From: ingles@engin.umich.edu (Ray Ingles)
Subject: Re: Time Travel and Causality Violation
Date: Fri, 18 Dec 92 14:34:52 EST
Organization: University of Michigan Engineering, Ann Arbor

In article <1992Dec18.182141.18256@porthos.cc.bellcore.com> ccw@prefect.cc.bellcore.com (wood,christopher) writes:
>In article <1992Dec18.165816.25061@bnr.ca> bucknerb@bnr.ca (Brent Buckner) writes:
>>In article <1992Dec18.154730.15628@porthos.cc.bellcore.com> ccw@prefect.cc.bellcore.com (wood,christopher) writes:
>
>>>My favorite time paradox is this one:  One builds a time machine, of any
>>>sort.  It could have a receiver and a transmitter if you like.  One
>>>gathers together a group of people, and some unique object.  One
>>>announces that at precisely 8:00, one will put the unique object into
>>>the time machine, set to go back in time half an hour.  At 7:30, if an
>>>object emerges from the time machine, one destroys the time machine and
>>>the unique object.  Now what happens?
>
>>In the parallel universes scheme, there are two resulting universes.
>>In Universe 1, at 7:30 you received no object, so you put the
>>object into the time machine.  In Universe 2, at 7:30 you received
>>an object, so you destroy the time machine and the object.
>
>Where does the object in universe 1 go?  Where did the object I received
>in universe 2 come from?  Could I use this as a duplicator?  Does this
>teleport the object from one universe to the other?
>
>What happens if the object I declare that I am going to put through the
>time machine is a Gold Brick?  If I get one, I take both bricks and
>leave.  The next night, I do the same thing with two bricks. After 64
>nights, I have enough bricks to cover california to quite a depth.

 Yes, it teleports objects from one universe to another. In one universe,
you wind up with enough gold to gild the Grand Canyon, while a lot of other
universes lose a gold brick.

>>In the single-universe no-paradox scheme, you do not receive
>>the object at 7:30.  When you try to send the object at
>>8:00 you fail.   
>
>Since this is true no matter how the time machine works, then I have a
>proof that in the single-universe no-paradox scheme, no time machine
>could work....

>Chris Wood     Bellcore     ...!bellcore!prefect!ccw

 Uh, no. You simply have a proof that no paradoxes can occur; or more
precisely, that situations that lead to paradoxes will not arise.
Since this is part of the assumtions, you have a tautology.
** Note, verb tenses will get funny here, tt makes a hash of English. **
 (There is one other possiblity- you send it back, but then decide not
to destroy it.)
 There are several potential solutions to the problem. Either...
 a) you sent it back and didn't destroy it, so it was still there to send
back, or
 b) you didn't send it back, or
 c) you sent it back but then destroyed it so you couldn't send it back.
And (c) is disallowed because it's physically impossible.
 Think of it this way. I have a square plot with a fence around it, and
I want to know how long the fence is, but I only know that the plot is
25 sq. meters. So, I solve the equation x=4*sqrt(25). I find that x is
either +20 or -20. But, I know that you can't have a negative length on
the fence, it's physically meaningless, so I throw out the second answer.
 So, time travel is possible, as long as it won't lead to paradoxes.
Whether this preserves 'free will' is another issue.

Sincerely,
Ray Ingles
ingles@engin.umich.edu

 They said there'd be snow at Christmas; They said there'd be peace on Earth.
 Halelujah Noel, be it Heaven or Hell; The Christmas we get we deserve.
      -ELP, "I Believe in Father Christmas"