🚀 go-pugleaf

A while back Edgar Swank posted on a response from Frank Tipler about
time travel.  Tipler referenced an article.  If the article is correct
then time travel is possible within the framework of "single-universe"
physics, without the need to invoke a sideways progression across
multiple timelines to avoid paradoxes, which was proposed in New
Scientist recently: 'Time Travel without the paradoxes' (and reprinted on
this forum by Steve Strong).

Just as a digression on the New Scientist piece.  NS reference an article
by Dr David Deutch, Physical Review D, October 1991 (no article name
given).  The reference is 'Quantum mechanics near closed timelike lines',
by DD, Physical Review D, Vol 44, #10, *November* 1991.  Typical of NS to
get the dates wrong in a time travel article....

Back to the Tipler reference.  The full reference is 'Cauchy problem in
spacetimes with closed timelike curves', Physical Review D Vol 42, #6,
September 1990, by J Friedman, MS Morris, ID Novikov, F Echeverria, G
Klinkhammer, KP Thorne and U Yurtsever.

-------------- Start of a highly selective review ----------------

The article builds on some earlier work done on the possibility of
constructing a "time-machine" via wormholes connecting different regions
of space-time.  You construct a wormhole (this is the hard part of the
recipe!), placing the usual obligatory, imaginary clocks at both ends.
Accelerate one end away to relativistic speeds so that it time dilates.
Then bring it back.  If a traveller enters the wormhole at the
gone-away-and-come-back end he reappears out of the stayed-at-home end in
the past.  The amount that the traveller goes back in time is the
difference in the clock readings.

Now imagine that the wormhole is connecting two of the pockets of a
snooker /billard/ pool table.  So knocking a ball down one the pockets
causes it to reappear out of the other, at an earlier time.  A paradox
occurs when the path of the ball is planned so that it collides with an
earlier copy of itself, deflecting the earlier ball from the path
necessary for the later ball to collide with it to exist.  This is an
impossible state of affairs since the ball must travel on a well defined
path (even if it does loop back and forth through spacetime).  What the
article claims is that for every paradoxical path there is another
non-paradoxical path, with the same *starting* path for the ball.
Sometimes there are multiple non-paradoxical paths (we'll come back to
this later).  If you plotted a causality-violating path and sent your
ball off towards the pocket then as the ball approached the "entry"
pocket a copy of itself would emerge from the "exit" pocket BUT on a
slightly different path than you had calculated.  Just sufficiently
different so that, instead of knocking the earlier version completely off
course and missing the "entry" hole, it has a glancing collision with its
earlier self.  The deflected earlier ball now enters the "entry" pocket
on a slightly altered path, which accounts for the slightly different
path that the later ball had taken on exit from the other pocket.  Thus
the paradox has been resolved and causality.

Whilst the authors claim to have proved this only for the elastic
collisions of a time-travelling ball with itself, they are hopeful that
it can be extended to more complex situations to remove more complex
paradoxes.  It's fun to speculate on how time-travel paradoxes can be
averted by this mechanism [this is my own example]:

Suppose someone starts construction of a time machine, intent on
murdering their grandfather.  Instead of appearing back in his
grandfather's time an older version of the traveller appears to the
younger, homicidal traveller and persuades him to abort the original
mission and instead go use the machine to stop the murder (by talking his
younger self out of it....).  Presumably the older traveller would have
no problem persuading the younger version to change his mission
objectives, since he has memories of the encounter and understands his
earlier self very well.

The trouble with dreaming up these escape-from-paradox scenarios is that,
generally speaking, there are too many solutions to each potential
paradox.  The question arises, how does the universe 'choose' between the
different resolutions?  This is a problem in classical, Newtonian
physics, where balls (and atoms) are expected to follow a single path.
However in quantum theory you are allowed to consider all the possible
paths, via the Feynmann sum-over-histories path-integral approach, which
removes all ambiguities.  This is the solution proposed by the article.
All possible configurations contribute to the Feynmann integral.

-------------End of review-------------

Well, that's my version of the article.  Summary:
 o  There's no logical reason why we can't travel through time,
    although the physical possibilities have yet to be demonstrated.
 o  We won't be able to change history.
 o  The mechanism that prevents the altering of history is the
    presence of time travellers, either ourselves or others.

As for why we see no time travellers.  I think that this is probably
because the time machine can't move through time.  Of the three
proposed mechanisms for time travel (spinning blackholes, rotating
cylinders and wormholes) each one only permits travel over the life-time
of the machine, which itself doesn't travel through time.  So we never
get to photograph dinosaurs (sob!).  And similarly our descendants (and
ouselves!) can not come and visit us until we get some time machines built
ourselves.

Deutsch's article on the parallel timelines idea, which is very appealing
and asthetically attractive, doesn't seem to have taken some of the above
conclusions onboard (despite the fact that he references it).  There are
no grounds in quantum theory for identifying the parallel timelines with
the other worlds of the many-worlds interpretation of QM.  The other
worlds in quantum theory are generated by the requirement that the
quantum theory be logical and objective, treating observers as automata.
 The other worlds are also capable of interfering with eachother,
although usually not very much.  Conversely the parallel timelines for
time travel Deutsch and SF writers invoke can never interfere with
eachother, once separated.  As such the multiple timelines are presently
unscientific and require an extension of physics (i.e. a new mechanism
for time travel) to be otherwise.

Mike Price



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