Every time I hear the tired old river bit, I want to scream.

	"And then there is the Sea." -- final sentence of "The River of
Time" by David Brin

   Contemporary science holds that Time Travel is impossible. 
This is a good view for contemporary science to take, as it helps 
insure that a functional time device will never be invented.

	What about wormholes?

   The biggest argument against time travel is that it spawns 
paradox like a tenement spawns roaches. A humble example:

   You go back in time and shoot your own grandfather, before the 
old bugger met your grandma. This means that you will never have 
been born, which means that you can't go back and plug grandpa. 
BUT, this means that grandpa lives, and you ARE born, and you can 
go back and shoot him, which means...

   See the problem?

	The biggest causality violation introduced by time travel is not
having effect precede cause, but rather having effects that have no cause.
If you can accept that the latter is possible, then it's also possible for
you never to have been born; you just appeared out of thin air, shot
someone, and continued your existence from that point.  It could also be
that you can kill your grandfather and return to the time you started from,
only to find that all trace of your existence has vanished.  This can be
interpreted to mean that you've been shunted into a universe different from
the one you were born in.

----------------------------------------------------------
PART ONE - BITE ME, ASIMOV

	What's Asimov got to do with this?  (BTW, have you read Larry
Niven's essay "The Theory and Practice of Time Travel"?)

   One of the biggest pitfalls that sci-fi time travel has is the 
We-must-not-divert-the-time-stream line. Some people even 
make it impossible for history to be dramatically altered.

	This is referred to as backward determinism.  In discussions of
backward determinism "history" is usually used as a synonym for "the past",
but it doesn't have anything to do with recorded history.  Here's an example
of backward determinism at work: say you invent a time machine, and just as
you're about to take it for a test flight, a duplicate of you appears in an
identical time machine, having come from one minute in the future.  If time
is backward-deterministic, then even if you change your mind about using the
time machine, no matter how much you try to resist, you *will* get into that
time machine one minute later, and disappear.

In other word, the E-W-G model assumes that there are a infinite 
number of universes based on every state vector that has ever 
been. Travel to a specific one of these, one that resembled a past 
frame of your universe, would be to you like time travel.

   It wouldn't change anything in your old universe, of course.

	A strict application of the many-worlds interpretation makes time
travel pointless.  Looking at the Grandfather Paradox again:

 * There are an infinite number of universes in which you go back
   in time and kill your grandfather.
 * There are an infinite number of universes in which you go back
   in time and fail to kill your grandfather.
 * There are an infinite number of universes in which you don't go
   back in time at all.
 * There are an infinite number of universes in which your grandfather
   died of natural causes and you were never born.

...and so on.  If every possible alternate universe exists, then there's no
point in going back to the past and preventing something from happening,
because there are universes out there in which that event failed to happen
without your interference.

	To return the discussion to "Smoke and Mirrors," one could have gone
up to Kasumi and said, "Hey, don't worry, your family is alive and well in
an infinite number of universes, but this universe just happens not to be
one of them," but I doubt she would have found that very reassuring.

	The point is, I don't think it's a good idea to explain time travel
using the Everett interpretation, because if the Everett interpretation were
correct, you wouldn't *need* time time travel.

	And personally, I think having the time-traveling Kasumi get killed
by the bomb blast was taking the easy way out.  Since the time-traveling
Kasumi is desperate to get her family back, and we know she's capable of
murder, why couldn't she have killed her double and taken her place?  It
would look like Kasumi had aged a month or so spontaneously, but no one
would ever notice.

	Of course, there are an infinite number of universes in which this
was precisely what happened.



-- 
"I will chide no breather in the world but myself,                 _____
 against whom I know most faults." - As You Like It         |_____|_____
                                     Act 3, Scene 2    _____|     |
            Julian Fong: fong_jh@sunset.bph.jhu.edu