From: jblum@hamlet.umd.edu (Hi ho -- Kermit the Frog here...)
Newsgroups: rec.arts.startrek.current
Subject: Re: time-travel backwards possible? perhaps
Date: 2 Oct 92 02:26:15 GMT
Organization: University of Maryland, College Park

In article <1992Sep30.224257.721@altair.selu.edu> scsd1005@altair.selu.edu (Terry Cooper) writes:
>In article <g92n3258.22.717595816@giraffe.ru.ac.za>, g92n3258@giraffe.ru.ac.za (MR RL NICOL) writes:
>What I found most curious, though, was that some folks either
>haven't thought about causality and the possibility of paradox
>that backward time travel opens up, or they just dismissed it out
>of hand.

I don't think redefining the concept of "paradox" as being possibly more
subtle than currently observed counts as ignoring it out of hand.

>I won't reject the notion that there may be parallel universes,
>alternate time lines, and such. But that's all they are: notions,
>not theories (yet!)

Agreed.  But the framework has neither been proved nor disproved.  It's
too early to reject them out of hand.

>Inventing the steam engine, flying in a heavier-than-air vehicle,
>telephones, TV, computers, going to the moon, transporting, even
>traveling to Epsilon Eridani in 2 days time -- none of those
>things need lead to paradox given the right conditions. Neither
>does being on earth today and near Rigil Kent two weeks later.

Yes, it does.  I didn't want to make this technical, because I'm sure
I would probably end up sticking my foot in my mouth bigtime, but let
me mention Bell's Inequality.  The "paradox" it suggested was the basis
of Einstein's objections to quantum mechanics.  It was solved by making
the concept of what exactly was meant by causality clearer.

...wait a moment, I don't have my PHYS 368 textbook here in the lab.
Here's a simpler explanation of why FTL travel violates causality.

Suppose, for the sake of illustration, that in order to bump your starship
to FTL speeds, you need to set off a supernova.  You find a nice star,
torch it, and are catapulted at warp one zillion straight to Earth.

The light from the supernova doesn't reach Earth for another thousand years.

This means that, from the point of view of an observer on Earth, the effect
-- your arrival -- has happened before the cause -- the supernova.

THAT'S causality, as defined by relativity.  The transmission of information
in a sensible order to observers.  Since, on the quantum level, the speed
of light is the only medium for the transmission of information, its speed
becomes the speed limit of the universe.  And it's that which FTL travel
_and_ time travel both violate.

(Ah! you say.  But the supernova really DID go off before I arrived!  Well,
yes it did... from YOUR viewpoint.  How can anyone else measure this?
Remember, there is no absolute time-scale for the universe, just like there
are no fixed coordinates.  Look at the twin paradox for an example.  It
shows that the measurement of time is -- guess what, Einstein? -- relative.
That's why they need an absolute upper bound on information transmission,
and that bound is light speed.)

Now...  I happen to grudgingly think that both time travel and warp speed
are probably beyond our capabilities.  *sigh*  However, I am not going
to insist that they are theoretically impossible -- simply BECAUSE theories
have been reinterpreted to allow for the impossible before.  Theories have
even predicted the impossible, like nuclear fission, and it has later been
found.  So, while I'm not holding my breath, I'm not going to dismiss
hypotheses that would allow such happenings out of hand.

And (ObStarTrek) I'm not going to base my literary criticism of stories
like "Time's Arrow" on which theory of the universe is currently
fashionable.


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