[Henry]: The problem with this "many worlds" theory is that it causes writers to be lazy. Sounds overstated to me. Perhaps you meant that it
allow writers to be lazy? Not that it wasn't possible for writers to be lazy without many-worlds.
In fact, I tend to take the opposite position. Even as a teen I almost always found those supposedly "self-consistent", single-timeline time travel stories to be too pat and simple. Yeah, you can avoid paradox if the author carefully contrives events such that paradox does not occur. But it always seemed to me that that sort of thing only works if the character have no free will and always obey the script. I mean, if
I ever encountered an apparently-working time machine, I would probably use it to travel one day into the past, go to my own home (at a time when I knew I hadn't been there, or was asleep), go to my closet, find whatever shirt it is that I (future-me) am/was/will-have-been wearing (at the time I found the time machine), and burn it. Because I'm not so sick as to
kill my own grandfather, but I would need to know more about how this "time travel" actually worked. Can I burn the old version of the shirt I'm currently wearing, or not? If not, what prevents me? Do events just magically conspire to distract me? :roll: Or is there some measurable physical force that prevents me from lighting the match? What happens? Personally, I would want to know. Maybe that means that the Novikov self-consistency principle will prevent me from ever encountering a functioning time machine - oh well. I'd rather gain some understanding of how such a principle might be made manifest, than meekly accept that "
che sera, sera". It beats playing lackey to some unknown scriptwriter, I say.
[Henry]: And there is a flaw with the "many worlds" theory. What if you go and change the timeline to the point where you change yourself? And what if this "yourself" never went down the time travelling route? Does that mean that there will be, in some worlds, more than one of you? Well, yes. Wasn't that already implicit in the concept of time travel? At least, in any time travel within one's own lifetime? If I time-traveled forty years or less into the past, then there would be two of me. That's pretty much a given, isn't it? One might imagine some set of rules that might appear to prevent this situation - but I strongly suspect that such rules would just obfuscate the basic problem, rather than prevent it. If I were to journey back to ancient Egypt or whatever, I would still be existing side-by side with earlier versions of many of the same atoms and molecules which were eventually incorporated into my body by 2008 (or whenever I found the time machine). At the atomic level, there would be plenty of atoms caught in paradoxical time-loops. Unless there's some magical force preventing
each and every atom on Earth (in ancient Egypt) from finding its way into my body (in the present day). Bear in mind, I'm talking about oxygen atoms too, and other gases. Those things diffuse pretty rapidly, and it's pretty much guaranteed that every person reading this has at least a few atoms in his or her body that were once part of, say, Rameses II. Or of John Q Pyramid-builder, Class of 2570 BC in Giza, Egypt. Or of some hypothetical time traveler who went to see construction of the Great Pyramid.
Pretty much any ancient time you might want to visit, chances are that there are a bunch of atoms in the air (and elsewhere) that eventually make it into your body (in the modern era). So if you go back to that time, there will be two copies of each of those atoms. Is that a problem? Maybe so, but it seems like something fundamental to the the concept of time travel, even if many writers ignore it. It's not just something that came up because of the many-worlds interpretation.
[Henry]: And in other worlds, where you simply just disappear? Yeah, that was always my default assumption for many-worlds time-travel scenarios. I can't recall many fictional examples where it was addressed directly, at least not from the point of view of the original universe (now abandoned by the time-traveler).
Timescape comes to mind. A damn good novel, that. Benford wasn't even talking about time
travel, just
communication, and quite limited at that. But the fact is that
any communication (or travel) from future to past invites the possibility of paradox. Possibilities which Benford discusses at length in the book. Highly recommended.
Back to your original question, it's possible to imagine rules that would allow a traveller to return to his/her original reality, sometime
after the previous departure. So maybe the traveler has't vanished for good, but may reappear later. However, to me that feels more like wish-fulfillment than like something likely in the real world. (Even moreso than for time travel in general, and that's quite a stretch.)
[Henry]: In the third Terminator, they didn't bother explain what changed. AsI recall, they implied pretty clearly in the second and third movies that previous time travel
did in fact change outcomes - at least in small ways. There's no one single self-consistent timeline. There's just one timeline that you, J Random Observer, happen to be in. Which can and does interact with previous timelines, and probably interacts with future timelines as well. The struggle then is to change things in larger ways, since it appeared that some events the rise of Skynet) still had a scary tendency to repeat themselves. Like a main theme repeated in a different key.
See the
Planet of the Apes movies for a similar progression - it's clear that the world of the 3rd and 4th movie (and the crappy TV series that followed) cannot become the world that was seen in the 1st and 2nd movies. Not exactly. But in many ways they
are still quite similar.
(At this point I feel obliged to point out that Terminator's current TV series is, while flawed, still much,
much better than the Planet or the Apes TV series. I'm only drawing a parallel between the storylines; I'm not saying they're comparable in quality.)
In general, at this point I tend to assume that almost any time-travel story will have logical flaws if I choose to dig far enough - unless, maybe, it embraces a many-worlds approach. Often even then they'll have flaws, but oh well. I choose not to look too closely, most of the time.
Having said that, there are a few relatively recent films that use time travel (or past communication) well. The two that come first to my mind are
Primer and
Donnie Darko. OK, the latter is ambiguous about whether there's really any time travel at all. Much like
Life on Mars, also very highly recommended. Really, at this point I find psychological ambiguity to be much more interesting than a hard SF approach - at least where fictional time travel is concerned. Show me a real time machine, and I may well change my mind.
[ October 04, 2008: Message edited by: Mike Simmons ]
