment' Grandad's alive, at a subsequent hypermoment you're travelling
back to kill him, and at a third hypermoment he's dead and you were never
born, except maybe you're still around. or maybe you 'poof out.'
 Finally, causality is folded, spindled, and mutilated. In 'T2,' eventually,
thanks to the help of Arnie the Good Cyborg, Skynet never came to be. So
Arnie the Good Cyborg never existed, and so Skynet won't be prevented, and
so... Some authors find this version somewhat repugnant, and look upon those
who use it as, er, lazy at best.
 
	III. Change the Past? Yeah, Right.
 Many stories hold that the past cannot be changed. Any change you try to
make is already part of the past you came from. The logic may be involved, but
everything adds up with no loose ends. Grandad will live, no matter what you
do. (Well, maybe. See below.) In fact, your attempt on his life back then may
make him hate redheads, so when he has a redheaded grandkid he rides the poor
child hard, so you grow up hating him...
 Examples: Stories: Heinlein's "Lifeline, " "All You Zombies," and "By His
Bootstraps" (all excellent stories that I recommend highly) Movies: "Time
After Time," "The Final Countdown," "The Terminator*," "Bill and Ted's
Excellent Adventure," and many more.
 *"The Terminator" (plot stolen from Harlan Ellison) is a near-perfect
example of this...
 This model preserves the L.C.E. and needs no hypertime. It also avoids
paradoxes and cosmic censors. But it requires, at best, a reformulation of
the concept of free will.
 In the 'many worlds' model, you do not travel into *the* past, you travel
into *a* past, not the one that you came from. In this model, you travel
into *the* past, since it's the only past that exists. Moreover, in a
sense you *already* went back there. To put it in perspective, if not in
grammatical English: If you are going to meet the love of your life in 18th
century New Orleans, then you are already back there with them.
 The key is to abandon the concept of a privileged present. All moments
are equal in the continuum, and they coexist in a four-dimensional static
sculpture. The future and the past already exist, and are as real as 'now,'
but are off in directions not normally accessible. The practical consequence
is that changing the past is not possible. It's fixed. Any change you try to
make already was part of the past.
 Perhaps this can best be explained by example. You set out to kill Grandpa
Many things could happen. For example...
 a) You appear, and get mugged and beaten to death in some alley before you
manage to do anything of note.
 b) You get there, pretend to befriend him so as to get him to let down his
guard, and find out he's not so bad a dude after all and decide not to do it.
 c) You kill him, and hide the body. You take his wallet, as you have none
of your own, and go to the bar to celebrate. You meet this cute little
redhead, fall in love, and decide to stay and get married, taking on your
Granddad's identity. A few years later you father a son and name him after
your dad...

 Well, this is getting long enough as it is, and this is just a first stab.
Any suggestions, comment, flames? Care to try to write one yourself?

 Sincerely,
 Raymond Ingles
 ingles@engin.umich.edu

 They said there's be snow at Christmas; they said there'd be peace on Earth.
 Hallelujah Noel, be it Heaven or Hell; the Christmas we get we deserve.
  - ELP, "I Believe in Father Christmas"
Newsgroups: rec.arts.sf.science