"Eliezer S. Yudkowsky" wrote:
>
>
> Actually, I've always maintained that scientists refusing to believe in
> naked singularities or closed timelike lines is a case of simple
> chauvinism. "We don't want to believe in paradoxes because, er, they're
> so ugly." It is absolutely no different, in form or content, from
> people refusing to believe that the Earth is not the center of the
> Universe. They cite no justifications from experiment or physical law.
> They don't want to believe in it, so it can't exist.
>
> I happen to believe that time travel is explicitly permitted by General
> Relativity, which forbids a single correct direction of time just as
> Special Relativity forbids an "ether" or single correct reference frame.
>
This reminds me of a _Scientific American_ article that I read, maybe five or so years ago, where the author (it was physicist David Deutsch, IIRC), was diagramming how time travel would work, assuming the quantum Many Worlds idea is correct. Apparently Deutch's book _The Fabric of Reality_ talks about this, the web site promoting the book is at http://www.qubit.org/people/david/FabricOfReality/FoR.html . A relevant quote from Deutch can be found in Bryce DeWitt's review at the site, Deutch says "Time travel may be achieved one day, or it may not. But if it is, it should not require any fundamental change in world-view, at least for those who broadly share the world view I am presenting in this book". So it sounds as though some interesting or consistent development of the closed time loop idea may be possible, basically along the lines of the kind of sf story where you get access to alternate time lines!
Notice that even if this radical idea should happen to work out for real, it doesn't remove the need for causality as such. For instance, the whole point of getting multiple timelines involved in this is to open up the "closed" time loops, so that the causal effects extend forward into another history, instead of actually going around in circles. Anyway, it's certainly difficult to say for sure what these strange GR "time warpy" ideas might come to! For instance, maybe someone could use time loops to arrange for some reliable system of FTL transfer, with all the protocols needed for dealing with the complication that you don't always arrive in your own historical universe? OTOH, maybe sheer biased conservatism will turn out to be correct, and the time travel/FTL thing will never happen, perhaps due to the special energy densities required being strictly impossible, in the end?
David Blenkinsop <blenl@sk.sympatico.ca>