starship-design: Re: Paradox

[Steve VanDevender <stevev@efn.org>]

Ken Wharton writes:
 > 
 > 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,
 >
 > 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.
 >
 > 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.
 >
 > There you have it.  Anyone for an option D)?

One thing you have to deal with in all of these possibilities is that
time travel, as usually conceived, produces local problems with
conservation of mass.  If you can actually go back in time and meet
yourself, then there's one more you worth of mass in the universe for
some amount of time.

Talking about somehow rewinding the worldlines of all your atoms to
prevent this is also problematic.  Short trips back in time would
involve meeting a heavily discombobulated self (missing most of its
atoms) as well as a lot of other partially discombobulated plants and
animals (you had to eat, after all :-).  So the only way to travel with
any hope of safety is to travel way back in time so that you minimize
the damage to innocent organisms who only miss a few atoms here or
there.

If you really can send mass back in time without affecting the mass that
was already there, then you could conceivably do all sorts of horrible
things, like send enough mass back to produce a very, very closed
universe that collapses before the future from which you sent back the
extra mass.  Before Isaac starts whacking me again I'll explicitly
disclaim that I don't know enough general relativity to even begin
evaluating this concept for plausibility.