From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: alt.sci.physics.new-theories
Subject: Re: time travell ??
Date: 8 Nov 92 23:03:25 GMT
Organization: Harvard University Science Center

In article <xeno.721200434@aviator.cc.iastate.edu> xeno@iastate.edu (Gary L Snethen) writes:
>
>Have you stopped to consider whether gravity has forward/reverse time
>symmetry?  If not, give it a try...  Perhaps photons 'move' backward in
>time...  Have they proven that anti-particle entropy always increases
>as our time goes forward?  Let me *propose* that it does not, but that the
>inverse rule applies:  Anti-particle entropy NEVER increases, but either
>stays the same or decreases.  Now, prove that anti-particles aren't
>'standard' particles moving through time in the opposite direction.

This is an interesting idea-- not one I'd consider *likely*, but since
nobody's ever had a big lump of antimatter to play with, the experiment
hasn't been done!  Of course, in the formalism of Feynman diagrams, 
antiparticles *are* to some extent treated as particles moving backward
in time, in the sense that the solution of the equation of motion selected
in the derivation of propagators is advanced rather than retarded.  It
turns out that this means less than one might think at first; it doesn't,
at least in that theory, imply that antiparticles can be used to send
messages to the past.

Nevertheless, backwards time travel is far from a crank notion-- it's
not even too far outside the mainstream, in the sense that the best
theory of gravity we currently have, general relativity, seems actually
to allow it!  Solutions to the equations exist which possess what are
called "closed timelike loops"-- paths along which one could in principle
travel into one's own past.  Most of these solutions appear to have very
little to do with the real world, and some are useless, such as 
situations in which the loop resides entirely within a black hole.
Nevertheless, given sufficient technology, some of them seem to have some
possibility of realization, like Tipler's large, rotating cylinder, or
the recent, very simple design which relies on the existence of cosmic
strings.  Often if you examine these more closely some obstacle tends
to crop up, like killer black holes or instabilities which destroy the
loop if anyone tries to use it.  Some physicists have gone so far as
to propose a "Chronology Protection Conjecture" which keeps a working
time machine from being constructed.  But so far there doesn't seem to
be any known physical principle which forbids such a thing.

This, of course, immediately brings to mind all of the paradoxical
issues which science-fiction writers have been playing with for
decades.  Nobody really knows what the proper means of dealing with
them might be; whether history always turns out to be self-consistent,
or if the time traveler could end up in another classical-like branch
of the universal wave function, or some such thing.  There are many
possibilities, few of which can be dismissed out of hand.

At any rate, all of these GR time machines seem to be extremely
hard to build and not very practical.  Nevertheless, if such a thing
exists, it raises all of the questions immediately.
-- 
Matt McIrvin
Newsgroups: alt.sci.physics.new-theories