davidson@sdcsvax.UUCP (Greg Davidson) (08/30/85)
< SPOILER WARNING ! SPOILER WARNING ! SPOILER WARNING ! > SF time travel buffs will recognise that the Marty1 who grew up with the nerdly father, alcoholic mother, etc., in future1 is a different person from the Marty2 that we saw briefly in future2 towards the end. Marty2's family and friends (including Jennifer) and even Doc will have a hard time dealing with the change in personality. Marty2 was probably a much more self-confident person, and the change will be disturbing. Also, Marty2 will behave differently in the past than Marty1 did, and he may or may not interfere with his parents' first meeting. If he does, he may not succeed in fixing it. Its certainly going to be a surprise and a shock if he meets his father and Biff, since he has a completely different expectation of them. If he's lucky, and doesn't run into his father before Doc can warn him, imagine what it will be like for him when he returns to future3 (yes, not future1, even if it differs only minutely, see below) and discovers that for no known reason his wonderful family has turned into deadbeats. Ugh! All the former assumes a branching worlds model. In a single line universe which allows revision, as in James Hogan's ``Thrice Upon a Time'', Martys 1&2 and futures 1&2 are only temporary, and in fact, time will only proceed onwards when a future is generated in which no one goes back. I don't like to think of time travel as destroying the universe in such a manner, but branching universes also disturb me (as you know if you've read my submissions to net.physics) with their inability to really change anything. In ``The Man Who Folded Himself'' (highly recommended) David Gerrold presents a branching universe time travel model, and makes it clear that in universes that invent time travel, backwards time travellers simply disappear, never to come back. So Jennifer1, Family1, etc. never heard from Marty again (of course, Doc1 couldn't, since he died). Quite sad. Its logically possible for forwards time travel to not branch universes, but it seems to me that a gadget would need to use a different time travel mechanism for non-branching forward travel. If backward time travel causes branching on arrival, its probably the arrival from non-adjacent spacetime that does it, and it would happen going in either ``direction''. All of this analysis leaves honest SF writers with a nasty choice for their stories: Either (1) characters can't change their past, but can leave and create a new one, (2) they can change the past by destroying the entire universe, or (3) they can't change the past. Note that the latter still allows time travel, as Heinlein and Anderson use this model in their stories. But I never believed their rationalizations of how one just wouldn't wind up changing things even if one tried. It strains any reasonable notion of causality. Can anyone think of any alternative to these models? BTW, I thought BTTF was a fabulous movie. Every detail; the story, the characterization, the background, etc. was fully exploited and well integrated. I've seen it three times so far, and expect to see it many more times. BTW2, I don't see how backwards erasure of time travellers who undermine their future existence can be supported by any reasonable model. (But read Robert Silverberg's ``Up The Line'' for a similar idea.) And I took the silly business with the family photograph as a clever model to help the general public understand what was going on, at the price of a few sniggers from the sophisticated viewer. _Greg Davidson Virtual Infinity Systems, San Diego