Date: Sun, 17 Mar 2002 00:52:09 -0500 (EST)
From: "Keith F. Lynch" <kfl at keithlynch.net>
To: WSFA members <WSFAlist at keithlynch.net>
Subject: [WSFA] Re: Waste of "Time Machine"
Reply-To: WSFA members <WSFAlist at keithlynch.net>

ronkean at juno.com wrote:
> Time travel poses a major philosophical stumbling block, in the
> implications of changing the past, which, presumably, could change
> the present.

I'm aware of five major self-consistent approaches to dealing with the
classic "grandfather paradox" of time travel (i.e. what it you go back
and kill your grandfather before your father was conceived):

* Backward time travel is impossible.  This is the most widely
  believed both among laymen and scientists, but has the serious
  flaw that it doesn't lead to interesting time travel stories.

* Events will somehow conspire to remain consistent.  If you go back
  to the Battle of Gettysburg, you will have been there the first time.
  Heinlein uses this approach in several books.

* History branches.  Change the past, and you simply create a new
  timeline.  L. Neil Smith prefers this approach.  James Hogan
  (_The Proteus Operation_) has also used it to good effect.  Some
  physicists think history branches anyway, even without the aid of
  meddling time travelers.  Some stories describe "sideways" time
  travel in which one can travel into parallel time-streams, but
  not into the past or future.  H. Beam Piper (_Lord Kalvan of
  Otherwhen_), F.M. Busby (_All These Earths_), Isaac Asimov (_Earth
  is Room Enough_), and Murray Leinster ("Sidewise in Time") are
  good examples.

* The stack theory:  Change the past and history starts over from that
  point.  The previous future is erased, except for whatever parts of
  it the time traveler brought back with him.  James Hogan (_Thrice
  Upon a Time_), John Cramer (_Einstein's Bridge_), and Orson Scott
  Card (_Pastwatch: The Redemption of Christopher Columbus_) have
  used this to good effect.

* Niven's law:  Time machines will never be invented since use of time
  machines would repeatedly alter the past until it was altered to
  cause the time machine not to have been invented.  In one of Larry
  Niven's short stories with a long title, one side in an interstellar
  war invents a time machine, but, believing in this law, decides to
  leak the information to their opponents instead of trying to build
  one, expecting that this will cause their civilization to collapse.

Of course many time travel books and movies either evade the issue or
have an inconsistent mish-mash.

> But suppose we ask whether it is possible to simply _look_ into the
> past, without actually 'being there', ...

It's true that this wouldn't lead to paradox.  It could lead to social
problems, as in Isaac Asimov's classic novella "The Dead Past".  Clarke
and Baxter's _The Light of Other Days_ is a good recent treatment.

> Suppose there were a webcam on a planet orbiting a nearby star,
> say, 5 light years away.  The camera and the computer to which it
> is connected would record events as they happen, and, assuming we
> had an internet connection to that computer, we could watch those
> events, but we would see them 5 years after they actually happened,
> because the internet transfers data at roughly the speed of light.

Yes, this could work, if they had a *really* strong telescope, and if
you didn't mind waiting ten years for the web page to load.

A simpler approach is to have an earth-sized flat mirror a few light
years away, and aim a nearby telescope at it.

More practical would be lots of cameras in low earth orbit, and lots
of videotapes.

We're already starting to get video cameras everywhere.  I have no
great objection to this (so long as they don't start peeking into
houses), but I think access to this should be available to everyone,
not just to the government.  Imagine being able to key in a location
and a date and time on your PC, and being shown what was visible then
and there (assuming a camera was there at the time).

> But suppose we had an internet connection which was many times
> faster than the speed of light, ...

Unless there are major flaws in special relativity (which seems
unlikely) faster than light communication implies an easy way to also
get back through time communication, which leads back to the classic
paradox.  Imagine being able to enter a location and a *future* date
and time on your PC and being shown what will appear then and there.
--
Keith F. Lynch - kfl at keithlynch.net - http://keithlynch.net/
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