From: pwestlak@eoe.co.uk (Peter Westlake)
Newsgroups: rec.arts.sf.science
Subject: Re: Time Travel and Causality Violation
Date: 21 Dec 92 18:34:50 GMT
Organization: EO Europe Limited, Cambridge, UK
X-Newsreader: TIN [version 1.1 PL6]

Peter K. Suk (pksuk@mothra.syr.EDU) wrote:
: 
: 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.  
: 
: >Scott
: 
: 	--Peter Kwangjun Suk

There was an article in _New Scientist_ a few months ago (I can try and dig
it out of my house-moving boxes if anyone is particularly keen for the reference)
that discussed time travel in the light of quantum mechanics.  The idea is to avoid
paradoxes by observing that things don't simply happen or not happen:  everything
has a probability, and things can happen at random, without causality.  For instance,
a particle travelling from A to B might take the straight line route between them,
or it might detour to the side or indeed *anywhere* on its way.  The direct route
simply happens to be the most probable (Richard Feynman's book _QED: the Strange
Theory of Light and Matter_ describes this extremely clearly, BTW).  So, if a time
machine is constructed (the article suggests using wormholes, if they exist)
and a paradox attempted, the only thing that is ruled out by the reductio ad absurdam
that Peter refers to is the *exact* sequence of events being attempted - shooting an
ancestor, or whatever - and any variations on it that also cause a contradiction.
There is a whole class of stories based on informal versions of this idea, e.g. Fritz
Leiber's _Try and Change the Past_.

As an example, consider the case where you sends a signal into the past.  The signal
switches on a light, and you determine to create a paradox by only sending the signal
if you do not see the light come on.  Clearly there are several ways for this to fail:
the time signal might not get through, the light bulb might be broken, or you might
just fail to notice it.  Even if everything is in perfect working order when you start
the experiment, and it "would have worked" if you had been doing anything other than
sending a paradox (this ^^^^^^^^^^^^^^^^^ is a very dodgy concept, I know), then something
will happen to prevent things working out as you intend.  Now comes the really interesting
question:  which low-probability events are most probable?  A random surge of electrons
sufficient to blow the bulb seems very unlikely; any electronics involved might be more
sensitive, unless you used valves :-) ; and just how sensitive to small changes is the
investigator's own brain?  Could it be that the most likely consequence is that you
simply change your mind, out of fear at the idea of paradox, or for no apparent reason?
I suspect one would have to try it to find out.

Now take this to its logical extreme:  can we influence the outcome to our
own advantage?  Can we set up the experiment so that some very low-probability
event becomes reasonably likely?  Suppose that instead of a simple signal, you
send back a screenful of text generated at random - really at random, that is, not
by a pseudo-random sequence.  A scintillation counter would be a good source of
true noise, or the proverbial cup of really hot tea :-)  Then make the condition for
sending back the signal be that the text contains something that makes it worth
having, like the secret of eternal youth.  The problem then becomes that of selecting
the *right* low-probability event - in this particular case, where the choice is over
all possible messages, there are probably too many bogus ones to make it a good idea.
The number of ways of writing down chemical formulae to counteract all the effects of
aging are almost certainly outnumbered by the number of messages that are so
funny/alarming/generally heartstopping that you drop your pen/coffee/head onto the
signalling switch by mistake.  A much safer bet would be a random choice from a fixed
list, say of Derby winners.  Here the desired event - picking one name out of a couple
of dozen - is very much more likely than any of the things that could go wrong.

Finally, suppose that temporal communication and probabilistic "miracles" become
not only possible but commonplace.  When every decision can be undone;  with everyone
with access to a simple time signaller trying to guide the future into *their* earthly
paradise;  what kind of world would we live in then?

Peter.
-- 
Peter M Westlake                        EO Europe Ltd.,
Tel: +44 223-843131                     Abberley House,  Granhams Road,
Fax: +44 223-843032                     Great Shelford,
email:  pwestlak@eoe.co.uk              Cambridge CB2 5LQ,   England