From: ingles@engin.umich.edu (Ray Ingles)
Subject: Re: Time Travel and Causality Violation
Date: Thu, 17 Dec 92 00:48:27 EST
Organization: University of Michigan Engineering, Ann Arbor

In article <1992Dec16.111449.15440@newstand.syr.edu> pksuk@mothra.syr.EDU (Peter K. Suk) writes:
>Dear Scott,
>
>	I'm a-gonna toast you a little.  It's nothing personal, and we'd 
>love to hear your input in the future.  But I've got to singe you just
>a bit right now.
>
>In article <Bz9pw5.Csn@newcastle.ac.uk> S.A.McIntyre@durham.ac.uk writes:
>
>>Personally I think that time travel is an attractive concept and
>>perhaps possible, but the concept of paradoxes makes me consider
>>whether or not it will ever be possible in the frame of reality we are
>>now experiencing.
>
>It should make you discount the possibility.  The principle is the same
>as in Einstein's gedankenexperiments.  
>
>	--Peter Kwangjun Suk

 The following is an excerpt from a series of articles I posted back at
the beginning of the term...
 ------------------------------------------------------------
 I just want to point out that physicists have been noting that time
travel (technically, "Closed Timelike Curves") shows up in a lot of
solutions to general relativistic equations. I.e., time travel *might*
be physically possible. Maybe.
 I don't pretend to understand the math in these papers, but for the
curious... (this is only a short list, there's more out there)
 
 Loosely "pro" time travel:
 
 "Complete Analytic Extension of the Symmetry Axis of Kerr's Solution
of Einstein's Equations," Brandon Carter, Physical Review vol. 141 #4,
Jan. 1966
 
 "Global Structure of the Kerr Family of Gravitational Fields," Brandon
Carter, Phys. Rev. vol 174 #5, 25 Oct. 1968
 
 "Rotating Cylinders and the Possibility of Global Causality Violation,"*
Frank J. Tipler, Physical Review D vol. 9 #8, 15 April 1974
 
 "Quantum Mechanics Near Closed Timelike Lines," David Deutsch, Phys.
Rev. D vol. 44 #10, 15 Nov. 1991
 
 [I dunno the title], J. R. Gott, Phys. Rev. Letters  p. 1126 (1991)
 
*yes, Niven wrote a story with this title.
 
 "Anti" time travel:
 
 "An Obstacle to Building a Time Machine," Carroll, Farhi, and Guth,
Phys. Rev. Letters, vol. 68 #3, 20 Jan. 1992
 
 "Physical Cosmic Strings Do Not Generate Closed Timelike Curves," Deser,
Jackiw, and Hooft, Phys. Rev. Letters, vol. 68 #3, 20 Jan. 1992
---------------------------------------------------------------------

 Anyway, David Gerrold explained a way of looking at this in his _The_Man_
Who_Folded_Himself_. He called it "The Paradox Paradox." It goes something
like...
 If paradoxes are possible, then time travel is impossible. But...
 If time travel is possible, then paradoxes must be impossible.

 Even in Einstein's gedankenexperiments, when he came to a contradiction,
he could go with one side or another. Either light moved at different
speeds for different reference frames... or time & space dilation & other
funny stuff happened.
 As it turned out, there was evidence for the second proposition, and that
is what Einstein went with, and what sure seems to be the case.
 At present, we have *no* conclusive evidence one way or another concerning
time travel, so we're free to make whatever assumptions we like about it.
Gerrold made his in _TMWFH_ and, while I don't personally like them, he
was consistent with them. I'm sort of a pseudo-predestinist, kinda, and
consistent models can be done in that framework too.
 
 Sorry to flame you a bit, Peter, but I think you were a tad premature in
saying time travel a) is impossible, and b) of necessity leads to paradox.
Shame on you! Haven't you ever read "By His Bootstraps," "Lifeline," and
"All You Zombies..." by Heinlein? (BTW, paper #4 above explores the
'multiple-universes' line from a QM perspective... interesting.)

 :->
 
 Sincerely,
 Ray Ingles
 ingles@engin.umich.edu
 
 "The biggest problem with time travel is grammar." - (or something like
that) - Douglas Adams
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