starship-design: Re: Paradox

[wharton@physics.ucla.edu (Ken Wharton)]

Lee writes:

>Does anyone know of any arguments that would tend to prove or disprove
>the case for paradoxes? I don't mean FTL in particular, just time travel
>paradoxes.

Ooh-- the philosophy of time travel.  I've run across three possibilities:

A) All future time travel is ALREADY taken into account in our present,
and is therefore self-consistent.  The first Terminator movie is the
example that comes to mind; the Terminator comes back in time, but instead
of changing the future, he actually helps create the future from which he
comes from.

In this scenario, there has to be something preventing you from killing your
grandfather, or the time-travel won't be self-consistent.  This "something"
is a little philisophically shaky.  Another shaky thing about A) is that
it makes the universe much more non-deterministic than it already is.
Under this scenario, my future self could appear in front of me, and teach
me how to build a time machine so I could go back and tell myself how to 
build a time machine.  All completely consistent, but it's also completely
consistent if I never do such a thing.  How can the universe decide which
consistent reality to make if both are equally acceptable?  Tough questions.

B)  The second possibility is the alternate reality, where by going back in
time the universe branches off into a "new" version of what's happening.
This isn't quite as philisophically unweidly as A), but it does have some
problems.  (Strangely enough, this is the version of time-travel presented
in the second Terminator movie, where they DO actually change the future)
This B) scenario splits into two futher possibilities: either the time-travel
CREATES the alternate universe, or else the alternate universes already
exist, and you are merely travelling between them.  The latter option has
many more philisophical dilemmas than the first, but neither one particularly
makes sense to me.  But at least in this case, you CAN kill your grandfather;
you don't need to invent some physical mechanism to keep you from doing 
whatever you want.

C) The third, and most philisophically pleasing possibility has been hitting
the bookstands in the last few years, in SF like "Pastwatch" by Orson Scott
Card and "Einstein's Bridge" by John Cramer.  In this, by travelling backwards
in time you destroy the entire universe that exists between the time you
leave and the time you arrive, and start creating a new one.  You can have
knowledge of the destroyed part of the universe, and use it to help build
the new universe, but there are no "restrictions" on what you can do, and
there are no alternate universes to deal with.  Personally, I stil have a 
probelm with even this version of time-travel, mainly because I don't think
time is the linear progression that we experience; I think the fundamental
nature of time is very different from our perception of it.

There you have it.  Anyone for an option D)?

Ken